If this new hypothesis thus connected together all these appearances

as happily as that of Ptolemy, there were others which it connected

together much better. The three superior Planets, when nearly in

conjunction with the Sun, appear always at the greatest distance from

the Earth, are smallest, and least sensible to the eye, and seem to

revolve forward in their direct motion with the greatest rapidity. On

the contrary, when in opposition to the Sun, that is, when in their

meridian about midnight, they appear nearest the Earth, are largest,

and most sensible to the eye, and seem to revolve backwards in their

retrograde motion. To explain these appearances, the system of Ptolemy

supposed {358} each of these Planets to be at the upper part of their

several Epicycles, in the one case; and at the lower, in the other.

But it afforded no satisfactory principle of connection, which could

lead the mind easily to conceive how the Epicycles of those Planets,

whose spheres were so distant from the sphere of the Sun, should thus,

if one may say so, keep time to his motion. The system of Copernicus

afforded this easily, and like a more simple machine, without the

assistance of Epicycles, connected together, by fewer movements, the

complex appearances of the heavens. When the superior Planets appear

nearly in conjunction with the Sun, they are then in the side of their

orbits, which is almost opposite to, and most distant from the Earth,

and therefore appear smallest, and least sensible to the eye. But, as

they then revolve in a direction which is almost contrary to that of

the Earth, they appear to advance forward with double velocity; as a

ship, that sails in a contrary direction to another, appears from that

other, to sail both with its own velocity, and the velocity of that

from which it is seen. On the contrary, when those Planets are in

opposition to the Sun, they are on the same side of the Sun with the

Earth, are nearest it, most sensible to the eye, and revolve in the

same direction with it; but, as their revolutions round the Sun are

slower than that of the Earth, they are necessarily left behind by it,

and therefore seem to revolve backwards; as a ship which sails slower

than another, though it sails in the same direction, appears from that

other to sail backwards. After the same manner, by the same annual

revolution of the Earth, he connected together the direct and

retrograde motions of the two inferior Planets, as well as the

stationary appearances of all the Five.

There are some other particular phenomena of the two inferior Planets,

which correspond still better to this system, and still worse to that

of Ptolemy. Venus and Mercury seem to attend constantly upon the

motion of the Sun, appearing, sometimes on the one side, and sometimes

on the other, of that great luminary; Mercury being almost always

buried in his rays, and Venus never receding above forty-eight degrees

from him, contrary to what is observed in the other three Planets,

which are often seen in the opposite side of the heavens, at the

greatest possible distance from the Sun. The system of Ptolemy

accounted for this, by supposing that the centres of the Epicycles of

these two Planets were always in the same line with those of the Sun

and the Earth; that they appeared therefore in conjunction with the

Sun, when either in the upper or lower part of their Epicycles, and at

the greatest distance from him, when in the sides of them. It

assigned, however, no reason why the Epicycles of these two Planets

should observe so different a rule from that which takes place in

those of the other three, nor for the enormous Epicycle of Venus,

whose sides must have been forty-eight degrees distant from the Sun,

while its centre was in conjunction with him, and whose diameter must

have covered {359} more than a quadrant of the Great Circle. But how

easily all these appearances coincide with the hypothesis, which

represents those two inferior Planets revolving round the Sun in

orbits comprehended within the orbit of the Earth, is too obvious to

require an explanation.

Thus far did this new account of things render the appearances of the

heavens more completely coherent than had been done by any of the

former systems. It did this, too, by a more simple and intelligible,

as well as more beautiful machinery. It represented the Sun, the great

enlightener of the universe, whose body was alone larger than all the

Planets taken together, as established immovable in the centre,

shedding light and heat on all the worlds that circulated around him

in one uniform direction, but in longer or shorter periods, according

to their different distances. It took away the diurnal revolution of

the firmament, whose rapidity, upon the old hypothesis, was beyond

what even thought could conceive. It not only delivered the

imagination from the embarrassment of Epicycles, but from the

difficulty of conceiving these two opposite motions going on at the

same time, which the system of Ptolemy and Aristotle bestowed upon all

the Planets; I mean, their diurnal westward, and periodical eastward

revolutions. The Earth's revolution round its own axis took away the

necessity for supposing the first, and the second was easily conceived

when by itself. The Five Planets, which seem, upon all other systems,

to be objects of a species by themselves, unlike to every thing to

which the imagination has been accustomed, when supposed to revolve

along with the Earth round the Sun, were naturally apprehended to be

objects of the same kind with the Earth, habitable, opaque, and

enlightened only by the rays of the Sun. And thus this hypothesis, by

classing them in the same species of things, with an object that is of

all others the most familiar to us, took off that wonder and that

uncertainty which the strangeness and singularity of their appearance

had excited; and thus far, too, better answered the great end of

Philosophy.

Neither did the beauty and simplicity of this system alone recommend

it to the imagination; the novelty and unexpectedness of that view of

nature, which it opened to the fancy, excited more wonder and surprise

than the strangest of those appearances, which it had been invented to

render natural and familiar, and these sentiments still more endeared

it. For, though it is the end of Philosophy, to allay that wonder,

which either the unusual or seemingly disjointed appearances of nature

excite, yet she never triumphs so much, as when, in order to connect

together a few, in themselves, perhaps, inconsiderable objects, she

has, if I may say so, created another constitution of things, more

natural, indeed, and such as the imagination can more easily attend

to, but more new, more contrary to common opinion and expectation,

than any of those appearances themselves. As, in the instance before

us, in order to connect together some seeming irregularities in the

motions of {360} the Planets, the most inconsiderable objects in the

heavens, and of which the greater part of mankind have no occasion to

take any notice during the whole course of their lives, she has, to

talk in the hyperbolical language of Tycho Brahe, moved the Earth from

its foundations, stopped the revolution of the Firmament, made the Sun

stand still, and subverted the whole order of the Universe.

Such were the advantages of this new hypothesis, as they appeared to

its author, when he first invented it. But, though that love of

paradox, so natural to the learned, and that pleasure, which they are

so apt to take in exciting, by the novelties of their supposed

discoveries, the amazement of mankind, may, notwithstanding what one

of his disciples tells us to the contrary, have had its weight in

prompting Copernicus to adopt this system; yet, when he had completed

his Treatise of Revolutions, and began coolly to consider what a

strange doctrine he was about to offer to the world, he so much

dreaded the prejudice of mankind against it, that, by a species of

continence, of all others the most difficult to a philosopher, he

detained it in his closet for thirty years together. At last, in the

extremity of old age, he allowed it to be extorted from him, but he

died as soon as it was printed, and before it was published to the

world.

When it appeared in the world, it was almost universally disapproved

of, by the learned as well as by the ignorant. The natural prejudices

of sense, confirmed by education, prevailed too much with both, to

allow them to give it a fair examination. A few disciples only, whom

he himself had instructed in his doctrine, received it with esteem and

admiration. One of them, Reinholdus, formed, upon this hypothesis,

larger and more accurate astronomical tables, than what accompanied

the Treatise of Revolutions, in which Copernicus had been guilty of

some errors in calculation. It soon appeared, that these Prutenic

Tables, as they were called, corresponded more exactly with the

heavens, than the Tables of Alphonsus. This ought naturally to have

formed a prejudice in favour of the diligence and accuracy of

Copernicus in observing the heavens. But it ought to have formed none

in favour of his hypothesis; since the same observations, and the

result of the same calculations, might have been accommodated to the

system of Ptolemy, without making any greater alteration in that

system than what Ptolemy had foreseen, and had even foretold should be

made. It formed, however, a prejudice in favour of both, and the

learned began to examine, with some attention, an hypothesis which

afforded the easiest methods of calculation, and upon which the most

exact predictions had been made. The superior degree of coherence,

which it bestowed upon the celestial appearances, the simplicity and

uniformity which it introduced into the real directions and velocities

of the Planets, soon disposed many astronomers, first to favour, and

at last to embrace a system, which thus connected together so happily,

{361} the most disjointed of those objects that chiefly occupied their

thoughts. Nor can any thing more evidently demonstrate, how easily the

learned give up the evidence of their senses to preserve the coherence

of the ideas of their imagination, than the readiness with which this,

the most violent paradox in all philosophy, was adopted by many

ingenious astronomers, notwithstanding its inconsistency with every

system of physics then known in the world, and notwithstanding the

great number of other more real objections, to which, as Copernicus

left it, this account of things was most justly exposed.

It was adopted, however, nor can this be wondered at, by astronomers

only. The learned in all other sciences, continued to regard it with

the same contempt as the vulgar. Even astronomers were divided about

its merit; and many of them rejected a doctrine, which not only

contradicted the established system of Natural Philosophy, but which,

considered astronomically only, seemed, to them, to labour under

several difficulties.

Some of the objections against the motion of the Earth, that were

drawn from the prejudices of sense, the patrons of this system,

indeed, easily enough got over. They represented, that the Earth might

really be in motion, though, to its inhabitants, it seemed to be at

rest; and that the Sun and Fixed Stars might really be at rest, though

from the Earth they seemed to be in motion; in the same manner as a

ship, which sails through a smooth sea, seems to those who are in it,

to be at rest, though really in motion; while the objects which she

passes along, seem to be in motion, though really at rest.

