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A History of Science, Volume 2 by Henry Smith Williams

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leaves of gold, silver, paper, etc." "Thus this globe," he says,
"when brought rather near drops of water causes them to swell and
puff up. It likewise attracts air, smoke, etc."[9] Before the
time of Guericke's demonstrations, Cabaeus had noted that chaff
leaped back from an "electric," but he did not interpret the
phenomenon as electrical repulsion. Von Guericke, however,
recognized it as such, and refers to it as what he calls
"expulsive virtue." "Even expulsive virtue is seen in this
globe," he says, "for it not only attracts, but also REPELS again
from itself little bodies of this sort, nor does it receive them
until they have touched something else." It will be observed from
this that he was very close to discovering the discharge of the
electrification of attracted bodies by contact with some other
object, after which they are reattracted by the electric.

He performed a most interesting experiment with his sulphur globe
and a feather, and in doing so came near anticipating Benjamin
Franklin in his discovery of the effects of pointed conductors in
drawing off the discharge. Having revolved and stroked his globe
until it repelled a bit of down, he removed the globe from its
rack and advancing it towards the now repellent down, drove it
before him about the room. In this chase he observed that the
down preferred to alight against "the points of any object
whatsoever." He noticed that should the down chance to be driven
within a few inches of a lighted candle, its attitude towards the
globe suddenly changed, and instead of running away from it, it
now "flew to it for protection" --the charge on the down having
been dissipated by the hot air. He also noted that if one face of
a feather had been first attracted and then repelled by the
sulphur ball, that the surface so affected was always turned
towards the globe; so that if the positions of the two were
reversed, the sides of the feather reversed also.

Still another important discovery, that of electrical conduction,
was made by Von Guericke. Until his discovery no one had observed
the transference of electricity from one body to another,
although Gilbert had some time before noted that a rod rendered
magnetic at one end became so at the other. Von Guericke's
experiments were made upon a linen thread with his sulphur globe,
which, he says, "having been previously excited by rubbing, can
exercise likewise its virtue through a linen thread an ell or
more long, and there attract something." But this discovery, and
his equally important one that the sulphur ball becomes luminous
when rubbed, were practically forgotten until again brought to
notice by the discoveries of Francis Hauksbee and Stephen Gray
early in the eighteenth century. From this we may gather that Von
Guericke himself did not realize the import of his discoveries,
for otherwise he would certainly have carried his investigations
still further. But as it was he turned his attention to other
fields of research.

ROBERT HOOKE

A slender, crooked, shrivelled-limbed, cantankerous little man,
with dishevelled hair and haggard countenance, bad-tempered and
irritable, penurious and dishonest, at least in his claims for
priority in discoveries--this is the picture usually drawn, alike
by friends and enemies, of Robert Hooke (1635-1703), a man with
an almost unparalleled genius for scientific discoveries in
almost all branches of science. History gives few examples so
striking of a man whose really great achievements in science
would alone have made his name immortal, and yet who had the
pusillanimous spirit of a charlatan--an almost insane mania, as
it seems--for claiming the credit of discoveries made by others.
This attitude of mind can hardly be explained except as a mania:
it is certainly more charitable so to regard it. For his own
discoveries and inventions were so numerous that a few more or
less would hardly have added to his fame, as his reputation as a
philosopher was well established. Admiration for his ability and
his philosophical knowledge must always be marred by the
recollection of his arrogant claims to the discoveries of other
philosophers.

It seems pretty definitely determined that Hooke should be
credited with the invention of the balance-spring for regulating
watches; but for a long time a heated controversy was waged
between Hooke and Huygens as to who was the real inventor. It
appears that Hooke conceived the idea of the balance-spring,
while to Huygens belongs the credit of having adapted the COILED
spring in a working model. He thus made practical Hooke's
conception, which is without value except as applied by the
coiled spring; but, nevertheless, the inventor, as well as the
perfector, should receive credit. In this controversy, unlike
many others, the blame cannot be laid at Hooke's door.

Hooke was the first curator of the Royal Society, and when
anything was to be investigated, usually invented the mechanical
devices for doing so. Astronomical apparatus, instruments for
measuring specific weights, clocks and chronometers, methods of
measuring the velocity of falling bodies, freezing and boiling
points, strength of gunpowder, magnetic instruments--in short,
all kinds of ingenious mechanical devices in all branches of
science and mechanics. It was he who made the famous air-pump of
Robert Boyle, based on Boyle's plans. Incidentally, Hooke claimed
to be the inventor of the first air-pump himself, although this
claim is now entirely discredited.

Within a period of two years he devised no less than thirty
different methods of flying, all of which, of course, came to
nothing, but go to show the fertile imagination of the man, and
his tireless energy. He experimented with electricity and made
some novel suggestions upon the difference between the electric
spark and the glow, although on the whole his contributions in
this field are unimportant. He also first pointed out that the
motions of the heavenly bodies must be looked upon as a
mechanical problem, and was almost within grasping distance of
the exact theory of gravitation, himself originating the idea of
making use of the pendulum in measuring gravity. Likewise, he
first proposed the wave theory of light; although it was Huygens
who established it on its present foundation.

Hooke published, among other things, a book of plates and
descriptions of his Microscopical Observations, which gives an
idea of the advance that had already been made in microscopy in
his time. Two of these plates are given here, which, even in this
age of microscopy, are both interesting and instructive. These
plates are made from prints of Hooke's original copper plates,
and show that excellent lenses were made even at that time. They
illustrate, also, how much might have been accomplished in the
field of medicine if more attention had been given to microscopy
by physicians. Even a century later, had physicians made better
use of their microscopes, they could hardly have overlooked such
an easily found parasite as the itch mite, which is quite as
easily detected as the cheese mite, pictured in Hooke's book.

In justice to Hooke, and in extenuation of his otherwise
inexcusable peculiarities of mind, it should be remembered that
for many years he suffered from a painful and wasting disease.
This may have affected his mental equilibrium, without
appreciably affecting his ingenuity. In his own time this
condition would hardly have been considered a disease; but
to-day, with our advanced ideas as to mental diseases, we should
be more inclined to ascribe his unfortunate attitude of mind to a
pathological condition, rather than to any manifestation of
normal mentality. From this point of view his mental deformity
seems not unlike that of Cavendish's, later, except that in the
case of Cavendish it manifested itself as an abnormal
sensitiveness instead of an abnormal irritability.

CHRISTIAN HUYGENS

If for nothing else, the world is indebted to the man who
invented the pendulum clock, Christian Huygens (1629-1695), of
the Hague, inventor, mathematician, mechanician, astronomer, and
physicist. Huygens was the descendant of a noble and
distinguished family, his father, Sir Constantine Huygens, being
a well-known poet and diplomatist. Early in life young Huygens
began his career in the legal profession, completing his
education in the juridical school at Breda; but his taste for
mathematics soon led him to neglect his legal studies, and his
aptitude for scientific researches was so marked that Descartes
predicted great things of him even while he was a mere tyro in
the field of scientific investigation.

One of his first endeavors in science was to attempt an
improvement of the telescope. Reflecting upon the process of
making lenses then in vogue, young Huygens and his brother
Constantine attempted a new method of grinding and polishing,
whereby they overcame a great deal of the spherical and chromatic
aberration. With this new telescope a much clearer field of
vision was obtained, so much so that Huygens was able to detect,
among other things, a hitherto unknown satellite of Saturn. It
was these astronomical researches that led him to apply the
pendulum to regulate the movements of clocks. The need for some
more exact method of measuring time in his observations of the
stars was keenly felt by the young astronomer, and after several
experiments along different lines, Huygens hit upon the use of a
swinging weight; and in 1656 made his invention of the pendulum
clock. The year following, his clock was presented to the
states-general. Accuracy as to time is absolutely essential in
astronomy, but until the invention of Huygens's clock there was
no precise, nor even approximately precise, means of measuring
short intervals.

Huygens was one of the first to adapt the micrometer to the
telescope--a mechanical device on which all the nice
determination of minute distances depends. He also took up the
controversy against Hooke as to the superiority of telescopic
over plain sights to quadrants, Hooke contending in favor of the
plain. In this controversy, the subject of which attracted wide
attention, Huygens was completely victorious; and Hooke, being
unable to refute Huygens's arguments, exhibited such irritability
that he increased his already general unpopularity. All of the
arguments for and against the telescope sight are too numerous to
be given here. In contending in its favor Huygens pointed out
that the unaided eye is unable to appreciate an angular space in
the sky less than about thirty seconds. Even in the best quadrant
with a plain sight, therefore, the altitude must be uncertain by
that quantity. If in place of the plain sight a telescope is
substituted, even if it magnify only thirty times, it will enable
the observer to fix the position to one second, with
progressively increased accuracy as the magnifying power of the
telescope is increased. This was only one of the many telling
arguments advanced by Huygens.

In the field of optics, also, Huygens has added considerably to
science, and his work, Dioptrics, is said to have been a favorite
book with Newton. During the later part of his life, however,
Huygens again devoted himself to inventing and constructing
telescopes, grinding the lenses, and devising, if not actually
making, the frame for holding them. These telescopes were of
enormous lengths, three of his object-glasses, now in possession
of the Royal Society, being of 123, 180, and 210 feet focal
length respectively. Such instruments, if constructed in the
ordinary form of the long tube, were very unmanageable, and to
obviate this Huygens adopted the plan of dispensing with the tube
altogether, mounting his lenses on long poles manipulated by
machinery. Even these were unwieldy enough, but the difficulties
of manipulation were fully compensated by the results obtained.

It had been discovered, among other things, that in oblique
refraction light is separated into colors. Therefore, any small
portion of the convex lens of the telescope, being a prism, the
rays proceed to the focus, separated into prismatic colors, which
make the image thus formed edged with a fringe of color and
indistinct. But, fortunately for the early telescope makers, the
degree of this aberration is independent of the focal length of
the lens; so that, by increasing this focal length and using the
appropriate eye-piece, the image can be greatly magnified, while
the fringe of colors remains about the same as when a less
powerful lens is used. Hence the advantage of Huygens's long
telescope. He did not confine his efforts to simply lengthening
the focal length of his telescopes, however, but also added to
their efficiency by inventing an almost perfect achromatic
eye-piece.

In 1663 he was elected a fellow of the Royal Society of London,
and in 1669 he gave to that body a concise statement of the laws
governing the collision of elastic bodies. Although the same
views had been given by Wallis and Wren a few weeks earlier,
there is no doubt that Huygens's views were reached
independently; and it is probable that he had arrived at his
conclusions several years before. In the Philosophical
Transactions for 1669 it is recorded that the society, being
interested in the laws of the principles of motion, a request was
made that M. Huygens, Dr. Wallis, and Sir Christopher Wren submit
their views on the subject. Wallis submitted his paper first,
November 15, 1668. A month later, December 17th, Wren imparted to
the society his laws as to the nature of the collision of bodies.
And a few days later, January 5, 1669, Huygens sent in his "Rules
Concerning the Motion of Bodies after Mutual Impulse." Although
Huygens's report was received last, he was anticipated by such a
brief space of time, and his views are so clearly stated--on the
whole rather more so than those of the other two--that we give
them in part here:

"1. If a hard body should strike against a body equally hard at
rest, after contact the former will rest and the latter acquire a
velocity equal to that of the moving body.

