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 he 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, he 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

Leeuwenhoek, 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, he

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 like a stroke of lightning," and for a moment made

him believe that "he was done for." Continuing his experiments,

nevertheless, he found that if the jar were placed on a piece of metal

on the table, a shock would be received by touching this piece of metal

with one hand and touching the wire with the other--that is, a path was

made for the electrical discharge through the body. This was practically

the same experiment as made by Von Kleist with his bottle and nail,

but carried one step farther, as it showed that the "jar" need not

necessarily be held in the hand, as believed by Von Kleist. Further

experiments, continued by many philosophers at the time, revealed what

Von Kleist had already pointed out, that the electrified jar remained

charged for some time.

Soon after this Daniel Gralath, wishing to obtain stronger discharges

than could be had from a single Leyden jar, conceived the idea of

combining several jars, thus for the first time grouping the generators

in a "battery" which produced a discharge strong enough to kill birds

and small animals. He also attempted to measure the strength of the

discharges, but soon gave it up in despair, and the solution of this

problem was left for late nineteenth-century scientists.

The advent of the Leyden jar, which made it possible to produce strong

electrical discharges from a small and comparatively simple device, was

followed by more spectacular demonstrations of various kinds all

over Europe. These exhibitions aroused the interest of the kings and

noblemen, so that electricity no longer remained a "plaything of the

philosophers" alone, but of kings as well. A favorite demonstration was

that of sending the electrical discharge through long lines of soldiers

linked together by pieces of wire, the discharge causing them to "spring

into the air simultaneously" in a most astonishing manner. A certain

monk in Paris prepared a most elaborate series of demonstrations for

the amusement of the king, among other things linking together an entire

regiment of nine hundred men, causing them to perform simultaneous

springs and contortions in a manner most amusing to the royal guests.

But not all the experiments being made were of a purely spectacular

character, although most of them accomplished little except in a

negative way. The famous Abbe Nollet, for example, combined useful

experiments with spectacular demonstrations, thus keeping up popular

interest while aiding the cause of scientific electricity.

WILLIAM WATSON

Naturally, the new discoveries made necessary a new nomenclature, new

words and electrical terms being constantly employed by the various

writers of that day. Among these writers was the English scientist

William Watson, who was not only a most prolific writer but a tireless

investigator. Many of the words coined by him are now obsolete, but one

at least, "circuit," still remains in use.

In 1746, a French scientist, Louis Guillaume le Monnier, bad made a

circuit including metal and water by laying a chain half-way around the

edge of a pond, a man at either end holding it. One of these men dipped

his free hand in the water, the other presenting a Leyden jar to a

rod suspended on a cork float on the water, both men receiving a shock

simultaneously. Watson, a year later, attempted the same experiment on

a larger scale. He laid a wire about twelve hundred feet long across

Westminster Bridge over the Thames, bringing the ends to the water's

edge on the opposite banks, a man at one end holding the wire and

touching the water. A second man on the opposite side held the wire and

a Leyden jar; and a third touched the jar with one hand, while with the

other he grasped a wire that extended into the river. In this way they

not only received the shock, but fired alcohol as readily across the

stream as could be done in the laboratory. In this experiment Watson

discovered the superiority of wire over chain as a conductor, rightly

ascribing this superiority to the continuity of the metal.

Watson continued making similar experiments over longer watercourses,

some of them as long as eight thousand feet, and while engaged in making

one of these he made the discovery so essential to later inventions,

that the earth could be used as part of the circuit in the same manner

as bodies of water. Lengthening his wires he continued his experiments

until a circuit of four miles was made, and still the electricity seemed

to traverse the course instantaneously, and with apparently undiminished

force, if the insulation was perfect.

BENJAMIN FRANKLIN

Watson's writings now carried the field of active discovery across

the Atlantic, and for the first time an American scientist appeared--a

scientist who not only rivalled, but excelled, his European

contemporaries. Benjamin Franklin, of Philadelphia, coming into

possession of some of Watson's books, became so interested in the

experiments described in them that he began at once experimenting with

electricity. In Watson's book were given directions for making

various experiments, and these assisted Franklin in repeating the old

experiments, and eventually adding new ones. Associated with Franklin,

and equally interested and enthusiastic, if not equally successful in

making discoveries, were three other men, Thomas Hopkinson, Philip Sing,

and Ebenezer Kinnersley. These men worked together constantly, although

it appears to have been Franklin who made independently the important

discoveries, and formulated the famous Franklinian theory.