But there were some other objections, which, though grounded upon the

same natural prejudices, they found it more difficult to get over. The

earth had always presented itself to the senses, not only as at rest,

but as inert, ponderous, and even averse to motion. The imagination

had always been accustomed to conceive it as such, and suffered the

greatest violence, when obliged to pursue, and attend it, in that

rapid motion which the system of Copernicus bestowed upon it. To

enforce their objection, the adversaries of this hypothesis were at

pains to calculate the extreme rapidity of this motion. They

represented, that the circumference of the Earth had been computed to

be above twenty-thousand miles: if the Earth, therefore, was supposed

to revolve every day round its axis, every point of it near the

equator would pass over above twenty-three thousand miles in a day;

and consequently, near a thousand miles in an hour, and about sixteen

miles in a minute; a motion more rapid than that of a cannon ball, or

even than the swifter progress of sound. The rapidity of its

periodical revolution was yet more violent than that of its diurnal

rotation. How, therefore, could the imagination ever conceive so

ponderous a body to be naturally endowed with so dreadful a movement?

The Peripatetic Philosophy, the only philosophy then known in the

world, still further confirmed {362} this prejudice. That philosophy,

by a very natural, though, perhaps, groundless distinction, divided

all motion into Natural and Violent. Natural motion was that which

flowed from an innate tendency in the body, as when a stone fell

downwards: Violent motion, that which arose from external force, and

which was, in some measure, contrary to the natural tendency of the

body, as when a stone was thrown upwards, or horizontally. No violent

motion could be lasting; for, being constantly weakened by the natural

tendency of the body, it would soon be destroyed. The natural motion

of the Earth, as was evident in all its parts, was downwards, in a

straight line to the centre; as that of fire and air was upwards, in a

straight line from the centre. It was the heavens only that revolved

naturally in a circle. Neither, therefore, the supposed revolution of

the Earth round its own centre, nor that round the Sun, could be

natural motions; they must therefore be violent, and consequently

could be of no long continuance. It was in vain that Copernicus

replied, that gravity was, probably, nothing else besides a tendency

in the different parts of the same Planet, to unite themselves to one

another; that this tendency took place, probably, in the parts of the

other Planets, as well as in those of the Earth; that it could very

well be united with a circular motion; that it might be equally

natural to the whole body of the Planet, and to every part of it; that

his adversaries themselves allowed, that a circular motion was natural

to the heavens, whose diurnal revolution was infinitely more rapid

than even that motion which he had bestowed upon the Earth; that

though a like motion was natural to the Earth, it would still appear

to be at rest to its inhabitants, and all the parts of it to tend in a

straight line to the centre, in the same manner as at present. But

this answer, how satisfactory soever it may appear to be now, neither

did nor could appear to be satisfactory then. By admitting the

distinction betwixt natural and violent motions, it was founded upon

the same ignorance of mechanical principles with the objection. The

systems of Aristotle and Hipparchus supposed, indeed, the diurnal

motion of the heavenly bodies to be infinitely more rapid than even

that dreadful movement which Copernicus bestowed upon the Earth. But

they supposed, at the same time, that those bodies were objects of a

quite different species, from any we are acquainted with, near the

surface of the Earth, and to which, therefore, it was less difficult

to conceive that any sort of motion might be natural. Those objects,

besides, had never presented themselves to the senses, as moving

otherwise, or with less rapidity, than these systems represented them.

The imagination, therefore, could feel no difficulty in following a

representation which the senses had rendered quite familiar to it. But

when the Planets came to be regarded as so many Earths, the case was

quite altered. The imagination had been accustomed to conceive such

objects as tending rather to rest than motion; and this idea of their

natural {363} inertness, encumbered, if one may say so, and clogged

its flight whenever it endeavoured to pursue them in their periodical

courses, and to conceive them as continually rushing through the

celestial spaces, with such violent and unremitting rapidity.

Nor were the first followers of Copernicus more fortunate in their

answers to some other objections, which were founded indeed in the

same ignorance of the laws of motion, but which, at the same time,

were necessarily connected with that way of conceiving things, which

then prevailed universally in the learned world.

If the earth, it was said, revolved so rapidly from west to east, a

perpetual wind would set in from east to west, more violent than what

blows in the greatest hurricanes; a stone, thrown westwards would fly

to a much greater distance than one thrown with the same force

eastwards; as what moved in a direction, contrary to the motion of the

Earth, would necessarily pass over a greater portion of its surface,

than what, with the same velocity, moved along with it. A ball, it was

said, dropped from the mast of a ship under sail, does not fall

precisely at the foot of the mast, but behind it; and in the same

manner, a stone dropped from a high tower would not, upon the

supposition of the Earth's motion, fall precisely at the bottom of the

tower, but west of it, the Earth being, in the mean time, carried away

eastward from below it. It is amusing to observe, by what, subtile and

metaphysical evasions the followers of Copernicus endeavoured to elude

this objection, which before the doctrine of the Composition of Motion

had been explained by Galileo, was altogether unanswerable. They

allowed, that a ball dropped from the mast of a ship under sail would

not fall at the foot of the mast, but behind it; because the ball,

they said, was no part of the ship, and because the motion of the ship

was natural neither to itself nor to the ball. But the stone was a

part of the earth, and the diurnal and annual revolutions of the Earth

were natural to the whole, and to every part of it, and therefore to

the stone. The stone, therefore, having naturally the same motion with

the Earth, fell precisely at the bottom of the tower. But this answer

could not satisfy the imagination, which still found it difficult to

conceive how these motions could be natural to the earth; or how a

body, which had always presented itself to the senses as inert,

ponderous, and averse to motion, should naturally be continually

wheeling about both its own axis and the Sun, with such violent

rapidity. It was, besides, argued by Tycho Brahe, upon the principles

of the same philosophy which had afforded both the objection and the

answer, that even upon the supposition, that any such motion was

natural to the whole body of the Earth, yet the stone, which was

separated from it, could no longer be actuated by that motion. The

limb, which is cut off from an animal, loses those animal motions

which were natural to the whole. The branch, which is cut off from the

trunk, loses that vegetative {364} motion which is natural to the

whole tree. Even the metals, minerals, and stones, which were dug out

from the bosom of the Earth, lose those motions which occasioned their

production and increase, and which were natural to them in their

original state. Though the diurnal and annual motion of the Earth,

therefore, had been natural to them while they were contained in its

bosom, it could no longer be so when they were separated from it.

Tycho Brahe, the great restorer of the science of the heavens, who had

spent his life, and wasted his fortune upon the advancement of

Astronomy, whose observations were both more numerous and more

accurate than those of all the astronomers who had gone before him,

was himself so much affected by the force of this objection, that,

though he had never mentioned the system of Copernicus without some

note of high admiration he had conceived for its author, he could

never himself be induced to embrace it; yet all his astronomical

observations tended to confirm it. They demonstrated, that Venus and

Mercury were sometimes above, and sometimes below the Sun; and that,

consequently, the Sun, and not the Earth, was the centre of their

periodical revolutions. They showed, that Mars, when in his meridian

at midnight, was nearer to the Earth than the Earth is to the Sun;

though, when in conjunction with the Sun, he was much more remote from

the Earth than that luminary; a discovery which was absolutely

inconsistent with the system of Ptolemy, which proved, that the Sun,

and not the Earth, was the centre of the periodical revolutions of

Mars, as well as of Venus and Mercury; and which demonstrated that the

Earth was placed betwixt the orbits of Mars and Venus. They made the

same thing probable with regard to Jupiter and Saturn; that they, too,

revolved round the Sun; and that, therefore, the Sun, if not the

centre of the universe, was at least, that of the planetary system.

They proved that Comets were superior to the Moon, and moved through

the heavens in all possible directions; an observation incompatible

with the Solid Spheres of Aristotle and Purbach, and which, therefore,

overturned the physical part, at least, of the established systems of

Astronomy.

All these observations, joined to his aversion to the system, and

perhaps, notwithstanding the generosity of his character, some little

jealousy for the fame of Copernicus, suggested to Tycho the idea of a

new hypothesis, in which the Earth continued to be, as in the old

account, the immovable centre of the universe, round which the

firmament revolved every day from east to west, and, by some secret

virtue, carried the Sun, the Moon, and the Five Planets along with it,

notwithstanding their immense distance, and notwithstanding that there

was nothing betwixt it and them but the most fluid ether. But,

although all these seven bodies thus obeyed the diurnal revolution of

the Firmament, they had each of them, as in the old system, too, a

{365} contrary periodical eastward revolution of their own, which made

them appear to be every day, more or less, left behind by the

Firmament. The Sun was the centre of the periodical revolutions of the

Five Planets; the Earth, that of the Sun and Moon. The Five Planets

followed the Sun in his periodical revolution round the Earth, as they

did the Firmament in its diurnal rotation. The three superior Planets

comprehended the Earth within the orbit in which they revolved round

the Sun, and had each of them an Epicycle to connect together, in the

same manner as in the system of Ptolemy, their direct, retrograde, and

stationary appearances. As, notwithstanding their immense distance,

they followed the Sun in his periodical revolution round the Earth,

keeping always at an equal distance from him, they were necessarily

brought much nearer to the Earth when in opposition to the Sun, than

than when in conjunction with him. Mars, the nearest of them, when in

his meridian at midnight, came within the orbit which the Sun

described round the Earth, and consequently was then nearer to the

Earth than the Earth was to the Sun. The appearances of the two

inferior Planets were explained, in the same manner, as in the system

of Copernicus, and consequently required no Epicycle to connect them.

The circles in which the Five Planets performed their periodical

revolutions round the Sun, as well as those in which the Sun and Moon

performed theirs round the Earth, were, as both in the old and new

hypothesis, Eccentric Circles, to connect together their differently

accelerated and retarded motions.