"2. But if that other equal body be likewise in motion, and
moving in the same direction, after contact they will move with
reciprocal velocities.

"3. A body, however great, is moved by a body however small
impelled with any velocity whatsoever.

"5. The quantity of motion of two bodies may be either increased
or diminished by their shock; but the same quantity towards the
same part remains, after subtracting the quantity of the contrary
motion.

"6. The sum of the products arising from multiplying the mass of
any hard body into the squares of its velocity is the same both
before and after the stroke.

"7. A hard body at rest will receive a greater quantity of motion
from another hard body, either greater or less than itself, by
the interposition of any third body of a mean quantity, than if
it was immediately struck by the body itself; and if the
interposing body be a mean proportional between the other two,
its action upon the quiescent body will be the greatest of
all."[10]

This was only one of several interesting and important
communications sent to the Royal Society during his lifetime. One
of these was a report on what he calls "Pneumatical Experiments."
"Upon including in a vacuum an insect resembling a beetle, but
somewhat larger," he says, "when it seemed to be dead, the air
was readmitted, and soon after it revived; putting it again in
the vacuum, and leaving it for an hour, after which the air was
readmitted, it was observed that the insect required a longer
time to recover; including it the third time for two days, after
which the air was admitted, it was ten hours before it began to
stir; but, putting it in a fourth time, for eight days, it never
afterwards recovered.... Several birds, rats, mice, rabbits, and
cats were killed in a vacuum, but if the air was admitted before
the engine was quite exhausted some of them would recover; yet
none revived that had been in a perfect vacuum.... Upon putting
the weight of eighteen grains of powder with a gauge into a
receiver that held several pounds of water, and firing the
powder, it raised the mercury an inch and a half; from which it
appears that there is one-fifth of air in gunpowder, upon the
supposition that air is about one thousand times lighter than
water; for in this experiment the mercury rose to the eighteenth
part of the height at which the air commonly sustains it, and
consequently the weight of eighteen grains of powder yielded air
enough to fill the eighteenth part of a receiver that contained
seven pounds of water; now this eighteenth part contains
forty-nine drachms of water; wherefore the air, that takes up an
equal space, being a thousand times lighter, weighs
one-thousandth part of forty-nine drachms, which is more than
three grains and a half; it follows, therefore, that the weight
of eighteen grains of powder contains more than three and a half
of air, which is about one-fifth of eighteen grains...."

From 1665 to 1681, accepting the tempting offer made him through
Colbert, by Louis XIV., Huygens pursued his studies at the
Bibliotheque du Roi as a resident of France. Here he published
his Horologium Oscillatorium, dedicated to the king, containing,
among other things, his solution of the problem of the "centre of
oscillation." This in itself was an important step in the history
of mechanics. Assuming as true that the centre of gravity of any
number of interdependent bodies cannot rise higher than the point
from which it falls, he reached correct conclusions as to the
general principle of the conservation of vis viva, although he
did not actually prove his conclusions. This was the first
attempt to deal with the dynamics of a system. In this work,
also, was the true determination of the relation between the
length of a pendulum and the time of its oscillation.

In 1681 he returned to Holland, influenced, it is believed, by
the attitude that was being taken in France against his religion.
Here he continued his investigations, built his immense
telescopes, and, among other things, discovered "polarization,"
which is recorded in Traite de la Lumiere, published at Leyden in
1690. Five years later he died, bequeathing his manuscripts to
the University of Leyden. It is interesting to note that he never
accepted Newton's theory of gravitation as a universal property
of matter.

XI. NEWTON AND THE COMPOSITION OF LIGHT

Galileo, that giant in physical science of the early seventeenth
century, died in 1642. On Christmas day of the same year there
was born in England another intellectual giant who was destined
to carry forward the work of Copernicus, Kepler, and Galileo to a
marvellous consummation through the discovery of the great
unifying law in accordance with which the planetary motions are
performed. We refer, of course, to the greatest of English
physical scientists, Isaac Newton, the Shakespeare of the
scientific world. Born thus before the middle of the seventeenth
century, Newton lived beyond the first quarter of the eighteenth
(1727). For the last forty years of that period his was the
dominating scientific personality of the world. With full
propriety that time has been spoken of as the "Age of Newton."

Yet the man who was to achieve such distinction gave no early
premonition of future greatness. He was a sickly child from
birth, and a boy of little seeming promise. He was an indifferent
student, yet, on the other hand, he cared little for the common
amusements of boyhood. He early exhibited, however, a taste for
mechanical contrivances, and spent much time in devising
windmills, water-clocks, sun-dials, and kites. While other boys
were interested only in having kites that would fly, Newton--at
least so the stories of a later time would have us
understand--cared more for the investigation of the seeming
principles involved, or for testing the best methods of attaching
the strings, or the best materials to be used in construction.

Meanwhile the future philosopher was acquiring a taste for
reading and study, delving into old volumes whenever he found an
opportunity. These habits convinced his relatives that it was
useless to attempt to make a farmer of the youth, as had been
their intention. He was therefore sent back to school, and in the
summer of 1661 he matriculated at Trinity College, Cambridge.
Even at college Newton seems to have shown no unusual mental
capacity, and in 1664, when examined for a scholarship by Dr.
Barrow, that gentleman is said to have formed a poor opinion of
the applicant. It is said that the knowledge of the estimate
placed upon his abilities by his instructor piqued Newton, and
led him to take up in earnest the mathematical studies in which
he afterwards attained such distinction. The study of Euclid and
Descartes's "Geometry" roused in him a latent interest in
mathematics, and from that time forward his investigations were
carried on with enthusiasm. In 1667 he was elected Fellow of
Trinity College, taking the degree of M.A. the following spring.

It will thus appear that Newton's boyhood and early manhood were
passed during that troublous time in British political annals
which saw the overthrow of Charles I., the autocracy of Cromwell,
and the eventual restoration of the Stuarts. His maturer years
witnessed the overthrow of the last Stuart and the reign of the
Dutchman, William of Orange. In his old age he saw the first of
the Hanoverians mount the throne of England. Within a decade of
his death such scientific path-finders as Cavendish, Black, and
Priestley were born--men who lived on to the close of the
eighteenth century. In a full sense, then, the age of Newton
bridges the gap from that early time of scientific awakening
under Kepler and Galileo to the time which we of the twentieth
century think of as essentially modern.

THE COMPOSITION OF WHITE LIGHT

In December, 1672, Newton was elected a Fellow of the Royal
Society, and at this meeting a paper describing his invention of
the refracting telescope was read. A few days later he wrote to
the secretary, making some inquiries as to the weekly meetings of
the society, and intimating that he had an account of an
interesting discovery that he wished to lay before the society.
When this communication was made public, it proved to be an
explanation of the discovery of the composition of white light.
We have seen that the question as to the nature of color had
commanded the attention of such investigators as Huygens, but
that no very satisfactory solution of the question had been
attained. Newton proved by demonstrative experiments that white
light is composed of the blending of the rays of diverse colors,
and that the color that we ascribe to any object is merely due to
the fact that the object in question reflects rays of that color,
absorbing the rest. That white light is really made up of many
colors blended would seem incredible had not the experiments by
which this composition is demonstrated become familiar to every
one. The experiments were absolutely novel when Newton brought
them forward, and his demonstration of the composition of light
was one of the most striking expositions ever brought to the
attention of the Royal Society. It is hardly necessary to add
that, notwithstanding the conclusive character of Newton's work,
his explanations did not for a long time meet with general
acceptance.

Newton was led to his discovery by some experiments made with an
ordinary glass prism applied to a hole in the shutter of a
darkened room, the refracted rays of the sunlight being received
upon the opposite wall and forming there the familiar spectrum.
"It was a very pleasing diversion," he wrote, "to view the vivid
and intense colors produced thereby; and after a time, applying
myself to consider them very circumspectly, I became surprised to
see them in varying form, which, according to the received laws
of refraction, I expected should have been circular. They were
terminated at the sides with straight lines, but at the ends the
decay of light was so gradual that it was difficult to determine
justly what was their figure, yet they seemed semicircular.

"Comparing the length of this colored spectrum with its breadth,
I found it almost five times greater; a disproportion so
extravagant that it excited me to a more than ordinary curiosity
of examining from whence it might proceed. I could scarce think
that the various thicknesses of the glass, or the termination
with shadow or darkness, could have any influence on light to
produce such an effect; yet I thought it not amiss, first, to
examine those circumstances, and so tried what would happen by
transmitting light through parts of the glass of divers
thickness, or through holes in the window of divers bigness, or
by setting the prism without so that the light might pass through
it and be refracted before it was transmitted through the hole;
but I found none of those circumstances material. The fashion of
the colors was in all these cases the same.

"Then I suspected whether by any unevenness of the glass or other
contingent irregularity these colors might be thus dilated. And
to try this I took another prism like the former, and so placed
it that the light, passing through them both, might be refracted
contrary ways, and so by the latter returned into that course
from which the former diverted it. For, by this means, I thought,
the regular effects of the first prism would be destroyed by the
second prism, but the irregular ones more augmented by the
multiplicity of refractions. The event was that the light, which
by the first prism was diffused into an oblong form, was by the
second reduced into an orbicular one with as much regularity as
when it did not all pass through them. So that, whatever was the
cause of that length, 'twas not any contingent irregularity.

"I then proceeded to examine more critically what might be
effected by the difference of the incidence of rays coming from
divers parts of the sun; and to that end measured the several
lines and angles belonging to the image. Its distance from the
hole or prism was 22 feet; its utmost length 13 1/4 inches; its
breadth 2 5/8; the diameter of the hole 1/4 of an inch; the angle
which the rays, tending towards the middle of the image, made
with those lines, in which they would have proceeded without
refraction, was 44 degrees 56'; and the vertical angle of the
prism, 63 degrees 12'. Also the refractions on both sides of the
prism--that is, of the incident and emergent rays--were, as near
as I could make them, equal, and consequently about 54 degrees
4'; and the rays fell perpendicularly upon the wall. Now,
subducting the diameter of the hole from the length and breadth
of the image, there remains 13 inches the length, and 2 3/8 the
breadth, comprehended by those rays, which, passing through the
centre of the said hole, which that breadth subtended, was about
31', answerable to the sun's diameter; but the angle which its
length subtended was more than five such diameters, namely 2
degrees 49'.

"Having made these observations, I first computed from them the
refractive power of the glass, and found it measured by the ratio
of the sines 20 to 31. And then, by that ratio, I computed the
refractions of two rays flowing from opposite parts of the sun's
discus, so as to differ 31' in their obliquity of incidence, and
found that the emergent rays should have comprehended an angle of
31', as they did, before they were incident.