Working steadily, and keeping constantly in touch with the progress of

the European investigators, Franklin soon made some experiments which

he thought demonstrated some hitherto unknown phases of electrical

manifestation. This was the effect of pointed bodies "in DRAWING OFF

and THROWING OFF the electrical fire." In his description of this

phenomenon, Franklin writes:

"Place an iron shot of three or four inches diameter on the mouth of a

clean, dry, glass bottle. By a fine silken thread from the ceiling

right over the mouth of the bottle, suspend a small cork ball, about the

bigness of a marble; the thread of such a length that the cork ball may

rest against the side of the shot. Electrify the shot, and the ball

will be repelled to the distance of four or five inches, more or less,

according to the quantity of electricity. When in this state, if you

present to the shot the point of a long, slender shaft-bodkin, at six

or eight inches distance, the repellency is instantly destroyed, and the

cork flies to the shot. A blunt body must be brought within an inch, and

draw a spark, to produce the same effect.

"To prove that the electrical fire is DRAWN OFF by the point, if you

take the blade of the bodkin out of the wooden handle and fix it in a

stick of sealing-wax, and then present it at the distance aforesaid,

or if you bring it very near, no such effect follows; but sliding one

finger along the wax till you touch the blade, and the ball flies to

the shot immediately. If you present the point in the dark you will see,

sometimes at a foot distance, and more, a light gather upon it like that

of a fire-fly or glow-worm; the less sharp the point, the nearer you

must bring it to observe the light; and at whatever distance you see the

light, you may draw off the electrical fire and destroy the repellency.

If a cork ball so suspended be repelled by the tube, and a point

be presented quick to it, though at a considerable distance, 'tis

surprising to see how suddenly it flies back to the tube. Points of

wood will do as well as those of iron, provided the wood is not dry; for

perfectly dry wood will no more conduct electricity than sealing-wax.

"To show that points will THROW OFF as well as DRAW OFF the electrical

fire, lay a long, sharp needle upon the shot, and you cannot electrify

the shot so as to make it repel the cork ball. Or fix a needle to the

end of a suspended gun-barrel or iron rod, so as to point beyond it

like a little bayonet, and while it remains there, the gun-barrel or rod

cannot, by applying the tube to the other end, be electrified so as to

give a spark, the fire continually running out silently at the point. In

the dark you may see it make the same appearance as it does in the case

before mentioned."(3)

Von Guericke, Hauksbee, and Gray had noticed that pointed bodies

attracted electricity in a peculiar manner, but this demonstration

of the "drawing off" of "electrical fire" was original with Franklin.

Original also was the theory that he now suggested, which had at least

the merit of being thinkable even by non-philosophical minds. It assumes

that electricity is like a fluid, that will flow along conductors and

accumulate in proper receptacles, very much as ordinary fluids do. This

conception is probably entirely incorrect, but nevertheless it is likely

to remain a popular one, at least outside of scientific circles, or

until something equally tangible is substituted.

FRANKLIN'S THEORY OF ELECTRICITY

According to Franklin's theory, electricity exists in all bodies as a

"common stock," and tends to seek and remain in a state of equilibrium,

just as fluids naturally tend to seek a level. But it may, nevertheless,

be raised or lowered, and this equilibrium be thus disturbed. If a body

has more electricity than its normal amount it is said to be POSITIVELY

electrified; but if it has less, it is NEGATIVELY electrified. An

over-electrified or "plus" body tends to give its surplus stock to

a body containing the normal amount; while the "minus" or

under-electrified body will draw electricity from one containing the

normal amount.

Working along lines suggested by this theory, Franklin attempted to show

that electricity is not created by friction, but simply collected from

its diversified state, the rubbed glass globe attracting a certain

quantity of "electrical fire," but ever ready to give it up to any body

that has less. He explained the charged Leyden jar by showing that the

inner coating of tin-foil received more than the ordinary quantity of

electricity, and in consequence is POSITIVELY electrified, while the

outer coating, having the ordinary quantity of electricity diminished,

is electrified NEGATIVELY.