Such was the system of Tycho Brahe, compounded, as is evident, out of

these of Ptolemy and Copernicus; happier than that of Ptolemy, in the

account which it gives of the motions of the two inferior Planets;

more complex, by supposing the different revolutions of all the Five

to be performed round two different centres; the diurnal round the

Earth, the periodical round the Sun, but, in every respect, more

complex and more incoherent than that of Copernicus. Such, however,

was the difficulty that mankind felt in conceiving the motion of the

Earth, that it long balanced the reputation of that otherwise more

beautiful system. It may be said, that those who considered the

heavens only, favoured the system of Copernicus, which connected so

happily all the appearances which presented themselves there; but that

those who looked upon the Earth, adopted the account of Tycho Brahe,

which, leaving it at rest in the centre of the universe, did less

violence to the usual habits of the imagination. The learned were,

indeed, sensible of the intricacy, and of the many incoherences of

that system; that it gave no account why the Sun, Moon, and Five

Planets, should follow the revolution of the Firmament; or why the

Five Planets, notwithstanding the immense distance of the three

superior ones, should obey the periodical motion of the Sun; or why

the Earth, though placed between the orbits of Mars and Venus, should

remain immovable in the centre {366} of the Firmament, and constantly

resist the influence of whatever it was, which carried bodies that

were so much larger than itself, and that were placed on all sides of

it, periodically round the Sun. Tycho Brahe died before he had fully

explained his system. His great and merited renown disposed many of

the learned to believe, that, had his life been longer, he would have

connected together many of these incoherences, and knew methods of

adapting his system to some other appearances, with which none of his

followers could connect it.

The objection to the system of Copernicus, which was drawn from the

nature of motion, and that was most insisted on by Tycho Brahe, was at

last fully answered by Galileo; not, however, till about thirty years

after the death of Tycho, and about a hundred after that of

Copernicus. It was then that Galileo, by explaining the nature of the

composition of motion, by showing, both from reason and experience,

that a ball dropped from the mast of a ship under sail would fall

precisely at the foot of the mast, and by rendering this doctrine,

from a great number of other instances, quite familiar to the

imagination, took off, perhaps, the principal objection which had been

made to this hypothesis of the astronomers.

Several other astronomical difficulties, which encumbered this account

of things, were removed by the same philosopher. Copernicus, after

altering the centre of the world, and making the Earth, and all the

Planets revolve round the Sun, was obliged to leave the Moon to

revolve round the Earth as before. But no example of any such

secondary Planet having then been discovered in the heavens, there

seemed still to be this irregularity remaining in the system. Galileo,

who first applied telescopes to Astronomy, discovered, by their

assistance, the Satellites of Jupiter, which, revolving round that

Planet, at the same time that they were carried along with it in its

revolution, round either the Earth, or the Sun, made it seem less

contrary to the analogy of nature, that the Moon should both revolve

round the Earth, and accompany her in her revolution round the Sun.

It had been objected to Copernicus, that, if Venus and Mercury

revolved round the Sun in an orbit comprehended within the orbit of

the Earth, they would show all the same phases with the Moon; present,

sometimes their darkened, and sometimes their enlightened sides to the

Earth, and sometimes part of the one, and part of the other. He

answered, that they undoubtedly did all this; but that their smallness

and distance hindered us from perceiving it. This very bold assertion

of Copernicus was confirmed by Galileo. His telescopes rendered the

phases of Venus quite sensible, and thus demonstrated, more evidently

than had been done, even by the observations of Tycho Brahe, the

revolutions of these two Planets round the Sun, as well as so far

destroyed the system of Ptolemy.

The mountains and seas, which, by the help of the same instrument,

{367} he discovered, or imagined he had discovered in the Moon,

rendering that Planet, in every respect, similar to the Earth, made it

seem less contrary to the analogy of nature, that, as the Moon

revolved round the Earth, the Earth should revolve round the Sun.

The spots which, in the same manner, he discovered in the Sun,

demonstrating, by their motion, the revolution of the Sun round his

axis, made it seem less improbable that the Earth, a body so much

smaller than the Sun, should likewise revolve round her axis in the

same manner.

Succeeding telescopical observations, discovered, in each of the Five

Planets, spots not unlike those which Galileo had observed in the

Moon, and thereby seemed to demonstrate what Copernicus had only

conjectured, that the Planets were naturally opaque, enlightened only

by the rays of the Sun, habitable, diversified by seas and mountains,

and, in every respect, bodies of the same kind with the earth; and

thus added one other probability to this system. By discovering, too,

that each of the Planets revolved round its own axis, at the same

time that it was carried round either the Earth or the Sun, they made

it seem quite agreeable to the analogy of nature, that the Earth,

which, in every other respect, resembled the Planets, should, like

them too, revolve round its own axis, and at the same time perform its

periodical motion round the Sun.

While, in Italy, the unfortunate Galileo was adding so many

probabilities to the system of Copernicus, there was another

philosopher employing himself in Germany, to ascertain, correct, and

improve it; Kepler, with great genius, but without the taste, or the

order and method of Galileo, possessed, like all his other countrymen,

the most laborious industry, joined to that passion for discovering

proportions and resemblances betwixt the different parts of nature,

which, though common to all philosophers, seems, in him, to have been

excessive. He had been instructed, by Mæstlinus, in the system of

Copernicus; and his first curiosity was, as he tells us, to find out,

why the Planets, the Earth being counted for one, were Six in number;

why they were placed at such irregular distances from the Sun; and

whether there was any uniform proportion betwixt their several

distances, and the times employed in their periodical revolutions.

Till some reason, or proportion of this kind, could be discovered, the

system did not appear to him to be completely coherent. He

endeavoured, first, to find it in the proportions of numbers, and

plain figures; afterwards, in those of the regular solids; and, last

of all, in those of the musical divisions of the Octave. Whatever was

the science which Kepler was studying, he seems constantly to have

pleased himself with finding some analogy betwixt it and the system of

the universe; and thus, arithmetic and music, plane and solid

geometry, came all of them by turns to illustrate the doctrine of the

Sphere, in the explaining of which he was, by his {368} profession,

principally employed. Tycho Brahe, to whom he had presented one of his

books, though he could not but disapprove of his system, was pleased,

however, with his genius, and with his indefatigable diligence in

making the most laborious calculations. That generous and magnificent

Dane invited the obscure and indigent Kepler to come and live with

him, and communicated to him, as soon as he arrived, his observations

upon Mars, in the arranging and methodizing of which his disciples

were at that time employed. Kepler, upon comparing them with one

another, found, that the orbit of Mars was not a perfect circle; that

one of its diameters was somewhat longer than the other; and that it

approached to an oval, or an ellipse, which had the Sun placed in one

of its foci. He found, too, that the motion of the Planet was not

equable; that it was swiftest when nearest the Sun, and slowest when

furthest from him; and that its velocity gradually increased, or

diminished, according as it approached or receded from him. The

observations of the same astronomer discovered to him, though not so

evidently, that the same things were true of all the other Planets;

that their orbits were elliptical, and that their motions were

swiftest when nearest the Sun, and slowest when furthest from him.

They showed the same things, too, of the Sun, if supposed to revolve

round the Earth; and consequently of the Earth, if it also was

supposed to revolve round the Sun.

That the motions of all the heavenly bodies were perfectly circular,

had been the fundamental idea upon which every astronomical

hypothesis, except the irregular one of the Stoics, had been built. A

circle, as the degree of its curvature is every where the same, is of

all curve lines the simplest and the most easily conceived. Since it

was evident, therefore, that the heavenly bodies did not move in

straight lines, the indolent imagination found, that it could most

easily attend to their motions if they were supposed to revolve in

perfect circles. It had, upon this account, determined that a circular

motion was the most perfect of all motions, and that none but the most

perfect motion could be worthy of such beautiful and divine objects;

and it had upon this account, so often, in vain, endeavoured to adjust

to the appearances, so many different systems, which all supposed them

to revolve in this perfect manner.

The equality of their motions was another fundamental idea, which, in

the same manner, and for the same reason, was supposed by all the

founders of astronomical systems. For an equal motion can be more

easily attended to, than one that is continually either accelerated or

retarded. All inconsistency, therefore, was declared to be unworthy

those bodies which revolved in the celestial regions, and to be fit

only for inferior and sublunary things. The calculations of Kepler

overturned, with regard to the Planets, both these natural prejudices

of the imagination; destroyed their circular orbits; and introduced

into their {369} real motions, such an equality as no equalizing

circle would remedy. It was, however, to render their motion perfectly

equable, without even the assistance of a equalizing circle, that

Copernicus, as he himself assures us, had originally invented his

system. Since the calculations of Kepler, therefore, overturned what

Copernicus had principally in view in establishing his system, we

cannot wonder that they should at first seem rather to embarrass than

improve it.

It is true, by these elliptical orbits and unequal motions, Kepler

disengaged the system from the embarrassment of those small Epicycles,

which Copernicus, in order to connect the seemingly accelerated and

retarded movements of the Planets, with their supposed real equality,

had been obliged to leave in it. For it is remarkable, that though

Copernicus had delivered the orbits of the Planets from the enormous

Epicycles of Hipparchus, that though in this consisted the great

superiority of his system above that of the ancient astronomers, he

was yet obliged, himself, to abandon, in some measure, this advantage,

and to make use of some small Epicycles, to join together those

seeming irregularities. His Epicycles indeed, like the irregularities

for whose sake they were introduced, were but small ones, and the

imaginations of his first followers seem, accordingly, either to have

slurred them over altogether, or scarcely to have observed them.