"But because this computation was founded on the hypothesis of
the proportionality of the sines of incidence and refraction,
which though by my own experience I could not imagine to be so
erroneous as to make that angle but 31', which in reality was 2
degrees 49', yet my curiosity caused me again to make my prism.
And having placed it at my window, as before, I observed that by
turning it a little about its axis to and fro, so as to vary its
obliquity to the light more than an angle of 4 degrees or 5
degrees, the colors were not thereby sensibly translated from
their place on the wall, and consequently by that variation of
incidence the quantity of refraction was not sensibly varied. By
this experiment, therefore, as well as by the former computation,
it was evident that the difference of the incidence of rays
flowing from divers parts of the sun could not make them after
decussation diverge at a sensibly greater angle than that at
which they before converged; which being, at most, but about 31'
or 32', there still remained some other cause to be found out,
from whence it could be 2 degrees 49'."

All this caused Newton to suspect that the rays, after their
trajection through the prism, moved in curved rather than in
straight lines, thus tending to be cast upon the wall at
different places according to the amount of this curve. His
suspicions were increased, also, by happening to recall that a
tennis-ball sometimes describes such a curve when "cut" by a
tennis-racket striking the ball obliquely.

"For a circular as well as a progressive motion being
communicated to it by the stroke," he says, "its parts on that
side where the motions conspire must press and beat the
contiguous air more violently than on the other, and there excite
a reluctancy and reaction of the air proportionately greater. And
for the same reason, if the rays of light should possibly be
globular bodies, and by their oblique passage out of one medium
into another acquire a circulating motion, they ought to feel the
greater resistance from the ambient ether on that side where the
motions conspire, and thence be continually bowed to the other.
But notwithstanding this plausible ground of suspicion, when I
came to examine it I could observe no such curvity in them. And,
besides (which was enough for my purpose), I observed that the
difference 'twixt the length of the image and diameter of the
hole through which the light was transmitted was proportionable
to their distance.

"The gradual removal of these suspicions at length led me to the
experimentum crucis, which was this: I took two boards, and,
placing one of them close behind the prism at the window, so that
the light must pass through a small hole, made in it for the
purpose, and fall on the other board, which I placed at about
twelve feet distance, having first made a small hole in it also,
for some of the incident light to pass through. Then I placed
another prism behind this second board, so that the light
trajected through both the boards might pass through that also,
and be again refracted before it arrived at the wall. This done,
I took the first prism in my hands and turned it to and fro
slowly about its axis, so much as to make the several parts of
the image, cast on the second board, successively pass through
the hole in it, that I might observe to what places on the wall
the second prism would refract them. And I saw by the variation
of these places that the light, tending to that end of the image
towards which the refraction of the first prism was made, did in
the second prism suffer a refraction considerably greater than
the light tending to the other end. And so the true cause of the
length of that image was detected to be no other than that LIGHT
consists of RAYS DIFFERENTLY REFRANGIBLE, which, without any
respect to a difference in their incidence, were, according to
their degrees of refrangibility, transmitted towards divers parts
of the wall."[1]

THE NATURE OF COLOR

Having thus proved the composition of light, Newton took up an
exhaustive discussion as to colors, which cannot be entered into
at length here. Some of his remarks on the subject of compound
colors, however, may be stated in part. Newton's views are of
particular interest in this connection, since, as we have already
pointed out, the question as to what constituted color could not
be agreed upon by the philosophers. Some held that color was an
integral part of the substance; others maintained that it was
simply a reflection from the surface; and no scientific
explanation had been generally accepted. Newton concludes his
paper as follows:

"I might add more instances of this nature, but I shall conclude
with the general one that the colors of all natural bodies have
no other origin than this, that they are variously qualified to
reflect one sort of light in greater plenty than another. And
this I have experimented in a dark room by illuminating those
bodies with uncompounded light of divers colors. For by that
means any body may be made to appear of any color. They have
there no appropriate color, but ever appear of the color of the
light cast upon them, but yet with this difference, that they are
most brisk and vivid in the light of their own daylight color.
Minium appeareth there of any color indifferently with which 'tis
illustrated, but yet most luminous in red; and so Bise appeareth
indifferently of any color with which 'tis illustrated, but yet
most luminous in blue. And therefore Minium reflecteth rays of
any color, but most copiously those indued with red; and
consequently, when illustrated with daylight--that is, with all
sorts of rays promiscuously blended--those qualified with red
shall abound most in the reflected light, and by their prevalence
cause it to appear of that color. And for the same reason, Bise,
reflecting blue most copiously, shall appear blue by the excess
of those rays in its reflected light; and the like of other
bodies. And that this is the entire and adequate cause of their
colors is manifest, because they have no power to change or alter
the colors of any sort of rays incident apart, but put on all
colors indifferently with which they are enlightened."[2]

This epoch-making paper aroused a storm of opposition. Some of
Newton's opponents criticised his methods, others even doubted
the truth of his experiments. There was one slight mistake in
Newton's belief that all prisms would give a spectrum of exactly
the same length, and it was some time before he corrected this
error. Meanwhile he patiently met and answered the arguments of
his opponents until he began to feel that patience was no longer
a virtue. At one time he even went so far as to declare that,
once he was "free of this business," he would renounce scientific
research forever, at least in a public way. Fortunately for the
world, however, he did not adhere to this determination, but went
on to even greater discoveries--which, it may be added, involved
still greater controversies.

In commenting on Newton's discovery of the composition of light,
Voltaire said: "Sir Isaac Newton has demonstrated to the eye, by
the bare assistance of a prism, that light is a composition of
colored rays, which, being united, form white color. A single ray
is by him divided into seven, which all fall upon a piece of
linen or a sheet of white paper, in their order one above the
other, and at equal distances. The first is red, the second
orange, the third yellow, the fourth green, the fifth blue, the
sixth indigo, the seventh a violet purple. Each of these rays
transmitted afterwards by a hundred other prisms will never
change the color it bears; in like manner as gold, when
completely purged from its dross, will never change afterwards in
the crucible."[3]

XII. NEWTON AND THE LAW OF GRAVITATION

We come now to the story of what is by common consent the
greatest of scientific achievements. The law of universal
gravitation is the most far-reaching principle as yet discovered.
It has application equally to the minutest particle of matter and
to the most distant suns in the universe, yet it is amazing in
its very simplicity. As usually phrased, the law is this: That
every particle of matter in the universe attracts every other
particle with a force that varies directly with the mass of the
particles and inversely as the squares of their mutual distance.
Newton did not vault at once to the full expression of this law,
though he had formulated it fully before he gave the results of
his investigations to the world. We have now to follow the steps
by which he reached this culminating achievement.

At the very beginning we must understand that the idea of
universal gravitation was not absolutely original with Newton.
Away back in the old Greek days, as we have seen, Anaxagoras
conceived and clearly expressed the idea that the force which
holds the heavenly bodies in their orbits may be the same that
operates upon substances at the surface of the earth. With
Anaxagoras this was scarcely more than a guess. After his day the
idea seems not to have been expressed by any one until the
seventeenth century's awakening of science. Then the
consideration of Kepler's Third Law of planetary motion suggested
to many minds perhaps independently the probability that the
force hitherto mentioned merely as centripetal, through the
operation of which the planets are held in their orbits is a
force varying inversely as the square of the distance from the
sun. This idea had come to Robert Hooke, to Wren, and perhaps to
Halley, as well as to Newton; but as yet no one had conceived a
method by which the validity of the suggestion might be tested.
It was claimed later on by Hooke that he had discovered a method
demonstrating the truth of the theory of inverse squares, and
after the full announcement of Newton's discovery a heated
controversy was precipitated in which Hooke put forward his
claims with accustomed acrimony. Hooke, however, never produced
his demonstration, and it may well be doubted whether he had
found a method which did more than vaguely suggest the law which
the observations of Kepler had partially revealed. Newton's great
merit lay not so much in conceiving the law of inverse squares as
in the demonstration of the law. He was led to this demonstration
through considering the orbital motion of the moon. According to
the familiar story, which has become one of the classic myths of
science, Newton was led to take up the problem through observing
the fall of an apple. Voltaire is responsible for the story,
which serves as well as another; its truth or falsity need not in
the least concern us. Suffice it that through pondering on the
familiar fact of terrestrial gravitation, Newton was led to
question whether this force which operates so tangibly here at
the earth's surface may not extend its influence out into the
depths of space, so as to include, for example, the moon.
Obviously some force pulls the moon constantly towards the earth;
otherwise that body would fly off at a tangent and never return.
May not this so-called centripetal force be identical with
terrestrial gravitation? Such was Newton's query. Probably many
another man since Anaxagoras had asked the same question, but
assuredly Newton was the first man to find an answer.

The thought that suggested itself to Newton's mind was this: If
we make a diagram illustrating the orbital course of the moon for
any given period, say one minute, we shall find that the course
of the moon departs from a straight line during that period by a
measurable distance--that: is to say, the moon has been virtually
pulled towards the earth by an amount that is represented by the
difference between its actual position at the end of the minute
under observation and the position it would occupy had its course
been tangential, as, according to the first law of motion, it
must have been had not some force deflected it towards the earth.
Measuring the deflection in question--which is equivalent to the
so-called versed sine of the arc traversed--we have a basis for
determining the strength of the deflecting force. Newton
constructed such a diagram, and, measuring the amount of the
moon's departure from a tangential rectilinear course in one
minute, determined this to be, by his calculation, thirteen feet.
Obviously, then, the force acting upon the moon is one that would
cause that body to fall towards the earth to the distance of
thirteen feet in the first minute of its fall. Would such be the
force of gravitation acting at the distance of the moon if the
power of gravitation varies inversely as the square of the
distance? That was the tangible form in which the problem
presented itself to Newton. The mathematical solution of the
problem was simple enough. It is based on a comparison of the
moon's distance with the length of the earth's radius. On making
this calculation, Newton found that the pull of gravitation--if
that were really the force that controls the moon--gives that
body a fall of slightly over fifteen feet in the first minute,
instead of thirteen feet. Here was surely a suggestive
approximation, yet, on the other band, the discrepancy seemed to
be too great to warrant him in the supposition that he had found
the true solution. He therefore dismissed the matter from his
mind for the time being, nor did he return to it definitely for
some years.

{illustration caption = DIAGRAM TO ILLUSTRATE NEWTON'S LAW OF
GRAVITATION (E represents the earth and A the moon. Were the
earth's pull on the moon to cease, the moon's inertia would cause
it to take the tangential course, AB. On the other hand, were the
moon's motion to be stopped for an instant, the moon would fall
directly towards the earth, along the line AD. The moon's actual
orbit, resulting from these component forces, is AC. Let AC
represent the actual flight of the moon in one minute. Then BC,
which is obviously equal to AD, represents the distance which the
moon virtually falls towards the earth in one minute. Actual
computation, based on measurements of the moon's orbit, showed
this distance to be about fifteen feet. Another computation
showed that this is the distance that the moon would fall towards
the earth under the influence of gravity, on the supposition that
the force of gravity decreases inversely with the square of the
distance; the basis of comparison being furnished by falling
bodies at the surface of the earth. Theory and observations thus
coinciding, Newton was justified in declaring that the force that
pulls the moon towards the earth and keeps it in its orbit, is
the familiar force of gravity, and that this varies inversely as
the square of the distance.)}

It was to appear in due time that Newton's hypothesis was
perfectly valid and that his method of attempted demonstration
was equally so. The difficulty was that the earth's proper
dimensions were not at that time known. A wrong estimate of the
earth's size vitiated all the other calculations involved, since
the measurement of the moon's distance depends upon the
observation of the parallax, which cannot lead to a correct
computation unless the length of the earth's radius is accurately
known. Newton's first calculation was made as early as 1666, and
it was not until 1682 that his attention was called to a new and
apparently accurate measurement of a degree of the earth's
meridian made by the French astronomer Picard. The new
measurement made a degree of the earth's surface 69.10 miles,
instead of sixty miles.