These studies of the Leyden jar, and the studies of pieces of glass

coated with sheet metal, led Franklin to invent his battery, constructed

of eleven large glass plates coated with sheets of lead. With this

machine, after overcoming some defects, he was able to produce

electrical manifestations of great force--a force that "knew no bounds,"

as he declared ("except in the matter of expense and of labor"), and

which could be made to exceed "the greatest know effects of common

lightning."

This reference to lightning would seem to show Franklin's belief, even

at that time, that lightning is electricity. Many eminent observers,

such as Hauksbee, Wall, Gray, and Nollet, had noticed the resemblance

between electric sparks and lightning, but none of these had more than

surmised that the two might be identical. In 1746, the surgeon, John

Freke, also asserted his belief in this identity. Winkler, shortly after

this time, expressed the same belief, and, assuming that they were

the same, declared that "there is no proof that they are of different

natures"; and still he did not prove that they were the same nature.

FRANKLIN INVENTS THE LIGHTNING-ROD

Even before Franklin proved conclusively the nature of lightning, his

experiments in drawing off the electric charge with points led to

some practical suggestions which resulted in the invention of the

lightning-rod. In the letter of July, 1750, which he wrote on the

subject, he gave careful instructions as to the way in which these rods

might be constructed. In part Franklin wrote: "May not the knowledge

of this power of points be of use to mankind in preserving houses,

churches, ships, etc., from the stroke of lightning by directing us to

fix on the highest parts of the edifices upright rods of iron made sharp

as a needle, and gilt to prevent rusting, and from the foot of these

rods a wire down the outside of the building into the grounds, or down

round one of the shrouds of a ship and down her side till it reaches the

water? Would not these pointed rods probably draw the electrical fire

silently out of a cloud before it came nigh enough to strike, and

thereby secure us from that most sudden and terrible mischief?

"To determine this question, whether the clouds that contain the

lightning are electrified or not, I propose an experiment to be tried

where it may be done conveniently. On the top of some high tower or

steeple, place a kind of sentry-box, big enough to contain a man and an

electrical stand. From the middle of the stand let an iron rod rise and

pass, bending out of the door, and then upright twenty or thirty feet,

pointed very sharp at the end. If the electrical stand be kept clean

and dry, a man standing on it when such clouds are passing low might be

electrified and afford sparks, the rod drawing fire to him from a cloud.

If any danger to the man be apprehended (though I think there would be

none), let him stand on the floor of his box and now and then bring near

to the rod the loop of a wire that has one end fastened to the leads,

he holding it by a wax handle; so the sparks, if the rod is electrified,

will strike from the rod to the wire and not effect him."(4)

Not satisfied with all the evidence that he had collected pointing to

the identity of lightning and electricity, he adds one more striking

and very suggestive piece of evidence. Lightning was known sometimes to

strike persons blind without killing them. In experimenting on pigeons

and pullets with his electrical machine, Franklin found that a fowl,

when not killed outright, was sometimes rendered blind. The report

of these experiments were incorporated in this famous letter of the

Philadelphia philosopher.

The attitude of the Royal Society towards this clearly stated letter,

with its useful suggestions, must always remain as a blot on the

record of this usually very receptive and liberal-minded body. Far from

publishing it or receiving it at all, they derided the whole matter as

too visionary for discussion by the society. How was it possible that

any great scientific discovery could be made by a self-educated colonial

newspaper editor, who knew nothing of European science except by

hearsay, when all the great scientific minds of Europe had failed to

make the discovery? How indeed! And yet it would seem that if any of the

influential members of the learned society had taken the trouble to read

over Franklin's clearly stated letter, they could hardly have failed

to see that his suggestions were worthy of consideration. But at all

events, whether they did or did not matters little. The fact remains

that they refused to consider the paper seriously at the time; and later

on, when its true value became known, were obliged to acknowledge their

error by a tardy report on the already well-known document.