Neither Galileo, nor Gassendi, the two most eloquent of his defenders,

take any notice of them. Nor does it seem to have been generally

attended to, that there was any such thing as Epicycles in the system

of Copernicus, till Kepler, in order to vindicate his own elliptical

orbits, insisted, that even, according to Copernicus, the body of the

Planet was to be found but at two different places in the

circumference of that circle which the centre of its Epicycle

described.

It is true, too, that an ellipse is, of all curve lines after a

circle, the simplest and most easily conceived; and it is true,

besides all this, that, while Kepler took from the motion of the

Planets the easiest of all proportions, that of equality, he did not

leave them absolutely without one, but ascertained the rule by which

their velocities continually varied; for a genius so fond of

analogies, when he had taken away one, would be sure to substitute

another in its room. Notwithstanding all this, notwithstanding that

his system was better supported by observations than any system had

ever been before, yet, such was the attachment to the equal motions

and circular orbits of the Planets, that it seems, for some time, to

have been in general but little attended to by the learned, to have

been altogether neglected by philosophers, and not much regarded even

by astronomers.

Gassendi, who began to figure in the world about the latter days of

Kepler, and who was himself no mean astronomer, seems indeed to have

conceived a good deal of esteem for his diligence and accuracy in

accommodating the observations of Tycho Brahe to the system of {370}

Copernicus. But Gassendi appears to have had no comprehension of the

importance of those alterations which Kepler had made in that system,

as is evident from his scarcely ever mentioning them in the whole

course of his voluminous writings upon Astronomy. Des Cartes, the

contemporary and rival of Gassendi, seems to have paid no attention to

them at all, but to have built his Theory of the Heavens, without any

regard to them. Even those astronomers, whom a serious attention had

convinced of the justness of his corrections, were still so enamoured

with the circular orbits and equal motion, that they endeavoured to

compound his system with those ancient but natural prejudices. Thus,

Ward endeavoured to show that, though the Planets moved in elliptical

orbits, which had the Sun in one of their foci, and though their

velocities in the elliptical line were continually varying, yet, if a

ray was supposed to be extended from the centre of any one of them to

the other focus, and to be carried along by the periodical motion of

the Planet, it would make equal angles in equal times, and

consequently cut off equal portions of the circle of which that other

focus was the centre. To one, therefore, placed in that focus, the

motion of the Planet would appear to be perfectly circular and

perfectly equable, in the same manner as in the Equalizing Circles of

Ptolemy and Hipparchus. Thus Bouillaud, who censured this hypothesis

of Ward, invented another of the same kind, infinitely more whimsical

and capricious. The Planets, according to that astronomer, always

revolve in circles; for that being the most perfect figure, it is

impossible they should revolve in any other. No one of them, however,

continues to move in any one circle, but is perpetually passing from

one to another, through an infinite number of circles, in the course

of each revolution; for an ellipse, said he, is an oblique section of

a cone, and in a cone, betwixt the two vortices of the ellipse there

is an infinite number of circles, out of the infinitely small portions

of which the elliptical line is compounded. The Planet, therefore

which moves in this line, is, in every point of it, moving in an

infinitely small portion of a certain circle. The motion of each

Planet, too, according to him, was necessarily, for the same reason,

perfectly equable. An equable motion being the most perfect of all

motions. It was not, however, in the elliptical line, that it was

equable, but in any one of the circles that were parallel to the base

of that cone, by whose section this elliptical line had been formed:

for, if a ray was extended from the Planet to any one of those

circles, and carried along by its periodical motion, it would cut off

equal portions of that circle in equal times; another most fantastical

equalising circle, supported by no other foundation besides the

frivolous connection between a cone and an ellipse, and recommended by

nothing but the natural passion for circular orbits and equable

motions. It may be regarded as the last effort of this passion, and

may serve to show the force of that principle which could {371} thus

oblige this accurate observer, and great improver of the Theory of the

Heavens, to adopt so strange an hypothesis. Such was the difficulty

and hesitation with which the followers of Copernicus adopted the

corrections of Kepler.

The rule, indeed, which Kepler ascertained for determining the gradual

acceleration or retardation in the movement of the Planets, was

intricate, and difficult to be comprehended; it could therefore but

little facilitate the progress of the imagination in tracing those

revolutions which were supposed to be conducted by it. According to

that astronomer, if a straight line was drawn from the centre of each

Planet to the Sun, and carried along by the periodical motion of the

Planet, it would describe equal areas in equal times, though the

Planet did not pass over equal spaces; and the same rule he found,

took place nearly with regard to the Moon. The imagination, when

acquainted with the law by which any motion is accelerated or

retarded, can follow and attend to it more easily, than when at a

loss, and, as it were, wandering in uncertainty with regard to the

proportion which regulates its varieties; the discovery of this

analogy therefore, no doubt, rendered the system of Kepler more

agreeable to the natural taste of mankind: it, was, however, an

analogy too difficult to be followed, or comprehended, to render it

completely so.

Kepler, besides this, introduced another new analogy into the system,

and first discovered, that there was one uniform relation observed

betwixt the distances of the Planets from the Sun, and the times

employed in their periodical motions. He found, that their periodical

times were greater than in proportion to their distances, and less

than in proportion to the squares of those distances; but, that they

were nearly as the mean proportionals betwixt their distances and the

squares of their distances; or, in other words, that the squares of

their periodical times were nearly as the cubes of their distances; an

analogy, which, though, like all others, it no doubt rendered the

system somewhat more distinct and comprehensible, was, however, as

well as the former, of too intricate a nature to facilitate very much

the effort of the imagination in conceiving it.

The truth of both these analogies, intricate as they were, was at last

fully established by the observations of Cassini. That astronomer

first discovered, that the secondary Planets of Jupiter and Saturn

revolved round their primary ones, according to the same laws which

Kepler had observed in the revolutions of the primary ones round the

Sun, and that of the Moon round the earth; that each of them described

equal areas in equal times, and that the squares of their periodic

times were as the cubes of their distances. When these two last

abstruse analogies, which, when Kepler at first observed them, were

but little regarded, had been thus found to take place in the

revolutions of the Four Satellites of Jupiter, and in those of the

Five of Saturn, they were {372} now thought not only to confirm the

doctrine of Kepler, but to add a new probability to the Copernican

hypothesis. The observations of Cassini seem to establish it as a law

of the system, that, when one body revolved round another, it

described equal areas in equal times; and that, when several revolved

round the same body, the squares of their periodic times were as the

cubes of their distances. If the Earth and the Five Planets were

supposed to revolve round the Sun, these laws, it was said, would take

place universally. But if, according to the system of Ptolemy, the

Sun, Moon, and Five Planets were supposed to revolve round the Earth,

the periodical motions of the Sun and Moon, would, indeed, observe the

first of these laws, would each of them describe equal areas in equal

times; but they would not observe the second, the squares of their

periodic times would not be as the cubes of their distances: and the

revolutions of the Five Planets would observe neither the one law nor

the other. Or if, according to the system of Tycho Brahe, the Five

Planets were supposed to revolve round the Sun, while the Sun and Moon

revolved round the Earth, the revolutions of the Five Planets round

the Sun, would, indeed, observe both these laws; but those of the

Sun and Moon round the Earth would observe only the first of them. The

analogy of nature, therefore, could be preserved completely, according

to no other system but that of Copernicus, which, upon that account,

must be the true one. This argument is regarded by Voltaire, and the

Cardinal of Polignac, as an irrefragable demonstration; even M'Laurin,

who was more capable of judging, nay, Newton himself, seems to mention

it as one of the principal evidences for the truth of that hypothesis.

Yet, an analogy of this kind, it would seem, far from a demonstration,

could afford, at most, but the shadow of a probability.

It is true, that though Cassini supposed the Planets to revolve in an

oblong curve, it was in a curve somewhat different from that of

Kepler. In the ellipse, the sum of the two lines which are drawn from

any one point in the circumference to the two foci, is always equal to

that of those which are drawn from any other point in the

circumference to the same foci. In the curve of Cassini, it is not the

sum of the lines, but the rectangles which are contained under the

lines, that are always equal. As this, however, was a proportion more

difficult to be comprehended by astronomers than the other, the curve

of Cassini has never had the vogue.

Nothing now embarrassed the system of Copernicus, but the difficulty

which the imagination felt in conceiving bodies so immensely ponderous

as the Earth and the other Planets revolving round the Sun with such

incredible rapidity. It was in vain that Copernicus pretended, that,

notwithstanding the prejudices of sense, this circular motion might be

as natural to the Planets, as it is to a stone to fall to the ground.

The imagination had been accustomed to conceive such {373} objects as

tending rather to rest than motion. This habitual idea of their

natural inertness was incompatible with that of their natural motion.

It was in vain that Kepler, in order to assist the fancy in connecting

together this natural inertness with their astonishing velocities,

talked of some vital and immaterial virtue, which was shed by the Sun

into the surrounding spaces, which was whirled about with his

revolution round his own axis, and which, taking hold of the Planets,

forced them, in spite of their ponderousness and strong propensity to

rest, thus to whirl about the centre of the system. The imagination

had no hold of this immaterial virtue, and could form no determinate

idea of what it consisted in. The imagination, indeed, felt a gap, or

interval, betwixt the constant motion and the supposed inertness of

the Planets, and had in this, as in all other cases, some general idea

or apprehension that there must be a connecting chain of intermediate

objects to link together these discordant qualities. Wherein this

connecting chain consisted, it was, indeed, at a loss to conceive; nor

did the doctrine of Kepler lend it any assistance in this respect.

That doctrine, like almost all those of the philosophy in fashion

during his time, bestowed a name upon this invisible chain, called it

an immaterial virtue, but afforded no determinate idea of what was its

nature.