Learning of this materially altered calculation as to the earth's
size, Newton was led to take up again his problem of the falling
moon. As he proceeded with his computation, it became more and
more certain that this time the result was to harmonize with the
observed facts. As the story goes, he was so completely
overwhelmed with emotion that he was forced to ask a friend to
complete the simple calculation. That story may well be true,
for, simple though the computation was, its result was perhaps
the most wonderful demonstration hitherto achieved in the entire
field of science. Now at last it was known that the force of
gravitation operates at the distance of the moon, and holds that
body in its elliptical orbit, and it required but a slight effort
of the imagination to assume that the force which operates
through such a reach of space extends its influence yet more
widely. That such is really the case was demonstrated presently
through calculations as to the moons of Jupiter and by similar
computations regarding the orbital motions of the various
planets. All results harmonizing, Newton was justified in
reaching the conclusion that gravitation is a universal property
of matter. It remained, as we shall see, for nineteenth-century
scientists to prove that the same force actually operates upon
the stars, though it should be added that this demonstration
merely fortified a belief that had already found full acceptance.

Having thus epitomized Newton's discovery, we must now take up
the steps of his progress somewhat in detail, and state his
theories and their demonstration in his own words. Proposition
IV., theorem 4, of his Principia is as follows:

"That the moon gravitates towards the earth and by the force of
gravity is continually drawn off from a rectilinear motion and
retained in its orbit.

"The mean distance of the moon from the earth, in the syzygies in
semi-diameters of the earth, is, according to Ptolemy and most
astronomers, 59; according to Vendelin and Huygens, 60; to
Copernicus, 60 1/3; to Street, 60 2/3; and to Tycho, 56 1/2. But
Tycho, and all that follow his tables of refractions, making the
refractions of the sun and moon (altogether against the nature of
light) to exceed the refractions of the fixed stars, and that by
four or five minutes NEAR THE HORIZON, did thereby increase the
moon's HORIZONTAL parallax by a like number of minutes, that is,
by a twelfth or fifteenth part of the whole parallax. Correct
this error and the distance will become about 60 1/2
semi-diameters of the earth, near to what others have assigned.
Let us assume the mean distance of 60 diameters in the syzygies;
and suppose one revolution of the moon, in respect to the fixed
stars, to be completed in 27d. 7h. 43', as astronomers have
determined; and the circumference of the earth to amount to
123,249,600 Paris feet, as the French have found by mensuration.
And now, if we imagine the moon, deprived of all motion, to be
let go, so as to descend towards the earth with the impulse of
all that force by which (by Cor. Prop. iii.) it is retained in
its orb, it will in the space of one minute of time describe in
its fall 15 1/12 Paris feet. For the versed sine of that arc
which the moon, in the space of one minute of time, would by its
mean motion describe at the distance of sixty semi-diameters of
the earth, is nearly 15 1/12 Paris feet, or more accurately 15
feet, 1 inch, 1 line 4/9. Wherefore, since that force, in
approaching the earth, increases in the reciprocal-duplicate
proportion of the distance, and upon that account, at the surface
of the earth, is 60 x 60 times greater than at the moon, a body
in our regions, falling with that force, ought in the space of
one minute of time to describe 60 x 60 x 15 1/12 Paris feet; and
in the space of one second of time, to describe 15 1/12 of those
feet, or more accurately, 15 feet, 1 inch, 1 line 4/9. And with
this very force we actually find that bodies here upon earth do
really descend; for a pendulum oscillating seconds in the
latitude of Paris will be 3 Paris feet, and 8 lines 1/2 in
length, as Mr. Huygens has observed. And the space which a heavy
body describes by falling in one second of time is to half the
length of the pendulum in the duplicate ratio of the
circumference of a circle to its diameter (as Mr. Huygens has
also shown), and is therefore 15 Paris feet, 1 inch, 1 line 4/9.
And therefore the force by which the moon is retained in its
orbit is that very same force which we commonly call gravity;
for, were gravity another force different from that, then bodies
descending to the earth with the joint impulse of both forces
would fall with a double velocity, and in the space of one second
of time would describe 30 1/6 Paris feet; altogether against
experience."[1]

All this is beautifully clear, and its validity has never in
recent generations been called in question; yet it should be
explained that the argument does not amount to an actually
indisputable demonstration. It is at least possible that the
coincidence between the observed and computed motion of the moon
may be a mere coincidence and nothing more. This probability,
however, is so remote that Newton is fully justified in
disregarding it, and, as has been said, all subsequent
generations have accepted the computation as demonstrative.

Let us produce now Newton's further computations as to the other
planetary bodies, passing on to his final conclusion that gravity
is a universal force.

"PROPOSITION V., THEOREM V.

"That the circumjovial planets gravitate towards Jupiter; the
circumsaturnal towards Saturn; the circumsolar towards the sun;
and by the forces of their gravity are drawn off from rectilinear
motions, and retained in curvilinear orbits.

"For the revolutions of the circumjovial planets about Jupiter,
of the circumsaturnal about Saturn, and of Mercury and Venus and
the other circumsolar planets about the sun, are appearances of
the same sort with the revolution of the moon about the earth;
and therefore, by Rule ii., must be owing to the same sort of
causes; especially since it has been demonstrated that the forces
upon which those revolutions depend tend to the centres of
Jupiter, of Saturn, and of the sun; and that those forces, in
receding from Jupiter, from Saturn, and from the sun, decrease in
the same proportion, and according to the same law, as the force
of gravity does in receding from the earth.

"COR. 1.--There is, therefore, a power of gravity tending to all
the planets; for doubtless Venus, Mercury, and the rest are
bodies of the same sort with Jupiter and Saturn. And since all
attraction (by Law iii.) is mutual, Jupiter will therefore
gravitate towards all his own satellites, Saturn towards his, the
earth towards the moon, and the sun towards all the primary
planets.

"COR. 2.--The force of gravity which tends to any one planet is
reciprocally as the square of the distance of places from the
planet's centre.

"COR. 3.--All the planets do mutually gravitate towards one
another, by Cor. 1 and 2, and hence it is that Jupiter and
Saturn, when near their conjunction, by their mutual attractions
sensibly disturb each other's motions. So the sun disturbs the
motions of the moon; and both sun and moon disturb our sea, as we
shall hereafter explain.

"SCHOLIUM

"The force which retains the celestial bodies in their orbits has
been hitherto called centripetal force; but it being now made
plain that it can be no other than a gravitating force, we shall
hereafter call it gravity. For the cause of the centripetal force
which retains the moon in its orbit will extend itself to all the
planets by Rules i., ii., and iii.

"PROPOSITION VI., THEOREM VI.

"That all bodies gravitate towards every planet; and that the
weights of the bodies towards any the same planet, at equal
distances from the centre of the planet, are proportional to the
quantities of matter which they severally contain.

"It has been now a long time observed by others that all sorts of
heavy bodies (allowance being made for the inability of
retardation which they suffer from a small power of resistance in
the air) descend to the earth FROM EQUAL HEIGHTS in equal times;
and that equality of times we may distinguish to a great accuracy
by help of pendulums. I tried the thing in gold, silver, lead,
glass, sand, common salt, wood, water, and wheat. I provided two
wooden boxes, round and equal: I filled the one with wood, and
suspended an equal weight of gold (as exactly as I could) in the
centre of oscillation of the other. The boxes hanging by eleven
feet, made a couple of pendulums exactly equal in weight and
figure, and equally receiving the resistance of the air. And,
placing the one by the other, I observed them to play together
forward and backward, for a long time, with equal vibrations. And
therefore the quantity of matter in gold was to the quantity of
matter in the wood as the action of the motive force (or vis
motrix) upon all the gold to the action of the same upon all the
wood--that is, as the weight of the one to the weight of the
other: and the like happened in the other bodies. By these
experiments, in bodies of the same weight, I could manifestly
have discovered a difference of matter less than the thousandth
part of the whole, had any such been. But, without all doubt, the
nature of gravity towards the planets is the same as towards the
earth. For, should we imagine our terrestrial bodies removed to
the orb of the moon, and there, together with the moon, deprived
of all motion, to be let go, so as to fall together towards the
earth, it is certain, from what we have demonstrated before,
that, in equal times, they would describe equal spaces with the
moon, and of consequence are to the moon, in quantity and matter,
as their weights to its weight.

"Moreover, since the satellites of Jupiter perform their
revolutions in times which observe the sesquiplicate proportion
of their distances from Jupiter's centre, their accelerative
gravities towards Jupiter will be reciprocally as the square of
their distances from Jupiter's centre--that is, equal, at equal
distances. And, therefore, these satellites, if supposed to fall
TOWARDS JUPITER from equal heights, would describe equal spaces
in equal times, in like manner as heavy bodies do on our earth.
And, by the same argument, if the circumsolar planets were
supposed to be let fall at equal distances from the sun, they
would, in their descent towards the sun, describe equal spaces in
equal times. But forces which equally accelerate unequal bodies
must be as those bodies--that is to say, the weights of the
planets (TOWARDS THE SUN must be as their quantities of matter.
Further, that the weights of Jupiter and his satellites towards
the sun are proportional to the several quantities of their
matter, appears from the exceedingly regular motions of the
satellites. For if some of these bodies were more strongly
attracted to the sun in proportion to their quantity of matter
than others, the motions of the satellites would be disturbed by
that inequality of attraction. If at equal distances from the sun
any satellite, in proportion to the quantity of its matter, did
gravitate towards the sun with a force greater than Jupiter in
proportion to his, according to any given proportion, suppose d
to e; then the distance between the centres of the sun and of the
satellite's orbit would be always greater than the distance
between the centres of the sun and of Jupiter nearly in the
subduplicate of that proportion: as by some computations I have
found. And if the satellite did gravitate towards the sun with a
force, lesser in the proportion of e to d, the distance of the
centre of the satellite's orb from the sun would be less than the
distance of the centre of Jupiter from the sun in the
subduplicate of the same proportion. Therefore, if at equal
distances from the sun, the accelerative gravity of any satellite
towards the sun were greater or less than the accelerative
gravity of Jupiter towards the sun by one-one-thousandth part of
the whole gravity, the distance of the centre of the satellite's
orbit from the sun would be greater or less than the distance of
Jupiter from the sun by one one-two-thousandth part of the whole
distance--that is, by a fifth part of the distance of the utmost
satellite from the centre of Jupiter; an eccentricity of the
orbit which would be very sensible. But the orbits of the
satellites are concentric to Jupiter, and therefore the
accelerative gravities of Jupiter and of all its satellites
towards the sun, at equal distances from the sun, are as their
several quantities of matter; and the weights of the moon and of
the earth towards the sun are either none, or accurately
proportional to the masses of matter which they contain.