But if English scientists were cold in their reception of Franklin's

theory and suggestions, the French scientists were not. Buffon,

perceiving at once the importance of some of Franklin's experiments,

took steps to have the famous letter translated into French, and soon

not only the savants, but members of the court and the king himself were

intensely interested. Two scientists, De Lor and D'Alibard, undertook to

test the truth of Franklin's suggestions as to pointed rods "drawing off

lightning." In a garden near Paris, the latter erected a pointed iron

rod fifty feet high and an inch in diameter. As no thunder-clouds

appeared for several days, a guard was stationed, armed with an

insulated brass wire, who was directed to test the iron rods with it in

case a storm came on during D'Alibard's absence. The storm did come on,

and the guard, not waiting for his employer's arrival, seized the wire

and touched the rod. Instantly there was a report. Sparks flew and the

guard received such a shock that he thought his time had come. Believing

from his outcry that he was mortally hurt, his friends rushed for a

spiritual adviser, who came running through rain and hail to administer

the last rites; but when he found the guard still alive and uninjured,

he turned his visit to account by testing the rod himself several times,

and later writing a report of his experiments to M. d'Alibard. This

scientist at once reported the affair to the French Academy, remarking

that "Franklin's idea was no longer a conjecture, but a reality."

FRANKLIN PROVES THAT LIGHTNING IS ELECTRICITY

Europe, hitherto somewhat sceptical of Franklin's views, was by this

time convinced of the identity of lightning and electricity. It was now

Franklin's turn to be sceptical. To him the fact that a rod, one hundred

feet high, became electrified during a storm did not necessarily prove

that the storm-clouds were electrified. A rod of that length was not

really projected into the cloud, for even a very low thunder-cloud was

more than a hundred feet above the ground. Irrefutable proof could

only be had, as he saw it, by "extracting" the lightning with something

actually sent up into the storm-cloud; and to accomplish this Franklin

made his silk kite, with which he finally demonstrated to his own and

the world's satisfaction that his theory was correct.

Taking his kite out into an open common on the approach of a

thunder-storm, he flew it well up into the threatening clouds, and then,

touching, the suspended key with his knuckle, received the electric

spark; and a little later he charged a Leyden jar from the electricity

drawn from the clouds with his kite.

In a brief but direct letter, he sent an account of his kite and his

experiment to England:

"Make a small cross of two light strips of cedar," he wrote, "the

arms so long as to reach to the four corners of a large, thin, silk

handkerchief when extended; tie the corners of the handkerchief to the

extremities of the cross so you have the body of a kite; which being

properly accommodated with a tail, loop, and string, will rise in the

air like those made of paper; but this being of silk is fitter to bear

the wind and wet of a thunder-gust without tearing. To the top of the

upright stick of the cross is to be fixed a very sharp-pointed wire,

rising a foot or more above the wood. To the end of the twine, next the

hand, is to be tied a silk ribbon; where the silk and twine join a key

may be fastened. This kite is to be raised when a thunder-gust appears

to be coming on, and the person who holds the string must stand within

a door or window or under some cover, so that the silk ribbon may not be

wet; and care must be taken that the twine does not touch the frame of

the door or window. As soon as any of the thunder-clouds come over the

kite, the pointed wire will draw the electric fire from them, and the

kite, with all the twine, will be electrified and the loose filaments

will stand out everywhere and be attracted by the approaching finger,

and when the rain has wet the kite and twine so that it can conduct the

electric fire freely, you will find it stream out plentifully from the

key on the approach of your knuckle, and with this key the phial may be

charged; and from electric fire thus obtained spirits may be kindled and

all other electric experiments performed which are usually done by the

help of a rubbed glass globe or tube, and thereby the sameness of the

electric matter with that of lightning completely demonstrated."(5)

In experimenting with lightning and Franklin's pointed rods in Europe,

several scientists received severe shocks, in one case with a fatal

result. Professor Richman, of St. Petersburg, while experimenting during

a thunder-storm, with an iron rod which he had erected on his house,

received a shock that killed him instantly.