Des Cartes was the first who attempted to ascertain, precisely,

wherein this invisible chain consisted, and to afford the imagination

a train of intermediate events, which, succeeding each other in an

order that was of all others the most familiar to it, should unite

those incoherent qualities, the rapid motion, and the natural

inertness of the Planets. Des Cartes was the first who explained

wherein consisted the real inertness of matter; that it was not in an

aversion to motion, or in a propensity to rest, but in a power of

continuing indifferently either at rest of in motion, and of

resisting, with a certain force, whatever endeavoured to change its

state from the one to the other. According to that ingenious and

fanciful philosopher, the whole of infinite space was full of matter,

for with him matter and extension were the same, and consequently

there could be no void. This immensity of matter, he supposed to be

divided into an infinite number of very small cubes; all of which,

being whirled about upon their own centres, necessarily gave occasion

to the production of two different elements. The first consisted of

those angular parts, which, having been necessarily rubbed off, and

grinded yet smaller by their mutual friction, constituted the most

subtle and movable part of matter. The second consisted of those

little globules that were formed by the rubbing off of the first. The

interstices betwixt these globules of the second element was filled up

by the particles of the first. But in the infinite collisions, which

must occur in an infinite space filled with matter, and all in motion,

it must necessarily happen that many of the globules of the second

element should be broken and grinded down into the first. The quantity

{374} of the first element having been thus increased beyond what was

sufficient to fill up the interstices of the second, it must, in many

places, have been heaped up together, without any mixture of the

second along with it. Such, according to Des Cartes, was the original

division of matter. Upon this infinitude of matter thus divided, a

certain quantity of motion was originally impressed by the Creator of

all things, and the laws of motion were so adjusted as always to

preserve the same quantity in it, without increase, and without

diminution. Whatever motion was lost by one part of matter, was

communicated to some other; and whatever was acquired by one part of

matter, was derived from some other: and thus, through an eternal

revolution, from rest to motion, and from motion to rest, in every

part of the universe, the quantity of motion in the whole was always

the same.

But, as there was no void, no one part of matter could be moved

without thrusting some other out of its place, nor that without

thrusting some other, and so on. To avoid, therefore, an infinite

progress, he supposed that the matter which any body pushed before it,

rolled immediately backwards, to supply the place of that matter which

flowed in behind it; and as we may observe in the swimming of a fish,

that the water which it pushes before it, immediately rolls backward,

to supply the place of what flows in behind it, and thus forms a small

circle or vortex round the body of the fish. It was, in the same

manner, that the motion originally impressed by the Creator upon the

infinitude of matter, necessarily produced in it an infinity of

greater and smaller vortices, or circular streams: and the law of

motion being so adjusted as always to preserve the same quantity of

motion in the universe, those vortices either continued for ever, or

by their dissolution gave birth to others of the same kind. There was,

thus, at all times, an infinite number of greater and smaller

vortices, or circular streams, revolving in the universe.

But, whatever moves in a circle, is constantly endeavouring to fly off

from the centre of its revolution. For the natural motion of all

bodies is in a straight line. All the particles of matter, therefore,

in each of those greater vortices, were continually pressing from the

centre to the circumference, with more or less force, according to the

different degrees of their bulk and solidity. The larger and more

solid globules of the second element forced themselves upwards to the

circumference, while the smaller, more yielding, and more active

particles of the first, which could flow, even through the interstices

of the second, were forced downwards to the centre. They were forced

downwards to the centre, notwithstanding their natural tendency was

upwards to the circumference; for the same reason that a piece of

wood, when plunged in water, is forced upwards to the surface,

notwithstanding its natural tendency is downwards to the bottom;

because its tendency downwards is less strong than that of the

particles of water, which, therefore, {375} if one may say so, press

in before it, and thus force it upwards. But there being a greater

quantity of the first element than what was necessary to fill up the

interstices of the second, it was necessarily accumulated in the

centre of each of these great circular streams, and formed there the

fiery and active substance of the Sun. For, according to that

philosopher, the Solar Systems were infinite in number, each Fixed

Star being the centre of one: and he is among the first of the

moderns, who thus took away the boundaries of the Universe; even

Copernicus and Kepler, themselves, having confined it within, what

they supposed, to be the vault of the Firmament.

The centre of each vortex being thus occupied by the most active and

movable parts of matter, there was necessarily among them, a more

violent agitation than in any other part of the vortex, and this

violent agitation of the centre cherished and supported the movement

of the whole. But, among the particles of the first element, which

fill up the interstices of the second, there are many, which, from the

pressure of the globules on all sides of them, necessarily receive an

angular form, and thus constitute a third element of particles less

fit for motion than those of the other two. As the particles, however,

of this third element were formed in the interstices of the second,

they are necessarily smaller than those of the second, and are,

therefore, along with those of the first, urged down towards the

centre, where, when a number of them happen to take hold of one

another, they form such spots upon the surface of the accumulated

particles of the first element, as are often discovered by telescopes

upon the face of that Sun which enlightens and animates our particular

system. Those spots are often broken and dispelled, by the violent

agitation of the particles of the first element, as has hitherto

happily been the case with those which have successively been formed

upon the face of our Sun. Sometimes, however, they encrust the whole

surface of that fire which is accumulated in the centre; and the

communication betwixt the most active and the most inert parts of the

vortex being thus interrupted, the rapidity of its motion immediately

begins to languish, and can no longer defend it from being swallowed

up and carried away by the superior violence of some other like

circular stream; and in this manner, what was once a Sun, becomes a

Planet. Thus, the time was, according to this system, when the Moon

was a body of the same kind with the Sun, the fiery centre of a

circular stream of ether, which flowed continually round her; but her

face having been crusted over by a congeries of angular particles, the

motion of this circular stream began to languish, and could no longer

defend itself from being absorbed by the more violent vortex of the

Earth, which was then, too, a Sun, and which chanced to be placed in

its neighbourhood. The Moon, therefore, became a Planet, and revolved

round the Earth. In process of time, the same fortune, which had thus

befallen the Moon, befell also {376} the Earth; its face was encrusted

by a gross and inactive substance; the motion of its vortex began to

languish, and it was absorbed by the greater vortex of the Sun: but

though the vortex of the Earth had thus become languid, it still had

force enough to occasion both the diurnal revolution of the Earth, and

the monthly motion of the Moon. For a small circular stream may easily

be conceived as flowing round the body of the Earth, at the same time

that it is carried along by that great ocean of ether which is

continually revolving round the Sun; in the same manner, as in a great

whirlpool of water, one may often see several small whirlpools, which

revolve round centres of their own, and at the same time are carried

round the centre of the great one. Such was the cause of the original

formation and consequent motions of the Planetary System. When a

solid body is turned round its centre, those parts of it, which are

nearest, and those which are remotest from the centre, complete their

revolutions in one and the same time. But it is otherwise with the

revolutions of a fluid; the parts of it which are nearest the centre

complete their revolutions in a shorter time, than those which are

remoter. The Planets, therefore, all floating, in that immense tide of

ether which is continually setting in from west to east round the body

of the Sun, complete their revolutions in a longer or a shorter time,

according to their nearness or distance from him. There was, however,

according to Des Cartes, no very exact proportion observed betwixt the

times of their revolutions and their distances from the centre. For

that nice analogy, which Kepler had discovered betwixt them, having

not yet been confirmed by the observations of Cassini, was, as I

before took notice, entirely disregarded by Des Cartes. According to

him, too, their orbits might not be perfectly circular, but be longer

the one way than the other, and thus approach to an Ellipse. Nor yet

was it necessary to suppose, that they described this figure with

geometrical accuracy, or even that they described always precisely the

same figure. It rarely happens, that nature can be mathematically

exact with regard to the figure of the objects she produces, upon

account of the infinite combinations of impulses, which must conspire

to the production of each of her effects. No two Planets, no two

animals of the same kind, have exactly the same figure, nor is that of

any one of them perfectly regular. It was in vain, therefore, that

astronomers laboured to find that perfect constancy and regularity in

the motions of the heavenly bodies, which is to be found in no other

parts of nature. These motions, like all others, must either languish

or be accelerated, according as the cause which produces them, the

revolution of the vortex of the Sun, either languishes, or is

accelerated; and there are innumerable events which may occasion

either the one or the other of those changes.

It was thus, that Des Cartes endeavoured to render familiar to the

imagination, the greatest difficulty in the Copernican system, the

rapid {377} motion of the enormous bodies of the Planets. When the

fancy had thus been taught to conceive them as floating in an immense

ocean of ether, it was quite agreeable to its usual habits to

conceive, that they should follow the stream of this ocean, how rapid

soever. This was an order of succession to which it had been long

accustomed, and with which it was, therefore, quite familiar. This

account, too, of the motions of the Heavens, was connected with a

vast, an immense system, which joined together a greater number of the

most discordant phenomena of nature, than had been united by any other

hypothesis; a system in which the principles of connection, though

perhaps equally imaginary, were, however, more distinct and

determinate, than any that had been known before; and which attempted

to trace to the imagination, not only the order of succession by which

the heavenly bodies were moved, but that by which they, and almost all

other natural objects, had originally been produced.--The Cartesian

philosophy begins now to be almost universally rejected, whilst the

Copernican system continues to be universally received. Yet it is not

easy to imagine, how much probability and coherence this admired

system was long supposed to derive from that exploded hypothesis. Till

Des Cartes had published his principles, the disjointed and incoherent

system of Tycho Brahe, though it was embraced heartily and completely

by scarce any body, was yet constantly talked of by all the learned,

as, in point of probability, upon a level with Copernicus. They took

notice, indeed, of its inferiority with regard to coherence and

connection, expressing hopes, however, that these defects might be

remedied by some future improvements. But when the world beheld that

complete, and almost perfect coherence, which the philosophy of Des

Cartes bestowed upon the system of Copernicus, the imaginations of

mankind could no longer refuse themselves the pleasure of going along

with so harmonious an account of things. The system of Tycho Brahe was

every day less and less talked of, till at last it was forgotten

altogether.