"COR. 5.--The power of gravity is of a different nature from the
power of magnetism; for the magnetic attraction is not as the
matter attracted. Some bodies are attracted more by the magnet;
others less; most bodies not at all. The power of magnetism in
one and the same body may be increased and diminished; and is
sometimes far stronger, for the quantity of matter, than the
power of gravity; and in receding from the magnet decreases not
in the duplicate, but almost in the triplicate proportion of the
distance, as nearly as I could judge from some rude observations.

"PROPOSITION VII., THEOREM VII.

"That there is a power of gravity tending to all bodies,
proportional to the several quantities of matter which they
contain.

That all the planets mutually gravitate one towards another we
have proved before; as well as that the force of gravity towards
every one of them considered apart, is reciprocally as the square
of the distance of places from the centre of the planet. And
thence it follows, that the gravity tending towards all the
planets is proportional to the matter which they contain.

"Moreover, since all the parts of any planet A gravitates towards
any other planet B; and the gravity of every part is to the
gravity of the whole as the matter of the part is to the matter
of the whole; and to every action corresponds a reaction;
therefore the planet B will, on the other hand, gravitate towards
all the parts of planet A, and its gravity towards any one part
will be to the gravity towards the whole as the matter of the
part to the matter of the whole. Q.E.D.

"HENCE IT WOULD APPEAR THAT the force of the whole must arise
from the force of the component parts."

Newton closes this remarkable Book iii. with the following words:

"Hitherto we have explained the phenomena of the heavens and of
our sea by the power of gravity, but have not yet assigned the
cause of this power. This is certain, that it must proceed from a
cause that penetrates to the very centre of the sun and planets,
without suffering the least diminution of its force; that
operates not according to the quantity of the surfaces of the
particles upon which it acts (as mechanical causes used to do),
but according to the quantity of solid matter which they contain,
and propagates its virtue on all sides to immense distances,
decreasing always in the duplicate proportions of the distances.
Gravitation towards the sun is made up out of the gravitations
towards the several particles of which the body of the sun is
composed; and in receding from the sun decreases accurately in
the duplicate proportion of the distances as far as the orb of
Saturn, as evidently appears from the quiescence of the aphelions
of the planets; nay, and even to the remotest aphelions of the
comets, if those aphelions are also quiescent. But hitherto I
have not been able to discover the cause of those properties of
gravity from phenomena, and I frame no hypothesis; for whatever
is not deduced from the phenomena is to be called an hypothesis;
and hypotheses, whether metaphysical or physical, whether of
occult qualities or mechanical, have no place in experimental
philosophy. . . . And to us it is enough that gravity does really
exist, and act according to the laws which we have explained, and
abundantly serves to account for all the motions of the celestial
bodies and of our sea."[2]

The very magnitude of the importance of the theory of universal
gravitation made its general acceptance a matter of considerable
time after the actual discovery. This opposition had of course
been foreseen by Newton, and, much as be dreaded controversy, he
was prepared to face it and combat it to the bitter end. He knew
that his theory was right; it remained for him to convince the
world of its truth. He knew that some of his contemporary
philosophers would accept it at once; others would at first
doubt, question, and dispute, but finally accept; while still
others would doubt and dispute until the end of their days. This
had been the history of other great discoveries; and this will
probably be the history of most great discoveries for all time.
But in this case the discoverer lived to see his theory accepted
by practically all the great minds of his time.

Delambre is authority for the following estimate of Newton by
Lagrange. "The celebrated Lagrange," he says, "who frequently
asserted that Newton was the greatest genius that ever existed,
used to add--'and the most fortunate, for we cannot find MORE
THAN ONCE a system of the world to establish.' " With pardonable
exaggeration the admiring followers of the great generalizer
pronounced this epitaph:

"Nature and Nature's laws lay hid in night;
God said `Let Newton be!' and all was light."

XIII. INSTRUMENTS OF PRECISION IN THE AGE OF NEWTON

During the Newtonian epoch there were numerous important
inventions of scientific instruments, as well as many
improvements made upon the older ones. Some of these discoveries
have been referred to briefly in other places, but their
importance in promoting scientific investigation warrants a
fuller treatment of some of the more significant.

Many of the errors that had arisen in various scientific
calculations before the seventeenth century may be ascribed to
the crudeness and inaccuracy in the construction of most
scientific instruments. Scientists had not as yet learned that an
approach to absolute accuracy was necessary in every
investigation in the field of science, and that such accuracy
must be extended to the construction of the instruments used in
these investigations and observations. In astronomy it is obvious
that instruments of delicate exactness are most essential; yet
Tycho Brahe, who lived in the sixteenth century, is credited with
being the first astronomer whose instruments show extreme care in
construction.

It seems practically settled that the first telescope was
invented in Holland in 1608; but three men, Hans Lippershey,
James Metius, and Zacharias Jansen, have been given the credit of
the invention at different times. It would seem from certain
papers, now in the library of the University of Leyden, and
included in Huygens's papers, that Lippershey was probably the
first to invent a telescope and to describe his invention. The
story is told that Lippershey, who was a spectacle-maker,
stumbled by accident upon the discovery that when two lenses are
held at a certain distance apart, objects at a distance appear
nearer and larger. Having made this discovery, be fitted two
lenses with a tube so as to maintain them at the proper distance,
and thus constructed the first telescope.

It was Galileo, however, as referred to in a preceding chapter,
who first constructed a telescope based on his knowledge of the
laws of refraction. In 1609, having heard that an instrument had
been invented, consisting of two lenses fixed in a tube, whereby
objects were made to appear larger and nearer, he set about
constructing such an instrument that should follow out the known
effects of refraction. His first telescope, made of two lenses
fixed in a lead pipe, was soon followed by others of improved
types, Galileo devoting much time and labor to perfecting lenses
and correcting errors. In fact, his work in developing the
instrument was so important that the telescope came gradually to
be known as the "Galilean telescope."

In the construction of his telescope Galileo made use of a convex
and a concave lens; but shortly after this Kepler invented an
instrument in which both the lenses used were convex. This
telescope gave a much larger field of view than the Galilean
telescope, but did not give as clear an image, and in consequence
did not come into general use until the middle of the seventeenth
century. The first powerful telescope of this type was made by
Huygens and his brother. It was of twelve feet focal length, and
enabled Huygens to discover a new satellite of Saturn, and to
determine also the true explanation of Saturn's ring.

It was Huygens, together with Malvasia and Auzout, who first
applied the micrometer to the telescope, although the inventor of
the first micrometer was William Gascoigne, of Yorkshire, about
1636. The micrometer as used in telescopes enables the observer
to measure accurately small angular distances. Before the
invention of the telescope such measurements were limited to the
angle that could be distinguished by the naked eye, and were, of
course, only approximately accurate. Even very careful observers,
such as Tycho Brahe, were able to obtain only fairly accurate
results. But by applying Gascoigne's invention to the telescope
almost absolute accuracy became at once possible. The principle
of Gascoigne's micrometer was that of two pointers lying
parallel, and in this position pointing to zero. These were
arranged so that the turning of a single screw separated or
approximated them at will, and the angle thus formed could be
determined with absolute accuracy.

Huygens's micrometer was a slip of metal of variable breadth
inserted at the focus of the telescope. By observing at what
point this exactly covered an object under examination, and
knowing the focal length of the telescope and the width of the
metal, he could then deduce the apparent angular breadth of the
object. Huygens discovered also that an object placed in the
common focus of the two lenses of a Kepler telescope appears
distinct and clearly defined. The micrometers of Malvasia, and
later of Auzout and Picard, are the development of this
discovery. Malvasia's micrometer, which he described in 1662,
consisted of fine silver wires placed at right-angles at the
focus of his telescope.

As telescopes increased in power, however, it was found that even
the finest wire, or silk filaments, were much too thick for
astronomical observations, as they obliterated the image, and so,
finally, the spider-web came into use and is still used in
micrometers and other similar instruments. Before that time,
however, the fine crossed wires had revolutionized astronomical
observations. "We may judge how great was the improvement which
these contrivances introduced into the art of observing," says
Whewell, "by finding that Hevelius refused to adopt them because
they would make all the old observations of no value. He had
spent a laborious and active life in the exercise of the old
methods, and could not bear to think that all the treasures which
he had accumulated had lost their worth by the discovery of a new
mine of richer ones."[1]

Until the time of Newton, all the telescopes in use were either
of the Galilean or Keplerian type, that is, refractors. But about
the year 1670 Newton constructed his first reflecting telescope,
which was greatly superior to, although much smaller than, the
telescopes then in use. He was led to this invention by his
experiments with light and colors. In 1671 he presented to the
Royal Society a second and somewhat larger telescope, which he
had made; and this type of instrument was little improved upon
until the introduction of the achromatic telescope, invented by
Chester Moor Hall in 1733.

As is generally known, the element of accurate measurements of
time plays an important part in the measurements of the movements
of the heavenly bodies. In fact, one was scarcely possible
without the other, and as it happened it was the same man,
Huygens, who perfected Kepler's telescope and invented the
pendulum clock. The general idea had been suggested by Galileo;
or, better perhaps, the equal time occupied by the successive
oscillations of the pendulum had been noted by him. He had not
been able, however, to put this discovery to practical account.
But in 1656 Huygens invented the necessary machinery for
maintaining the motion of the pendulum and perfected several
accurate clocks. These clocks were of invaluable assistance to
the astronomers, affording as they did a means of keeping time
"more accurate than the sun itself." When Picard had corrected
the variation caused by heat and cold acting upon the pendulum
rod by combining metals of different degrees of expansibility, a
high degree of accuracy was possible.

But while the pendulum clock was an unequalled stationary
time-piece, it was useless in such unstable situations as, for
example, on shipboard. But here again Huygens played a prominent
part by first applying the coiled balance-spring for regulating
watches and marine clocks. The idea of applying a spring to the
balance-wheel was not original with Huygens, however, as it had
been first conceived by Robert Hooke; but Huygens's application
made practical Hooke's idea. In England the importance of
securing accurate watches or marine clocks was so fully
appreciated that a reward of L20,000 sterling was offered by
Parliament as a stimulus to the inventor of such a time-piece.
The immediate incentive for this offer was the obvious fact that
with such an instrument the determination of the longitude of
places would be much simplified. Encouraged by these offers, a
certain carpenter named Harrison turned his attention to the
subject of watch-making, and, after many years of labor, in 1758
produced a spring time-keeper which, during a sea-voyage
occupying one hundred and sixty-one days, varied only one minute
and five seconds. This gained for Harrison a reward Of L5000
sterling at once, and a little later L10,000 more, from
Parliament.