About 1733, as we have seen, Dufay had demonstrated that there were two

apparently different kinds of electricity; one called VITREOUS because

produced by rubbing glass, and the other RESINOUS because produced

by rubbed resinous bodies. Dufay supposed that these two apparently

different electricities could only be produced by their respective

substances; but twenty years later, John Canton (1715-1772), an

Englishman, demonstrated that under certain conditions both might be

produced by rubbing the same substance. Canton's experiment, made upon

a glass tube with a roughened surface, proved that if the surface of the

tube were rubbed with oiled silk, vitreous or positive electricity was

produced, but if rubbed with flannel, resinous electricity was produced.

He discovered still further that both kinds could be excited on the same

tube simultaneously with a single rubber. To demonstrate this he used a

tube, one-half of which had a roughened the other a glazed surface.

With a single stroke of the rubber he was able to excite both kinds of

electricity on this tube. He found also that certain substances, such as

glass and amber, were electrified positively when taken out of mercury,

and this led to his important discovery that an amalgam of mercury

and tin, when used on the surface of the rubber, was very effective in

exciting glass.

XV. NATURAL HISTORY TO THE TIME OF LINNAEUS

Modern systematic botany and zoology are usually held to have their

beginnings with Linnaeus. But there were certain precursors of the

famous Swedish naturalist, some of them antedating him by more than a

century, whose work must not be altogether ignored--such men as Konrad

Gesner (1516-1565), Andreas Caesalpinus (1579-1603), Francisco Redi

(1618-1676), Giovanni Alfonso Borelli (1608-1679), John Ray (1628-1705),

Robert Hooke (1635-1703), John Swammerdam (1637-1680), Marcello Malpighi

(1628-1694), Nehemiah Grew (1628-1711), Joseph Tournefort (1656-1708),

Rudolf Jacob Camerarius (1665-1721), and Stephen Hales (1677-1761). The

last named of these was, to be sure, a contemporary of Linnaeus himself,

but Gesner and Caesalpinus belong, it will be observed, to so remote an

epoch as that of Copernicus.

Reference has been made in an earlier chapter to the microscopic

investigations of Marcello Malpighi, who, as there related, was the

first observer who actually saw blood corpuscles pass through the

capillaries. Another feat of this earliest of great microscopists was

to dissect muscular tissue, and thus become the father of microscopic

anatomy. But Malpighi did not confine his observations to animal

tissues. He dissected plants as well, and he is almost as fully entitled

to be called the father of vegetable anatomy, though here his honors are

shared by the Englishman Grew. In 1681, while Malpighi's work, Anatomia

plantarum, was on its way to the Royal Society for publication, Grew's

Anatomy of Vegetables was in the hands of the publishers, making its

appearance a few months earlier than the work of the great Italian.

Grew's book was epoch-marking in pointing out the sex-differences in

plants.

Robert Hooke developed the microscope, and took the first steps towards

studying vegetable anatomy, publishing in 1667, among other results,

the discovery of the cellular structure of cork. Hooke applied the

name "cell" for the first time in this connection. These discoveries of

Hooke, Malpighi, and Grew, and the discovery of the circulation of the

blood by William Harvey shortly before, had called attention to the

similarity of animal and vegetable structures. Hales made a series

of investigations upon animals to determine the force of the blood

pressure; and similarly he made numerous statical experiments to

determine the pressure of the flow of sap in vegetables. His Vegetable

Statics, published in 1727, was the first important work on the subject

of vegetable physiology, and for this reason Hales has been called the

father of this branch of science.

In botany, as well as in zoology, the classifications of Linnaeus of

course supplanted all preceding classifications, for the obvious reason

that they were much more satisfactory; but his work was a culmination of

many similar and more or less satisfactory attempts of his predecessors.

About the year 1670 Dr. Robert Morison (1620-1683), of Aberdeen,

published a classification of plants, his system taking into account the

woody or herbaceous structure, as well as the flowers and fruit. This

classification was supplanted twelve years later by the classification

of Ray, who arranged all known vegetables into thirty-three classes, the

basis of this classification being the fruit. A few years later Rivinus,

a professor of botany in the University of Leipzig, made still another

classification, determining the distinguishing character chiefly

from the flower, and Camerarius and Tournefort also made elaborate

classifications. On the Continent Tournefort's classification was the

most popular until the time of Linnaeus, his systematic arrangement

including about eight thousand species of plants, arranged chiefly

according to the form of the corolla.