The system of Des Cartes, however, though it connected together the

real motions of the heavenly bodies according to the system of

Copernicus, more happily than had been done before, did so only when

they were considered in the gross; but did not apply to them, when

they were regarded in the detail. Des Cartes, as was said before, had

never himself observed the Heavens with any particular application.

Though he was not ignorant, therefore, of any of the observations

which had been made before his time, he seems to have paid them no

great degree of attention; which, probably, proceeded from his own

inexperience in the study of Astronomy. So far, therefore, from

accommodating his system to all the minute irregularities, which

Kepler had ascertained in the movements of the Planets; or from

showing, particularly, how these irregularities, and no other, should

arise from it, he contented himself with observing, that perfect

uniformity could not {378} be expected in their motions, from the

nature of the causes which produced them; that certain irregularities

might take place in them, for a great number of successive

revolutions, and afterwards gave way to others of a different kind: a

remark which, happily, relieved him from the necessity of applying his

system to the observations of Kepler, and the other Astronomers.

But when the observations of Cassini had established the authority of

those laws, which Kepler had first discovered in the system, the

philosophy of Des Cartes, which could afford no reason why such

particular laws should be observed, might continue to amuse the

learned in other sciences, but could no longer satisfy those that were

skilled in Astronomy. Sir Isaac Newton first attempted to give a

physical account of the motions of the Planets, which should

accommodate itself to all the constant irregularities which

astronomers had ever observed in their motions. The physical

connection, by which Des Cartes had endeavoured to bind together the

movements of the Planets, was the laws of impulse; of all the orders

of succession, those which are most familiar to the imagination; as

they all flow from the inertness of matter. After this quality, there

is no other with which we are so well acquainted as that of gravity.

We never act upon matter, but we have occasion to observe it. The

superior genius and sagacity of Sir Isaac Newton, therefore, made the

most happy, and, we may now say, the greatest and most admirable

improvement that was ever made in philosophy, when he discovered, that

he could join together the movements of the Planets by so familiar a

principle of connection, which completely removed all the difficulties

the imagination had hitherto felt in attending to them. He

demonstrated, that, if the Planets were supposed to gravitate towards

the Sun, and to one another, and at the same time to have had a

projecting force originally impressed upon them, the primary ones

might all describe ellipses in one of the foci of which that great

luminary was placed; and the secondary ones might describe figures of

the same kind round their respective primaries, without being

disturbed by the continual motion of the centres of their revolutions.

That if the force, which retained each of them in their orbits, was

like that of gravity, and directed towards the Sun, they would, each

of them, describe equal areas in equal times. That if this attractive

power of the Sun, like all other qualities which are diffused in rays

from a centre, diminished in the same proportion as the squares of the

distances increased, their motions would be swiftest when nearest the

Sun, and slowest when farthest off from him, in the same proportion in

which, by observation, they are discovered to be; and that upon the

same supposition, of this gradual diminution of their respective

gravities, their periodic times would bear the same proportion to

their distances, which Kepler and Cassini had established betwixt

them. Having thus shown, that gravity might be the {379} connecting

principle which joined together the movements of the Planets, he

endeavoured next to prove that it really was so. Experience shows us,

what is the power of gravity near the surface of the Earth. That it is

such as to make a body fall, in the first second of its descent,

through about fifteen Parisian feet. The Moon is about sixty

semidiameters of the Earth distant from its surface. If gravity,

therefore, was supposed to diminish, as the squares of the distance

increase, a body, at the Moon, would fall towards the Earth in a

minute; that is, in sixty seconds, through the same space, which it

falls near its surface in one second. But the arch which the Moon

describes in a minute, falls, by observation, about fifteen Parisian

feet below the tangent drawn at the beginning of it. So far,

therefore, the Moon may be conceived as constantly falling towards the

Earth.

The system of Sir Isaac Newton corresponded to many other

irregularities which Astronomers had observed in the Heavens. It

assigned a reason, why the centres of the revolutions of the Planets

were not precisely in the centre of the Sun, but in the common centre

of gravity of the Sun and the Planets. From the mutual attraction of

the Planets, it gave a reason for some other irregularities in their

motions; irregularities, which are quite sensible in those of Jupiter

and Saturn, when those Planets are nearly in conjunction with one

another. But of all the irregularities in the Heavens, those of the

Moon had hitherto given the greatest perplexity to Astronomers; and

the system of Sir Isaac Newton corresponded, if possible, yet more

accurately with them than with any of the other Planets. The Moon,

when either in conjunction, or in opposition to the Sun, appears

furthest from the Earth, and nearest to it when in her quarters.

According to the system of that philosopher, when she is in

conjunction with the Sun, she is nearer the Sun than the Earth is;

consequently, more attracted to him, and, therefore, more separated

from the Earth. On the contrary, when in opposition to the Sun, she is

further from the Sun than the Earth. The Earth, therefore, is more

attracted to the Sun: and consequently, in this case, too, further

separated from the Moon. But, on the other hand, when the Moon is in

her quarters, the Earth and the Moon, being both at equal distance

from the Sun, are equally attracted to him. They would not, upon this

account alone, therefore, be brought nearer to one another. As it is

not in parallel lines however that they are attracted towards the Sun,

but in lines which meet in his centre, they are, thereby, still

further approached to one another. Sir Isaac Newton computed the

difference of the forces with which the Moon and the Earth ought, in

all those different situations, according to his theory, to be

impelled towards one another; and found, that the different degrees of

their approaches, as they had been observed by Astronomers,

corresponded exactly to his computations. As the attraction of the

Sun, in the conjunctions and oppositions, diminishes the gravity of

{380} the Moon towards the Earth, and, consequently, makes her

necessarily extend her orbit, and, therefore, require a longer

periodical time to finish it. But, when the Moon and the Earth are in

that part of the orbit which is nearest the Sun, this attraction of

the Sun will be the greatest; consequently, the gravity of the Moon

towards the Earth will there be most diminished; her orbit be most

extended; and her periodic time be, therefore, the longest. This is,

also, agreeable to experience, and in the very same proportion, in

which, by computation, from these principles, it might be expected.

The orbit of the Moon is not precisely in the same Plane with that of

the Earth; but makes a very small angle with it. The points of

intersection with those two Planes, are called, the Nodes of the Moon.

These Nodes of the Moon are in continual motion, and in eighteen or

nineteen years, revolve backwards, from east to west, through all the

different points of the Ecliptic. For the Moon, after having finished

her periodical revolution, generally intersects the orbit of the Earth

somewhat behind the point where she had intersected it before. But,

though the motion of the Nodes is thus generally retrograde, it is not

always so, but is sometimes direct, and sometimes they appear even

stationary; the Moon generally intersects the Plane of the Earth's

orbit behind the point where she had intersected it in her former

revolution; but she sometimes intersects it before that point, and

sometimes in the very same point. It is the situation of those Nodes

which determines the times of Eclipses, and their motions had, upon

this account, at all times, been particularly attended to by

Astronomers. Nothing, however, had perplexed them more, than to

account for these so inconsistent motions, and, at the same time,

preserve their so much sought-for regularity in the revolutions of the

Moon. For they had no other means of connecting the appearances

together than by supposing the motions which produced them, to be, in

reality, perfectly regular and equable. The history of Astronomy,

therefore, gives an account of a greater number of theories invented

for connecting together the motions of the Moon, than for connecting

together those of all the other heavenly bodies taken together. The

theory of gravity, connected together, in the most accurate manner, by

the different actions of the Sun and the Earth, all those irregular

motions; and it appears, by calculation, that the time, the quantity,

and the duration of those direct and retrograde motions of the Nodes,

as well as of their stationary appearances, might be expected to be

exactly such, as the observations of Astronomers have determined them.

The same principle, the attraction of the Sun, which thus accounts for

the motions of the Nodes, connects, too, another very perplexing

irregularity in the appearances of the Moon; the perpetual variation

in the inclination of her orbit to that of the Earth.

As the Moon revolves in an ellipse, which has the centre of the {381}

Earth in one of its foci, the longer axis of its orbit is called the

Line of its Apsides. This line is found, by observation, not to be

always directed towards the same points of the Firmament, but to

revolve forwards from west to east, so as to pass through all the

points of the Ecliptic, and to complete its period in about nine

years; another irregularity, which had very much perplexed

Astronomers, but which the theory of gravity sufficiently accounted

for.