While inventors were busy with the problem of accurate
chronometers, however, another instrument for taking longitude at
sea had been invented. This was the reflecting quadrant, or
sextant, as the improved instrument is now called, invented by
John Hadley in 1731, and independently by Thomas Godfrey, a poor
glazier of Philadelphia, in 1730. Godfrey's invention, which was
constructed on the same principle as that of the Hadley
instrument, was not generally recognized until two years after
Hadley's discovery, although the instrument was finished and
actually in use on a sea-voyage some months before Hadley
reported his invention. The principle of the sextant, however,
seems to have been known to Newton, who constructed an instrument
not very unlike that of Hadley; but this invention was lost sight
of until several years after the philosopher's death and some
time after Hadley's invention.

The introduction of the sextant greatly simplified taking
reckonings at sea as well as facilitating taking the correct
longitude of distant places. Before that time the mariner was
obliged to depend upon his compass, a cross-staff, or an
astrolabe, a table of the sun's declination and a correction for
the altitude of the polestar, and very inadequate and incorrect
charts. Such were the instruments used by Columbus and Vasco da
Gama and their immediate successors.

During the Newtonian period the microscopes generally in use were
those constructed of simple lenses, for although compound
microscopes were known, the difficulties of correcting aberration
had not been surmounted, and a much clearer field was given by
the simple instrument. The results obtained by the use of such
instruments, however, were very satisfactory in many ways. By
referring to certain plates in this volume, which reproduce
illustrations from Robert Hooke's work on the microscope, it will
be seen that quite a high degree of effectiveness had been
attained. And it should be recalled that Antony von Leeuwenboek,
whose death took place shortly before Newton's, had discovered
such micro-organisms as bacteria, had seen the blood corpuscles
in circulation, and examined and described other microscopic
structures of the body.

XIV. PROGRESS IN ELECTRICITY FROM GILBERT AND VON GUERICKE TO
FRANKLIN

We have seen how Gilbert, by his experiments with magnets, gave
an impetus to the study of magnetism and electricity. Gilbert
himself demonstrated some facts and advanced some theories, but
the system of general laws was to come later. To this end the
discovery of electrical repulsion, as well as attraction, by Von
Guericke, with his sulphur ball, was a step forward; but
something like a century passed after Gilbert's beginning before
anything of much importance was done in the field of electricity.

In 1705, however, Francis Hauksbee began a series of experiments
that resulted in some startling demonstrations. For many years it
had been observed that a peculiar light was seen sometimes in the
mercurial barometer, but Hauksbee and the other scientific
investigators supposed the radiance to be due to the mercury in a
vacuum, brought about, perhaps, by some agitation. That this
light might have any connection with electricity did not, at
first, occur to Hauksbee any more than it had to his
predecessors. The problem that interested him was whether the
vacuum in the tube of the barometer was essential to the light;
and in experimenting to determine this, he invented his
"mercurial fountain." Having exhausted the air in a receiver
containing some mercury, he found that by allowing air to rush
through the mercury the metal became a jet thrown in all
directions against the sides of the vessel, making a great,
flaming shower, "like flashes of lightning," as he said. But it
seemed to him that there was a difference between this light and
the glow noted in the barometer. This was a bright light, whereas
the barometer light was only a glow. Pondering over this,
Hauksbee tried various experiments, revolving pieces of amber,
flint, steel, and other substances in his exhausted air-pump
receiver, with negative, or unsatisfactory, results. Finally, it
occurred to him to revolve an exhausted glass tube itself.
Mounting such a globe of glass on an axis so that it could be
revolved rapidly by a belt running on a large wheel, he found
that by holding his fingers against the whirling globe a purplish
glow appeared, giving sufficient light so that coarse print could
be read, and the walls of a dark room sensibly lightened several
feet away. As air was admitted to the globe the light gradually
diminished, and it seemed to him that this diminished glow was
very similar in appearance to the pale light seen in the
mercurial barometer. Could it be that it was the glass, and not
the mercury, that caused it? Going to a barometer he proceeded to
rub the glass above the column of mercury over the vacuum,
without disturbing the mercury, when, to his astonishment, the
same faint light, to all appearances identical with the glow seen
in the whirling globe, was produced.

Turning these demonstrations over in his mind, he recalled the
well-known fact that rubbed glass attracted bits of paper,
leaf-brass, and other light substances, and that this phenomenon
was supposed to be electrical. This led him finally to determine
the hitherto unsuspected fact, that the glow in the barometer was
electrical as was also the glow seen in his whirling globe.
Continuing his investigations, he soon discovered that solid
glass rods when rubbed produced the same effects as the tube. By
mere chance, happening to hold a rubbed tube to his cheek, he
felt the effect of electricity upon the skin like "a number of
fine, limber hairs," and this suggested to him that, since the
mysterious manifestation was so plain, it could be made to show
its effects upon various substances. Suspending some woollen
threads over the whirling glass cylinder, he found that as soon
as he touched the glass with his hands the threads, which were
waved about by the wind of the revolution, suddenly straightened
themselves in a peculiar manner, and stood in a radical position,
pointing to the axis of the cylinder.

Encouraged by these successes, he continued his experiments with
breathless expectancy, and soon made another important discovery,
that of "induction," although the real significance of this
discovery was not appreciated by him or, for that matter, by any
one else for several generations following. This discovery was
made by placing two revolving cylinders within an inch of each
other, one with the air exhausted and the other unexhausted.
Placing his hand on the unexhausted tube caused the light to
appear not only upon it, but on the other tube as well. A little
later he discovered that it is not necessary to whirl the
exhausted tube to produce this effect, but simply to place it in
close proximity to the other whirling cylinder.

These demonstrations of Hauksbee attracted wide attention and
gave an impetus to investigators in the field of electricity; but
still no great advance was made for something like a quarter of a
century. Possibly the energies of the scientists were exhausted
for the moment in exploring the new fields thrown open to
investigation by the colossal work of Newton.

THE EXPERIMENTS OF STEPHEN GRAY

In 1729 Stephen Gray (died in 1736), an eccentric and irascible
old pensioner of the Charter House in London, undertook some
investigations along lines similar to those of Hauksbee. While
experimenting with a glass tube for producing electricity, as
Hauksbee had done, he noticed that the corks with which he had
stopped the ends of the tube to exclude the dust, seemed to
attract bits of paper and leaf-brass as well as the glass itself.
He surmised at once that this mysterious electricity, or
"virtue," as it was called, might be transmitted through other
substances as it seemed to be through glass.

"Having by me an ivory ball of about one and three-tenths of an
inch in diameter," he writes, "with a hole through it, this I
fixed upon a fir-stick about four inches long, thrusting the
other end into the cork, and upon rubbing the tube found that the
ball attracted and repelled the feather with more vigor than the
cork had done, repeating its attractions and repulsions for many
times together. I then fixed the ball on longer sticks, first
upon one of eight inches, and afterwards upon one of twenty-four
inches long, and found the effect the same. Then I made use of
iron, and then brass wire, to fix the ball on, inserting the
other end of the wire in the cork, as before, and found that the
attraction was the same as when the fir-sticks were made use of,
and that when the feather was held over against any part of the
wire it was attracted by it; but though it was then nearer the
tube, yet its attraction was not so strong as that of the ball.
When the wire of two or three feet long was used, its vibrations,
caused by the rubbing of the tube, made it somewhat troublesome
to be managed. This put me to thinking whether, if the ball was
hung by a pack-thread and suspended by a loop on the tube, the
electricity would not be carried down the line to the ball; I
found it to succeed accordingly; for upon suspending the ball on
the tube by a pack-thread about three feet long, when the tube
had been excited by rubbing, the ivory ball attracted and
repelled the leaf-brass over which it was held as freely as it
had done when it was suspended on sticks or wire, as did also a
ball of cork, and another of lead that weighed one pound and a
quarter."

Gray next attempted to determine what other bodies would attract
the bits of paper, and for this purpose he tried coins, pieces of
metal, and even a tea-kettle, "both empty and filled with hot or
cold water"; but he found that the attractive power appeared to
be the same regardless of the substance used.

"I next proceeded," he continues, "to try at what greater
distances the electric virtues might be carried, and having by me
a hollow walking-cane, which I suppose was part of a fishing-rod,
two feet seven inches long, I cut the great end of it to fit into
the bore of the tube, into which it went about five inches; then
when the cane was put into the end of the tube, and this excited,
the cane drew the leaf-brass to the height of more than two
inches, as did also the ivory ball, when by a cork and stick it
had been fixed to the end of the cane.... With several pieces of
Spanish cane and fir-sticks I afterwards made a rod, which,
together with the tube, was somewhat more than eighteen feet
long, which was the greatest length I could conveniently use in
my chamber, and found the attraction very nearly, if not
altogether, as strong as when the ball was placed on the shorter
rods."

This experiment exhausted the capacity of his small room, but on
going to the country a little later he was able to continue his
experiments. "To a pole of eighteen feet there was tied a line of
thirty-four feet in length, so that the pole and line together
were fifty-two feet. With the pole and tube I stood in the
balcony, the assistant below in the court, where he held the
board with the leaf-brass on it. Then the tube being excited, as
usual, the electric virtue passed from the tube up the pole and
down the line to the ivory ball, which attracted the leaf-brass,
and as the ball passed over it in its vibrations the leaf-brass
would follow it till it was carried off the board."

Gray next attempted to send the electricity over a line suspended
horizontally. To do this he suspended the pack-thread by pieces
of string looped over nails driven into beams for that purpose.
But when thus suspended he found that the ivory ball no longer
excited the leaf-brass, and he guessed correctly that the
explanation of this lay in the fact that "when the electric
virtue came to the loop that was suspended on the beam it went up
the same to the beam," none of it reaching the ball. As we shall
see from what follows, however, Gray had not as yet determined
that certain substances will conduct electricity while others
will not. But by a lucky accident he made the discovery that
silk, for example, was a poor conductor, and could be turned to
account in insulating the conducting-cord.

A certain Mr. Wheler had become much interested in the old
pensioner and his work, and, as a guest at the Wheler house, Gray
had been repeating some of his former experiments with the
fishing-rod, line, and ivory ball. He had finally exhausted the
heights from which these experiments could be made by climbing to
the clock-tower and exciting bits of leaf-brass on the ground
below.

"As we had no greater heights here," he says, "Mr. Wheler was
desirous to try whether we could not carry the electric virtue
horizontally. I then told him of the attempt I had made with that
design, but without success, telling him the method and materials
made use of, as mentioned above. He then proposed a silk line to
support the line by which the electric virtue was to pass. I told
him it might do better upon account of its smallness; so that
there would be less virtue carried from the line of
communication.

"The first experiment was made in the matted gallery, July 2,
1729, about ten in the morning. About four feet from the end of
the gallery there was a cross line that was fixed by its ends to
each side of the gallery by two nails; the middle part of the
line was silk, the rest at each end pack-thread; then the line to
which the ivory ball was hung and by which the electric virtue
was to be conveyed to it from the tube, being eighty and one-half
feet in length, was laid on the cross silk line, so that the ball
hung about nine feet below it. Then the other end of the line was
by a loop suspended on the glass cane, and the leaf-brass held
under the ball on a piece of white paper; when, the tube being
rubbed, the ball attracted the leaf-brass, and kept it suspended
on it for some time."