Most of these early workers gave attention to both vegetable and

animal kingdoms. They were called naturalists, and the field of their

investigations was spoken of as "natural history." The specialization of

knowledge had not reached that later stage in which botanist, zoologist,

and physiologist felt their labors to be sharply divided. Such a

division was becoming more and more necessary as the field of knowledge

extended; but it did not become imperative until long after the time

of Linnaeus. That naturalist himself, as we shall see, was equally

distinguished as botanist and as zoologist. His great task of organizing

knowledge was applied to the entire range of living things.

Carolus Linnaeus was born in the town of Rashult, in Sweden, on May 13,

1707. As a child he showed great aptitude in learning botanical names,

and remembering facts about various plants as told him by his father.

His eagerness for knowledge did not extend to the ordinary primary

studies, however, and, aside from the single exception of the study of

physiology, he proved himself an indifferent pupil. His backwardness was

a sore trial to his father, who was desirous that his son should enter

the ministry; but as the young Linnaeus showed no liking for that

calling, and as he had acquitted himself well in his study of

physiology, his father at last decided to allow him to take up the study

of medicine. Here at last was a field more to the liking of the boy,

who soon vied with the best of his fellow-students for first honors.

Meanwhile he kept steadily at work in his study of natural history,

acquiring considerable knowledge of ornithology, entomology, and botany,

and adding continually to his collection of botanical specimens. In 1729

his botanical knowledge was brought to the attention of Olaf Rudbeck,

professor of botany in the University of Upsala, by a short paper on the

sexes of plants which Linnaeus had prepared. Rudbeck was so impressed by

some of the ideas expressed in this paper that he appointed the author

as his assistant the following year.

This was the beginning of Linnaes's career as a botanist. The academic

gardens were thus thrown open to him, and he found time at his disposal

for pursuing his studies between lecture hours and in the evenings. It

was at this time that he began the preparation of his work the Systema

naturae, the first of his great works, containing a comprehensive sketch

of the whole field of natural history. When this work was published, the

clearness of the views expressed and the systematic arrangement of the

various classifications excited great astonishment and admiration, and

placed Linaeus at once in the foremost rank of naturalists. This

work was followed shortly by other publications, mostly on botanical

subjects, in which, among other things, he worked out in detail his

famous "system."

This system is founded on the sexes of plants, and is usually referred

to as an "artificial method" of classification because it takes into

account only a few marked characters of plants, without uniting them by

more general natural affinities. At the present time it is considered

only as a stepping-stone to the "natural" system; but at the time of its

promulgation it was epoch-marking in its directness and simplicity, and

therefore superiority, over any existing systems.

One of the great reforms effected by Linnaeus was in the matter of

scientific terminology. Technical terms are absolutely necessary to

scientific progress, and particularly so in botany, where obscurity,

ambiguity, or prolixity in descriptions are fatally misleading.

Linnaeus's work contains something like a thousand terms, whose meanings

and uses are carefully explained. Such an array seems at first glance

arbitrary and unnecessary, but the fact that it has remained in use

for something like two centuries is indisputable evidence of its

practicality. The descriptive language of botany, as employed by

Linnaeus, still stands as a model for all other subjects.

Closely allied to botanical terminology is the subject of botanical

nomenclature. The old method of using a number of Latin words to

describe each different plant is obviously too cumbersome, and several

attempts had been made prior to the time of Linnaeus to substitute

simpler methods. Linnaeus himself made several unsatisfactory attempts

before he finally hit upon his system of "trivial names," which

was developed in his Species plantarum, and which, with some, minor

alterations, remains in use to this day. The essence of the system is

the introduction of binomial nomenclature--that is to say, the use

of two names and no more to designate any single species of animal or

plant. The principle is quite the same as that according to which

in modern society a man has two names, let us say, John Doe, the one

designating his family, the other being individual. Similarly each

species of animal or plant, according to the Linnaeean system, received

a specific or "trivial" name; while various species, associated

according to their seeming natural affinities into groups called genera,

were given the same generic name. Thus the generic name given all

members of the cat tribe being Felis, the name Felis leo designates the

lion; Felis pardus, the leopard; Felis domestica, the house cat, and so

on. This seems perfectly simple and natural now, but to understand

how great a reform the binomial nomenclature introduced we have but to

consult the work of Linnaeus's predecessors. A single illustration will

suffice. There is, for example, a kind of grass, in referring to

which the naturalist anterior to Linnaeus, if he would be absolutely

unambiguous, was obliged to use the following descriptive formula:

Gramen Xerampelino, Miliacea, praetenuis ramosaque sparsa panicula,

sive Xerampelino congener, arvense, aestivum; gramen minutissimo semine.

Linnaeus gave to this plant the name Poa bulbosa--a name that sufficed,

according to the new system, to distinguish this from every other

species of vegetable. It does not require any special knowledge to

appreciate the advantage of such a simplification.

While visiting Paris in 1738 Linnaeus met and botanized with the two

botanists whose "natural method" of classification was later to supplant

his own "artificial system." These were Bernard and Antoine Laurent

de Jussieu. The efforts of these two scientists were directed towards

obtaining a system which should aim at clearness, simplicity, and

precision, and at the same time be governed by the natural affinities of

plants. The natural system, as finally propounded by them, is based on

the number of cotyledons, the structure of the seed, and the insertion

of the stamens. Succeeding writers on botany have made various

modifications of this system, but nevertheless it stands as the

foundation-stone of modern botanical classification.

APPENDIX

REFERENCE LIST

CHAPTER I

SCIENCE IN THE DARK AGE

(1) (p. 4). James Harvey Robinson, An Introduction to the History of

Western Europe, New York, 1898, p. 330.

(2) (p. 6). Henry Smith Williams, A Prefatory Characterization of The

History of Italy, in vol. IX. of The Historians' History of the World,

25 vols., London and New York, 1904.

CHAPTER III

MEDIAEVAL SCIENCE IN THE WEST

(1) (p. 47). Etigene Muntz, Leonardo do Vinci, Artist, Thinker, and Man

of Science, 2 vols., New York, 1892. Vol. II., p. 73.

CHAPTER IV

THE NEW COSMOLOGY--COPERNICUS TO KEPLER AND GALILEO

(1) (p. 62). Copernicus, uber die Kreisbewegungen der Welfkorper, trans.

from Dannemann's Geschichle du Naturwissenschaften, 2 vols., Leipzig,

1896.

(2) (p. 90). Galileo, Dialogo dei due Massimi Systemi del Mondo, trans.

from Dannemann, op. cit.

CHAPTER V

GALILEO AND THE NEW PHYSICS (1) (p. 101). Rothmann, History of Astronomy

(in the Library of Useful Knowledge), London, 1834.

(2) (p. 102). William Whewell, History of the Inductive Sciences, 3

Vols, London, 1847-Vol. II., p. 48.

(3) (p. 111). The Lives of Eminent Persons, by Biot, Jardine, Bethune,

etc., London, 1833.

(4) (p. 113). William Gilbert, De Magnete, translated by P. Fleury

Motteley, London, 1893. In the biographical memoir, p. xvi.

(5) (p. 114). Gilbert, op. cit., p. x1vii.

(6) (p. 114). Gilbert, op. cit., p. 24.

CHAPTER VI

TWO PSEUDO-SCIENCES--ALCHEMY AND ASTROLOGY

(1) (p. 125). Exodus xxxii, 20.

(2) (p. 126). Charles Mackay, Popular Delusions, 3 vols., London, 1850.

Vol. II., p. 280.

(3) (p. 140). Mackay, op. cit., Vol. 11., p. 289.

(4) (P. 145). John B. Schmalz, Astrology Vindicated, New York, 1898.

(5) (p. 146). William Lilly, The Starry Messenger, London, 1645, p. 63.

(6) (p. 149). Lilly, op. cit., p. 70.

(7) (p. 152). George Wharton, An Astrological judgement upon His

Majesty's Present March begun from Oxford, May 7, 1645, pp. 7-10.