The Earth had hitherto been regarded as perfectly globular, probably

for the same reason which had made men imagine, that the orbits of the

Planets must necessarily be perfectly circular. But Sir Isaac Newton,

from mechanical principles, concluded, that, as the parts of the Earth

must be more agitated by her diurnal revolution at the Equator, than

at the Poles, they must necessarily be somewhat elevated at the first,

and flattened at the second. The observation, that the oscillations of

pendulums were slower at the Equator than at the Poles, seeming to

demonstrate, that gravity was stronger at the Poles, and weaker at the

Equator, proved, he thought, that the Equator was further from the

centre than the Poles. All the measures, however, which had hitherto

been made of the Earth, seemed to show the contrary, that it was drawn

out towards the Poles, and flattened towards the Equator. Newton,

however, preferred his mechanical computations to the former measures

of Geographers and Astronomers; and in this he was confirmed by the

observations of Astronomers on the figure of Jupiter, whose diameter

at the Pole seems to be to his diameter at the Equator, as twelve to

thirteen; a much greater inequality than could be supposed to take

place betwixt the correspondent diameters of the Earth, but which was

exactly proportioned to the superior bulk of Jupiter, and the superior

rapidity with which he performs his diurnal revolutions. The

observations of Astronomers at Lapland and Peru have fully confirmed

Sir Isaac's system, and have not only demonstrated, that the figure of

the Earth is, in general, such as he supposed it; but that the

proportion of its axis to the diameter of its Equator is almost

precisely such as he had computed it. And of all the proofs that have

ever been adduced of the diurnal revolution of the Earth, this perhaps

is the most solid and most satisfactory.

Hipparchus, by comparing his own observations with those of some

former Astronomers, had found that the equinoctial points were not

always opposite to the same part of the Heavens, but that they

advanced gradually eastward by so slow a motion, as to be scarce

sensible in one hundred years, and which would require thirty-six

thousand to make a complete revolution of the Equinoxes, and to carry

them successively through all the different points of the Ecliptic.

More accurate observations discovered that this procession of the

Equinoxes was not so slow as Hipparchus had imagined it, and that it

required somewhat less than twenty-six thousand years to give them a

complete {382} revolution. While the ancient system of Astronomy,

which represented the Earth as the immovable centre of the universe,

took place, this appearance was necessarily accounted for, by

supposing that the Firmament, besides its rapid diurnal revolution

round the poles of the Equator, had likewise a slow periodical one

round those of the Ecliptic. And when the system of Hipparchus was by

the schoolmen united with the solid Spheres of Aristotle, they placed

a new crystalline Sphere above the Firmament, in order to join this

motion to the rest. In the Copernican system, this appearance had

hitherto been connected with the other parts of that hypothesis, by

supposing a small revolution in the Earth's axis from east to west.

Sir Isaac Newton connected this motion by the same principle of

gravity, by which he had united all the others, and showed, how the

elevation of the parts of the Earth at the Equator must, by the

attraction of the Sun, produce the same retrograde motion of the Nodes

of the Ecliptic, which it produced of the Nodes of the Moon. He

computed the quantity of motion which could arise from this action of

the Sun, and his calculations here too corresponded with the

observations of Astronomers.

Comets have hitherto, of all the appearances in the Heavens, been the

least attended to by Astronomers. The rarity and inconstancy of their

appearance, seemed to separate them entirely from the constant,

regular, and uniform objects in the Heavens, and to make them resemble

more the inconstant, transitory, and accidental phenomena of those

regions that are in the neighbourhood of the Earth. Aristotle,

Eudoxus, Hipparchus, Ptolemy, and Purbach, therefore, had all degraded

them below the Moon, and ranked them among the meteors of the upper

regions of the air. The observations of Tycho Brahe demonstrated, that

they ascended into the celestial regions, and were often higher than

Venus or the Sun. Des Cartes, at random, supposed them to be always

higher than even the orbit of Saturn; and seems, by the superior

elevation he thus bestowed upon them, to have been willing to

compensate that unjust degradation which they had suffered for so many

ages before. The observations of some later Astronomers demonstrated,

that they too revolved about the Sun, and might therefore be parts of

the Solar System. Newton accordingly applied his mechanical principle

of gravity to explain the motions of these bodies. That they described

equal areas in equal times, had been discovered by the observations of

some later Astronomers; and Newton endeavoured to show how from this

principle, and those observations, the nature and position of their

several orbits might be ascertained, and their periodic times

determined. His followers have, from his principles, ventured even to

predict the returns of several of them, particularly of one which is

to make its appearance in 1758.[1] We must wait for that time {383}

before we can determine, whether his philosophy corresponds as happily

to this part of the system as to all the others. In the meantime,

however, the ductility of this principle, which applied itself so

happily to these, the most irregular of all the celestial appearances,

and which has introduced such complete coherence into the motions of

all the Heavenly Bodies, has served not a little to recommend it to

the imaginations of mankind.

[Footnote 1: It must be observed, that the whole of this Essay was

written previous to the date here mentioned; and that the return of

the comet happened agreeably to the prediction.]

But of all the attempts of the Newtonian philosophy, that which would

appear to be the most above the reach of human reason and experience,

is the attempt to compute the weights and densities of the Sun, and of

the several Planets. An attempt, however, which was indispensably

necessary to complete the coherence of the Newtonian system. The power

of attraction which, according to the theory of gravity, each body

possesses, is in proportion to the quantity of matter contained in

that body. But the periodic time in which one body, at a given

distance, revolves round another that attracts it, is shorter in

proportion as this power is greater, and consequently as the quantity

of matter in the attracting body. If the densities of Jupiter and

Saturn were the same with that of the Earth, the periodic times of

their several Satellites would be shorter than by observation they are

found to be. Because the quantity of matter, and consequently the

attracting power of each of them, would be as the cubes of their

diameters. By comparing the bulks of those Planets, and the periodic

times of their Satellites, it is found that, upon the hypothesis of

gravity, the density of Jupiter must be greater than that of Saturn,

and the density of the Earth greater than that of Jupiter. This seems

to establish it as a law in the system, that the nearer the several

Planets approach to the Sun, the density of their matter is the

greater: a constitution of things which seems to be the most

advantageous of any that could have been established; as water of the

same density with that of our Earth, would freeze under the Equator of

Saturn, and boil under that of Mercury.

Such is the system of Sir Isaac Newton, a system whose parts are all

more strictly connected together, than those of any other

philosophical hypothesis. Allow his principle, the universality of

gravity, and that it decreases as the squares of the distance

increase, and all the appearances, which he joins together by it,

necessarily follow. Neither is their connection merely a general and

loose connection, as that of most other systems, in which either these

appearances, or some such like appearances, might indifferently have

been expected. It is everywhere the most precise and particular that

can be imagined, and ascertains the time, the place, the quantity, the

duration of each individual phenomenon, to be exactly such as, by

observation, they have been determined to be. Neither are the

principles of union, which it employs, such as the imagination can

find any difficulty in going along with. The gravity of matter is, of

all its qualities, after its inertness, {384} that which is most

familiar to us. We never act upon it without having occasion to

observe this property. The law too, by which it is supposed to

diminish as it recedes from its centre, is the same which takes place

in all other qualities which are propagated in rays from a centre, in

light, and in every thing else of the same kind. It is such, that we

not only find that it does take place in all such qualities, but we

arc necessarily determined to conceive that, from the nature of the

thing, it must take place. The opposition which was made in France,

and in some other foreign nations, to the prevalence of this system,

did not arise from any difficulty which mankind naturally felt in

conceiving gravity as an original and primary mover in the

constitution of the universe. The Cartesian system, which had

prevailed so generally before it, had accustomed mankind to conceive

motion as never beginning, but in consequence of impulse, and had

connected the descent of heavy bodies, near the surface of the Earth,

and the other Planets, by this more general bond of union; and it was

the attachment the world had conceived for this account of things,

which indisposed them to that of Sir Isaac Newton. His system,

however, now prevails over all opposition, and has advanced to the

acquisition of the most universal empire that was ever established in

philosophy. His principles, it must be acknowledged, have a degree of

firmness and solidity that we should in vain look for in any other

system. The most sceptical cannot avoid feeling this. They not only

connect together most perfectly all the phenomena of the Heavens,

which had been observed before his time; but those also which the

persevering industry and more perfect instruments of later Astronomers

have made known to us have been either easily and immediately

explained by the application of his principles, or have been explained

in consequence of more laborious and accurate calculations from these

principles, than had been instituted before. And even we, while we

have been endeavouring to represent all philosophical systems as mere

inventions of the imagination, to connect together the otherwise

disjointed and discordant phenomena of Nature, have insensibly been

drawn in, to make use of language expressing the connecting principles

of this one, as if they were the real chains which Nature makes use of

to bind together her several operations. Can we wonder then, that it

should have gained the general and complete approbation of mankind,

and that it should now be considered, not as an attempt to connect in

the imagination the phenomena of the Heavens, but as the greatest

discovery that ever was made by man, the discovery of an immense chain

of the most important and sublime truths, all closely connected

together, by one capital fact, of the reality of which we have daily

experience.

* * * * * * * *

* * * * * * * *

{385}

_Note by the Editors._

The Author, at the end of this Essay, left some Notes and Memorandums,

from which it appears, that he considered this last part of his

History of Astronomy as imperfect, and needing several additions. The

Editors, however, chose rather to publish than suppress it. It must

be viewed, not as a History or Account of Sir Isaac Newton's

Astronomy, but chiefly as an additional illustration of those

Principles in the Human Mind which Mr. Smith has pointed out to be the

universal motives of Philosophical Researches.

----------

THE PRINCIPLES

WHICH LEAD AND DIRECT

PHILOSOPHICAL ENQUIRIES;

ILLUSTRATED BY THE

HISTORY OF THE ANCIENT PHYSICS.