This experiment succeeded so well that the string was lengthened
until it was some two hundred and ninety-three feet long; and
still the attractive force continued, apparently as strong as
ever. On lengthening the string still more, however, the extra
weight proved too much for the strength of the silk
suspending-thread. "Upon this," says Gray, "having brought with
me both brass and iron wire, instead of the silk we put up small
iron wire; but this was too weak to bear the weight of the line.
We then took brass wire of a somewhat larger size than that of
iron. This supported our line of communication; but though the
tube was well rubbed, yet there was not the least motion or
attraction given by the ball, neither with the great tube, which
we made use of when we found the small solid cane to be
ineffectual; by which we were now convinced that the success we
had before depended upon the lines that supported the line of
communication being silk, and not upon their being small, as
before trial I had imagined it might be; the same effect
happening here as it did when the line that is to convey the
electric virtue is supported by pack-thread."

Soon after this Gray and his host suspended a pack-thread six
hundred and sixty-six feet long on poles across a field, these
poles being slightly inclined so that the thread could be
suspended from the top by small silk cords, thus securing the
necessary insulation. This pack-thread line, suspended upon poles
along which Gray was able to transmit the electricity, is very
suggestive of the modern telegraph, but the idea of signalling or
making use of it for communicating in any way seems not to have
occurred to any one at that time. Even the successors of Gray who
constructed lines some thousands of feet long made no attempt to
use them for anything but experimental purposes--simply to test
the distances that the current could be sent. Nevertheless, Gray
should probably be credited with the discovery of two of the most
important properties of electricity--that it can be conducted and
insulated, although, as we have seen, Gilbert and Von Guericke
had an inkling of both these properties.

EXPERIMENTS OF CISTERNAY DUFAY

So far England had produced the two foremost workers in
electricity. It was now France's turn to take a hand, and,
through the efforts of Charles Francois de Cisternay Dufay, to
advance the science of electricity very materially. Dufay was a
highly educated savant, who had been soldier and diplomat
betimes, but whose versatility and ability as a scientist is
shown by the fact that he was the only man who had ever
contributed to the annals of the academy investigations in every
one of the six subjects admitted by that institution as worthy of
recognition. Dufay upheld his reputation in this new field of
science, making many discoveries and correcting many mistakes of
former observers. In this work also he proved himself a great
diplomat by remaining on terms of intimate friendship with Dr.
Gray--a thing that few people were able to do.

Almost his first step was to overthrow the belief that certain
bodies are "electrics" and others "non-electrics"--that is, that
some substances when rubbed show certain peculiarities in
attracting pieces of paper and foil which others do not. Dufay
proved that all bodies possess this quality in a certain degree.

"I have found that all bodies (metallic, soft, or fluid ones
excepted)," he says, "may be made electric by first heating them
more or less and then rubbing them on any sort of cloth. So that
all kinds of stones, as well precious as common, all kinds of
wood, and, in general, everything that I have made trial of,
became electric by beating and rubbing, except such bodies as
grow soft by beat, as the gums, which dissolve in water, glue,
and such like substances. 'Tis also to be remarked that the
hardest stones or marbles require more chafing or heating than
others, and that the same rule obtains with regard to the woods;
so that box, lignum vitae, and such others must be chafed almost
to the degree of browning, whereas fir, lime-tree, and cork
require but a moderate heat.

"Having read in one of Mr. Gray's letters that water may be made
electrical by holding the excited glass tube near it (a dish of
water being fixed to a stand and that set on a plate of glass, or
on the brim of a drinking-glass, previously chafed, or otherwise
warmed), I have found, upon trial, that the same thing happened
to all bodies without exception, whether solid or fluid, and that
for that purpose 'twas sufficient to set them on a glass stand
slightly warmed, or only dried, and then by bringing the tube
near them they immediately became electrical. I made this
experiment with ice, with a lighted wood-coal, and with
everything that came into my mind; and I constantly remarked that
such bodies of themselves as were least electrical had the
greatest degree of electricity communicated to them at the
approval of the glass tube."

His next important discovery was that colors had nothing to do
with the conduction of electricity. "Mr. Gray says, towards the
end of one of his letters," he writes, "that bodies attract more
or less according to their colors. This led me to make several
very singular experiments. I took nine silk ribbons of equal
size, one white, one black, and the other seven of the seven
primitive colors, and having hung them all in order in the same
line, and then bringing the tube near them, the black one was
first attracted, the white one next, and others in order
successively to the red one, which was attracted least, and the
last of them all. I afterwards cut out nine square pieces of
gauze of the same colors with the ribbons, and having put them
one after another on a hoop of wood, with leaf-gold under them,
the leaf-gold was attracted through all the colored pieces of
gauze, but not through the white or black. This inclined me first
to think that colors contribute much to electricity, but three
experiments convinced me to the contrary. The first, that by
warming the pieces of gauze neither the black nor white pieces
obstructed the action of the electrical tube more than those of
the other colors. In like manner, the ribbons being warmed, the
black and white are not more strongly attracted than the rest.
The second is, the gauzes and ribbons being wetted, the ribbons
are all attracted equally, and all the pieces of gauze equally
intercept the action of electric bodies. The third is, that the
colors of a prism being thrown on a white gauze, there appear no
differences of attraction. Whence it proceeds that this
difference proceeds, not from the color, as a color, but from the
substances that are employed in the dyeing. For when I colored
ribbons by rubbing them with charcoal, carmine, and such other
substances, the differences no longer proved the same."

In connection with his experiments with his thread suspended on
glass poles, Dufay noted that a certain amount of the current is
lost, being given off to the surrounding air. He recommended,
therefore, that the cords experimented with be wrapped with some
non-conductor--that it should be "insulated" ("isolee"), as he
said, first making use of this term.

DUFAY DISCOVERS VITREOUS AND RESINOUS ELECTRICITY

It has been shown in an earlier chapter how Von Guericke
discovered that light substances like feathers, after being
attracted to the sulphur-ball electric-machine, were repelled by
it until they touched some object. Von Guericke noted this, but
failed to explain it satisfactorily. Dufay, repeating Von
Guericke's experiments, found that if, while the excited tube or
sulphur ball is driving the repelled feather before it, the ball
be touched or rubbed anew, the feather comes to it again, and is
repelled alternately, as, the hand touches the ball, or is
withdrawn. From this he concluded that electrified bodies first
attract bodies not electrified, "charge" them with electricity,
and then repel them, the body so charged not being attracted
again until it has discharged its electricity by touching
something.

"On making the experiment related by Otto von Guericke," he says,
"which consists in making a ball of sulphur rendered electrical
to repel a down feather, I perceived that the same effects were
produced not only by the tube, but by all electric bodies
whatsoever, and I discovered that which accounts for a great part
of the irregularities and, if I may use the term, of the caprices
that seem to accompany most of the experiments on electricity.
This principle is that electric bodies attract all that are not
so, and repel them as soon as they are become electric by the
vicinity or contact of the electric body. Thus gold-leaf is first
attracted by the tube, and acquires an electricity by approaching
it, and of consequence is immediately repelled by it. Nor is it
reattracted while it retains its electric quality. But if while
it is thus sustained in the air it chance to light on some other
body, it straightway loses its electricity, and in consequence is
reattracted by the tube, which, after having given it a new
electricity, repels it a second time, which continues as long as
the tube keeps its electricity. Upon applying this principle to
the various experiments of electricity, one will be surprised at
the number of obscure and puzzling facts that it clears up. For
Mr. Hauksbee's famous experiment of the glass globe, in which
silk threads are put, is a necessary consequence of it. When
these threads are arranged in the form of rays by the electricity
of the sides of the globe, if the finger be put near the outside
of the globe the silk threads within fly from it, as is well
known, which happens only because the finger or any other body
applied near the glass globe is thereby rendered electrical, and
consequently repels the silk threads which are endowed with the
same quality. With a little reflection we may in the same manner
account for most of the other phenomena, and which seem
inexplicable without attending to this principle.

"Chance has thrown in my way another principle, more universal
and remarkable than the preceding one, and which throws a new
light on the subject of electricity. This principle is that there
are two distinct electricities, very different from each other,
one of which I call vitreous electricity and the other resinous
electricity. The first is that of glass, rock-crystal, precious
stones, hair of animals, wool, and many other bodies. The second
is that of amber, copal, gumsack, silk thread, paper, and a
number of other substances. The characteristic of these two
electricities is that a body of the vitreous electricity, for
example, repels all such as are of the same electricity, and on
the contrary attracts all those of the resinous electricity; so
that the tube, made electrical, will repel glass, crystal, hair
of animals, etc., when rendered electric, and will attract silk
thread, paper, etc., though rendered electrical likewise. Amber,
on the contrary, will attract electric glass and other substances
of the same class, and will repel gum-sack, copal, silk thread,
etc. Two silk ribbons rendered electrical will repel each other;
two woollen threads will do the like; but a woollen thread and a
silken thread will mutually attract each other. This principle
very naturally explains why the ends of threads of silk or wool
recede from each other, in the form of pencil or broom, when they
have acquired an electric quality. From this principle one may
with the same ease deduce the explanation of a great number of
other phenomena; and it is probable that this truth will lead us
to the further discovery of many other things.

"In order to know immediately to which of the two classes of
electrics belongs any body whatsoever, one need only render
electric a silk thread, which is known to be of the resinuous
electricity, and see whether that body, rendered electrical,
attracts or repels it. If it attracts it, it is certainly of the
kind of electricity which I call VITREOUS; if, on the contrary,
it repels it, it is of the same kind of electricity with the
silk--that is, of the RESINOUS. I have likewise observed that
communicated electricity retains the same properties; for if a
ball of ivory or wood be set on a glass stand, and this ball be
rendered electric by the tube, it will repel such substances as
the tube repels; but if it be rendered electric by applying a
cylinder of gum-sack near it, it will produce quite contrary
effects--namely, precisely the same as gum-sack would produce. In
order to succeed in these experiments, it is requisite that the
two bodies which are put near each other, to find out the nature
of their electricity, be rendered as electrical as possible, for
if one of them was not at all or but weakly electrical, it would
be attracted by the other, though it be of that sort that should
naturally be repelled by it. But the experiment will always
succeed perfectly well if both bodies are sufficiently
electrical."[1]

As we now know, Dufay was wrong in supposing that there were two
different kinds of electricity, vitreous and resinous. A little
later the matter was explained by calling one "positive"
electricity and the other "negative," and it was believed that
certain substances produced only the one kind peculiar to that
particular substance. We shall see presently, however, that some
twenty years later an English scientist dispelled this illusion
by producing both positive (or vitreous) and negative (or
resinous) electricity on the same tube of glass at the same time.