(8) (p. 154). C. W. Roback, The Mysteries of Astrology, Boston, 1854, p.

29.

CHAPTER VII

FROM PARACELSUS TO HARVEY

(1) (p. 159). A. E. Waite, The Hermetic and Alchemical Writings of

Paracelsus, 2 vols., London, 1894. Vol. I., p. 21.

(2) (p. 167). E. T. Withington, Medical History from the Earliest Times,

London, 1894, p. 278.

(3) (p. 173). John Dalton, Doctrines of the Circulation, Philadelphia,

1884, p. 179.

(4) (p. 174). William Harvey, De Motu Cordis et Sanguinis, London, 1803,

chap. X.

(5) (p. 178). The Works of William Harvey, translated by Robert Willis,

London, 1847, p. 56.

CHAPTER VIII

MEDICINE IN THE SIXTEENTH AND SEVENTEENTH CENTURIES

(1) (p. 189). Hermann Baas, History of Medicine, translated by H. E.

Henderson, New York, 1894, p. 504.

(2) (p. 189). E. T. Withington, Medical History from the Earliest Times,

London, 1894, p. 320.

CHAPTER IX

PHILOSOPHER-SCIENTISTS AND NEW INSTITUTIONS OF LEARNING

(1) (p. 193). George L. Craik, Bacon and His Writings and Philosophy, 2

vols., London, 1846. Vol. II., p. 121.

(2) (p. 193). From Huxley's address On Descartes's Discourse Touching

the Method of Using One's Reason Rightly and of Seeking Scientific

Truth.

(3) (p. 195). Rene Descartes, Traite de l'Homme (Cousins's edition. in

ii vols.), Paris, 1824. Vol, VI., p. 347.

CHAPTER X

THE SUCCESSORS OF GALILEO IN PHYSICAL SCIENCE

(1) (p. 205). See The Phlogiston Theory, Vol, IV.

(2) (p. 205). Robert Boyle, Philosophical Works, 3 vols., London, 1738.

Vol. III., p. 41.

(3) (p. 206). Ibid., Vol. III., p. 47.

(4) (p. 206). Ibid., Vol. II., p. 92.

(5) (p. 207). Ibid., Vol. II., p. 2.

(6) (p. 209). Ibid., Vol. I., p. 8.

(7) (p. 209). Ibid., vol. III., p. 508.

(8) (p. 210). Ibid., Vol. III., p. 361.

(9) (p. 213). Otto von Guericke, in the Philosophical Transactions of

the Royal Society of London, No. 88, for 1672, p. 5103.

(10) (p. 222). Von Guericke, Phil. Trans. for 1669, Vol I., pp. 173,

174.

CHAPTER XI

NEWTON AND THE COMPOSITION OF LIGHT

(1) (p. 233). Phil. Trans. of Royal Soc. of London, No. 80, 1672, pp.

3076-3079. (2) (p 234). Ibid., pp. 3084, 3085.

(3) (p. 235). Voltaire, Letters Concerning the English Nation, London,

1811.

CHAPTER XII

NEWTON AND THE LAW OF GRAVITATION

(1) (p. 242). Sir Isaac Newton, Principia, translated by Andrew Motte,

New York, 1848, pp. 391, 392.

(2) (p. 250). Newton op. cit., pp. 506, 507.

CHAPTER XIV

PROGRESS IN ELECTRICITY FROM GILBERT AND VON GUERICKE TO FRANKLIN

(1) (p. 274). A letter from M. Dufay, F.R.S. and of the Royal Academy

of Sciences at Paris, etc., in the Phil. Trans. of the Royal Soc., vol.

XXXVIII., pp. 258-265.

(2) (p. 282). Dean von Kleist, in the Danzick Memoirs, Vol. I., p. 407.

From Joseph Priestley's History of Electricity, London, 1775, pp. 83,

84.

(3) (p. 288). Benjamin Franklin, New Experiments and Observations on

Electricity, London, 1760, pp. 107, 108.

(4) (p. 291). Franklin, op. cit., pp. 62, 63.

(5) (p. 295). Franklin, op. cit., pp. 107, 108.

(For notes and bibliography to vol. II. see vol. V.)