After the death of Dufay his work was continued by his
fellow-countryman Dr. Joseph Desaguliers, who was the first
experimenter to electrify running water, and who was probably the
first to suggest that clouds might be electrified bodies. But
about, this time--that is, just before the middle of the
eighteenth century--the field of greatest experimental activity
was transferred to Germany, although both England and France were
still active. The two German philosophers who accomplished most
at this time were Christian August Hansen and George Matthias
Bose, both professors in Leipsic. Both seem to have conceived the
idea, simultaneously and independently, of generating electricity
by revolving globes run by belt and wheel in much the same manner
as the apparatus of Hauksbee.

With such machines it was possible to generate a much greater
amount of electricity than Dufay had been able to do with the
rubbed tube, and so equipped, the two German professors were able
to generate electric sparks and jets of fire in a most startling
manner. Bose in particular had a love for the spectacular, which
he turned to account with his new electrical machine upon many
occasions. On one of these occasions he prepared an elaborate
dinner, to which a large number of distinguished guests were
invited. Before the arrival of the company, however, Bose
insulated the great banquet-table on cakes of pitch, and then
connected it with a huge electrical machine concealed in another
room. All being ready, and the guests in their places about to be
seated, Bose gave a secret signal for starting this machine,
when, to the astonishment of the party, flames of fire shot from
flowers, dishes, and viands, giving a most startling but
beautiful display.

To add still further to the astonishment of his guests, Bose then
presented a beautiful young lady, to whom each of the young men
of the party was introduced. In some mysterious manner she was
insulated and connected with the concealed electrical machine, so
that as each gallant touched her fingertips he received an
electric shock that "made him reel." Not content with this, the
host invited the young men to kiss the beautiful maid. But those
who were bold enough to attempt it received an electric shock
that nearly "knocked their teeth out," as the professor tells it.

LUDOLFF'S EXPERIMENT WITH THE ELECTRIC SPARK

But Bose was only one of several German scientists who were
making elaborate experiments. While Bose was constructing and
experimenting with his huge machine, another German, Christian
Friedrich Ludolff, demonstrated that electric sparks are actual
fire--a fact long suspected but hitherto unproved. Ludolff's
discovery, as it chanced, was made in the lecture-hall of the
reorganized Academy of Sciences at Berlin, before an audience of
scientists and great personages, at the opening lecture in 1744.

In the course of this lecture on electricity, during which some
of the well-known manifestations of electricity were being shown,
it occurred to Ludolff to attempt to ignite some inflammable
fluid by projecting an electric spark upon its surface with a
glass rod. This idea was suggested to him while performing the
familiar experiment of producing a spark on the surface of a bowl
of water by touching it with a charged glass rod. He announced to
his audience the experiment he was about to attempt, and having
warmed a spoonful of sulphuric ether, he touched its surface with
the glass rod, causing it to burst into flame. This experiment
left no room for doubt that the electric spark was actual fire.

As soon as this experiment of Ludolff's was made known to Bose,
he immediately claimed that he had previously made similar
demonstrations on various inflammable substances, both liquid and
solid; and it seems highly probable that he had done so, as he
was constantly experimenting with the sparks, and must almost
certainly have set certain substances ablaze by accident, if not
by intent. At all events, he carried on a series of experiments
along this line to good purpose, finally succeeding in exploding
gun-powder, and so making the first forerunner of the electric
fuses now so universally used in blasting, firing cannon, and
other similar purposes. It was Bose also who, observing some of
the peculiar manifestations in electrified tubes, and noticing
their resemblance to "northern lights," was one of the first, if
not the first, to suggest that the aurora borealis is of electric
origin.

These spectacular demonstrations had the effect of calling public
attention to the fact that electricity is a most wonderful and
mysterious thing, to say the least, and kept both scientists and
laymen agog with expectancy. Bose himself was aflame with
excitement, and so determined in his efforts to produce still
stronger electric currents, that he sacrificed the tube of his
twenty-foot telescope for the construction of a mammoth
electrical machine. With this great machine a discharge of
electricity was generated powerful enough to wound the skin when
it happened to strike it.

Until this time electricity had been little more than a plaything
of the scientists--or, at least, no practical use had been made
of it. As it was a practising physician, Gilbert, who first laid
the foundation for experimenting with the new substance, so again
it was a medical man who first attempted to put it to practical
use, and that in the field of his profession. Gottlieb Kruger, a
professor of medicine at Halle in 1743, suggested that
electricity might be of use in some branches of medicine; and the
year following Christian Gottlieb Kratzenstein made a first
experiment to determine the effects of electricity upon the body.
He found that "the action of the heart was accelerated, the
circulation increased, and that muscles were made to contract by
the discharge": and he began at once administering electricity in
the treatment of certain diseases. He found that it acted
beneficially in rheumatic affections, and that it was
particularly useful in certain nervous diseases, such as palsies.
This was over a century ago, and to-day about the most important
use made of the particular kind of electricity with which he
experimented (the static, or frictional) is for the treatment of
diseases affecting the nervous system.

By the middle of the century a perfect mania for making
electrical machines had spread over Europe, and the whirling,
hand-rubbed globes were gradually replaced by great cylinders
rubbed by woollen cloths or pads, and generating an "enormous
power of electricity." These cylinders were run by belts and
foot-treadles, and gave a more powerful, constant, and
satisfactory current than known heretofore. While making
experiments with one of these machines, Johann Heinrichs Winkler
attempted to measure the speed at which electricity travels. To
do this he extended a cord suspended on silk threads, with the
end attached to the machine and the end which was to attract the
bits of gold-leaf near enough together so that the operator could
watch and measure the interval of time that elapsed between the
starting of the current along the cord and its attracting the
gold-leaf. The length of the cord used in this experiment was
only a little over a hundred feet, and this was, of course,
entirely inadequate, the current travelling that space apparently
instantaneously.

The improved method of generating electricity that had come into
general use made several of the scientists again turn their
attention more particularly to attempt putting it to some
practical account. They were stimulated to these efforts by the
constant reproaches that were beginning to be heard on all sides
that electricity was merely a "philosopher's plaything." One of
the first to succeed in inventing something that approached a
practical mechanical contrivance was Andrew Gordon, a Scotch
Benedictine monk. He invented an electric bell which would ring
automatically, and a little "motor," if it may be so called. And
while neither of these inventions were of any practical
importance in themselves, they were attempts in the right
direction, and were the first ancestors of modern electric bells
and motors, although the principle upon which they worked was
entirely different from modern electrical machines. The motor was
simply a wheel with several protruding metal points around its
rim. These points were arranged to receive an electrical
discharge from a frictional machine, the discharge causing the
wheel to rotate. There was very little force given to this
rotation, however, not enough, in fact, to make it possible to
more than barely turn the wheel itself. Two more great
discoveries, galvanism and electro-magnetic induction, were
necessary before the practical motor became possible.

The sober Gordon had a taste for the spectacular almost equal to
that of Bose. It was he who ignited a bowl of alcohol by turning
a stream of electrified water upon it, thus presenting the
seeming paradox of fire produced by a stream of water. Gordon
also demonstrated the power of the electrical discharge by
killing small birds and animals at a distance of two hundred
ells, the electricity being conveyed that distance through small
wires.

THE LEYDEN JAR DISCOVERED

As yet no one had discovered that electricity could be stored, or
generated in any way other than by some friction device. But very
soon two experimenters, Dean von Kleist, of Camin, Pomerania, and
Pieter van Musschenbroek, the famous teacher of Leyden,
apparently independently, made the discovery of what has been
known ever since as the Leyden jar. And although Musschenbroek is
sometimes credited with being the discoverer, there can be no
doubt that Von Kleist's discovery antedated his by a few months
at least.

Von Kleist found that by a device made of a narrow-necked bottle
containing alcohol or mercury, into which an iron nail was
inserted, be was able to retain the charge of electricity, after
electrifying this apparatus with the frictional machine. He made
also a similar device, more closely resembling the modern Leyden
jar, from a thermometer tube partly filled with water and a wire
tipped with a ball of lead. With these devices he found that he
could retain the charge of electricity for several hours, and
could produce the usual electrical manifestations, even to
igniting spirits, quite as well as with the frictional machine.
These experiments were first made in October, 1745, and after a
month of further experimenting, Von Kleist sent the following
account of them to several of the leading scientists, among
others, Dr. Lieberkuhn, in Berlin, and Dr. Kruger, of Halle.

"When a nail, or a piece of thick brass wire, is put into a small
apothecary's phial and electrified, remarkable effects follow;
but the phial must be very dry, or warm. I commonly rub it over
beforehand with a finger on which I put some pounded chalk. If a
little mercury or a few drops of spirit of wine be put into it,
the experiment succeeds better. As soon as this phial and nail
are removed from the electrifying-glass, or the prime conductor,
to which it has been exposed, is taken away, it throws out a
pencil of flame so long that, with this burning machine in my
hand, I have taken above sixty steps in walking about my room.
When it is electrified strongly, I can take it into another room
and there fire spirits of wine with it. If while it is
electrifying I put my finger, or a piece of gold which I hold in
my hand, to the nail, I receive a shock which stuns my arms and
shoulders.

"A tin tube, or a man, placed upon electrics, is electrified much
stronger by this means than in the common way. When I present
this phial and nail to a tin tube, which I have, fifteen feet
long, nothing but experience can make a person believe how
strongly it is electrified. I am persuaded," he adds, "that in
this manner Mr. Bose would not have taken a second electrical
kiss. Two thin glasses have been broken by the shock of it. It
appears to me very extraordinary, that when this phial and nail
are in contact with either conducting or non-conducting matter,
the strong shock does not follow. I have cemented it to wood,
metal, glass, sealing-wax, etc., when I have electrified without
any great effect. The human body, therefore, must contribute
something to it. This opinion is confirmed by my observing that
unless I hold the phial in my hand I cannot fire spirits of wine
with it."[2]

But it seems that none of the men who saw this account were able
to repeat the experiment and produce the effects claimed by Von
Kleist, and probably for this reason the discovery of the obscure
Pomeranian was for a time lost sight of.

Musschenbroek's discovery was made within a short time after Von
Kleist's--in fact, only a matter of about two months later. But
the difference in the reputations of the two discoverers insured
a very different reception for their discoveries. Musschenbroek
was one of the foremost teachers of Europe, and so widely known
that the great universities vied with each other, and kings were
bidding, for his services. Naturally, any discovery made by such
a famous person would soon be heralded from one end of Europe to
the other. And so when this professor of Leyden made his
discovery, the apparatus came to be called the "Leyden jar," for
want of a better name. There can be little doubt that
Musschenbroek made his discovery entirely independently of any
knowledge of Von Kleist's, or, for that matter, without ever
having heard of the Pomeranian, and his actions in the matter are
entirely honorable.

His discovery was the result of an accident. While experimenting
to determine the strength of electricity he suspended a
gun-barrel, which he charged with electricity from a revolving
glass globe. From the end of the gun-barrel opposite the globe
was a brass wire, which extended into a glass jar partly filled
with water. Musschenbroek held in one hand this jar, while with
the other he attempted to draw sparks from the barrel. Suddenly
he received a shock in the hand holding the jar, that "shook him

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