-----

FROM arranging and methodizing the System of the Heavens, Philosophy

descended to the consideration of the inferior parts of Nature, of the

Earth, and of the bodies which immediately surround it. If the

objects, which were here presented to its view, were inferior in

greatness or beauty, and therefore less apt to attract the attention

of the mind, they were more apt, when they came to be attended to, to

embarrass and perplex it, by the variety of their species, and by the

intricacy and seeming irregularity of the laws or orders of their

succession. The species of objects in the Heavens are few in number;

the Sun, the Moon, the Planets, and the Fixed Stars, are all which

those philosophers could distinguish. All the changes too, which are

ever observed in these bodies, evidently arise from some difference in

the velocity and direction of their several motions; but the variety

of meteors in the air, of clouds, rainbows, thunder, lightning, winds,

rain, hail, snow, is vastly greater; and the order of their succession

seems to be still more irregular and inconstant. The species of

fossils, minerals, plants, animals, which are found in the Waters, and

near the surface of the Earth, are still more intricately diversified;

and if we regard the {386} different manners of their production,

their mutual influence in altering, destroying, supporting one

another, the orders of their succession seem to admit of an almost

infinite variety. If the imagination, therefore, when it considered

the appearances in the Heavens, was often perplexed, and driven out of

its natural career, it would be much more exposed to the same

embarrassment, when it directed its attention to the objects which the

Earth presented to it, and when it endeavoured to trace their progress

and successive revolutions.

To introduce order and coherence into the mind's conception of this

seeming chaos of dissimilar and disjointed appearances, it was

necessary to deduce all their qualities, operations, and laws of

succession, from those of some particular things, with which it was

perfectly acquainted and familiar, and along which its imagination

could glide smoothly and easily, and without interruption. But as we

would in vain attempt to deduce the heat of a stove from that of an

open chimney, unless we could show that the same fire which was

exposed in the one, lay concealed in the other; so it was impossible

to deduce the qualities and laws of succession, observed in the more

uncommon appearances of Nature, from those of such as were more

familiar, if those customary objects were not supposed, however

disguised in their appearance, to enter into the composition of those

rarer and more singular phenomena. To render, therefore, this lower

part of the great theatre of nature a coherent spectacle to the

imagination, it became necessary to suppose, first, That all the

strange objects of which it consisted were made up out of a few, with

which the mind was extremely familiar: and secondly, That all their

qualities, operations and rules of succession, were no more than

different diversifications of those to which it had long been

accustomed, in these primary and elementary objects.

Of all the bodies of which these inferior parts of the universe seem

to be composed, those with which we are most familiar, are the Earth,

which we tread upon; the Water, which we every day use; the Air, which

we constantly breathe; and the Fire, whose benign influence is not

only required for preparing the common necessaries of life, but for

the continual support of that vital principle which actuates both

plants and animals. These therefore, were by Empedocles, and the other

philosophers of the Italian school, supposed to be the elements, out

of which, at least, all the inferior parts of nature were composed.

The familiarity of those bodies to the mind, naturally disposed it to

look for some resemblance to them in whatever else was presented to

its consideration. The discovery of some such resemblance united the

new object to an assortment of things, with which the imagination was

perfectly acquainted. And if any analogy could be observed betwixt the

operations and laws of succession of the compound, and those of the

simple objects, the movement of the fancy, in tracing their progress,

{387} became quite smooth, and natural, and easy. This natural

anticipation, too, was still more confirmed by such a slight and

inaccurate analysis of things, as could be expected in the infancy of

science, when the curiosity of mankind, grasping at an account of all

things before it had got full satisfaction with regard to any one,

hurried on to build, in imagination, the immense fabric of the

universe. The heat, observed in both plants and animals, seemed to

demonstrate, that Fire made a part of their composition. Air was not

less necessary for the subsistence of both, and seemed, too, to enter

into the fabric of animals by respiration, and into that of plants by

some other means. The juices which circulated through them showed how

much of their texture was owing to Water. And their resolution into

Earth by putrefaction discovered that this element had not been left

out in their original formation. A similar analysis seemed to show the

same principles in most of the other compound bodies.

The vast extent of those bodies seemed to render them, upon another

account, proper to be the great stores out of which nature compounded

all the other species of things. Earth and Water divide almost the

whole of the terrestrial globe between them. The thin transparent

covering of the Air surrounds it to an immense height upon all sides.

Fire, with its attendant, light, seems to descend from the celestial

regions, and might, therefore, either be supposed to be diffused

through the whole of those etherial spaces, as well as to be condensed

and conglobated in those luminous bodies, which sparkle across them,

as by the Stoics; or, to be placed immediately under the sphere of the

Moon, in the region next below them, as by the Peripatetics, who could

not reconcile the devouring nature of Fire with the supposed

unchangeable essence of their solid and crystalline spheres.

The qualities, too, by which we are chiefly accustomed to characterize

and distinguish natural bodies, are all of them found, in the highest

degree in those Four Elements. The great divisions of the objects,

near the surface of the Earth, are those into hot and cold, moist and

dry, light and heavy. These are the most remarkable properties of

bodies; and it is upon them that many of their other most sensible

qualities and powers seem to depend. Of these, heat and cold were

naturally enough regarded by those first enquirers into nature, as the

active, moisture and dryness, as the passive qualities of matter. It

was the temperature of heat and cold which seemed to occasion the

growth and dissolution of plants and animals; as appeared evident from

the effects of the change of the seasons upon both. A proper degree of

moisture and dryness was not less necessary for these purposes; as was

evident from the different effects and productions of wet and dry

seasons and soils. It was the heat and cold, however, which actuated

and determined those two otherwise inert qualities of things, to a

state either of rest or motion. Gravity and levity were regarded {388}

as the two principles of motion, which directed all sublunary things

to their proper place: and all those six qualities, taken together,

were, upon such an inattentive view of nature, as must be expected in

the beginnings of philosophy, readily enough apprehended to be capable

of connecting together the most remarkable revolutions, which occur in

these inferior parts of the universe. Heat and dryness were the

qualities which characterized the element of Fire; heat and moisture

that of Air; moisture and cold that of Water; cold and dryness that of

Earth. The natural motion of two of these elements, Earth and Water,

was downwards, upon account of their gravity. This tendency, however,

was stronger in the one than in the other, upon account of the

superior gravity of Earth. The natural motion of the two other

elements, Fire and Air, was upwards, upon account of their levity; and

this tendency, too, was stronger in the one than in the other, upon

account of the superior levity of Fire. Let us not despise those

ancient philosophers, for thus supposing, that these two elements had

a positive levity, or a real tendency upwards. Let us remember, that

this notion has an appearance of being confirmed by the most obvious

observations; that those facts and experiments, which demonstrate the

weight of the Air, and which no superior sagacity, but chance alone,

presented to the moderns, were altogether unknown to them; and that,

what might, in some measure, have supplied the place of those

experiments, the reasonings concerning the causes of the ascent of

bodies, in fluids specifically heavier than themselves, seem to have

been unknown in the ancient world, till Archimedes discovered them,

long after their system of physics was completed, and had acquired an

established reputation: that those reasonings are far from being

obvious, and that by their inventor, they seem to have been thought

applicable only to the ascent of Solids in Water, and not even to that

of Solids in Air, much less to that of one fluid in another. But it is

this last only which could explain the ascent of flame, vapours, and

fiery exhalations, without the supposition of a specific levity.

Thus, each of those Four Elements had, in the system of the Universe,

a place which was peculiarly allotted to it, and to which it naturally

tended. Earth and Water rolled down to the centre; the Air spread

itself above them; while the Fire soared aloft, either to the

celestial region, or to that which was immediately below it. When each

of those simple bodies had thus obtained its proper sphere, there was

nothing in the nature of any one of them to make it pass into the

place of the other, to make the Fire descend into the Air, the Air

into the Water, or the Water into the Earth; or, on the contrary, to

bring up the Earth into the place of the Water, the Water into that of

the Air, or the Air into that of the Fire. All sublunary things,

therefore, if left to themselves, would have remained in an eternal

repose. The revolution of the heavens, those of the Sun, Moon, and

Five Planets, {389} by producing the vicissitudes of Day and Night,

and of the Seasons, prevented this torpor and inactivity from reigning

through the inferior parts of nature; inflamed by the rapidity of

their circumvolutions, the element of Fire, and forced it violently

downwards into the Air, into the Water, and into the Earth, and

thereby produced those mixtures of the different elements which kept

up the motion and circulation of the lower parts of Nature;

occasioned, sometimes, the entire transmutation of one element into

another, and sometimes the production of forms and species different

from them all, and in which, though the qualities of them all might be

found, they were so altered and attempered by the mixture, as scarce

to be distinguishable.

Thus, if a small quantity of Fire was mixed with a great quantity of

Air, the moisture and moderate warmth of the one entirely surmounted

and changed into their own essence the intense heat and dryness of the

other; and the whole aggregate became Air. The contrary of which

happened, if a small quantity of Air was mixed with a great quantity

of Fire: the whole, in this case, became Fire. In the same manner, if

a small quantity of Fire was mixed with a great quantity of Water,

then, either the moisture and cold of the Water might surmount the

heat and dryness of the Fire, so that the whole should become Water;

or, the moisture of the Water might surmount the dryness of the Fire,

while, in its turn, the heat of the Fire surmounted the coldness of

the Water, so as that the whole aggregate, its qualities being heat

and moisture, should become Air, which was regarded as the more

natural and easy metamorphosis of the two. In the same manner they

explained how like changes were produced by the different mixtures of

Fire and Earth, Earth and Water, Water and Air, Air and Earth; and

thus they connected together the successive transmutations of the

elements into one another.

Every mixture of the Elements, however, did not produce an entire

transmutation. They were sometimes so blended together, that the

qualities of the one, not being able to destroy, served only to

attemper those of the other. Thus Fire, when mixed with Water,

produced sometimes a watery vapour, whose qualities were heat and

moisture; which partook at once of the levity of the Fire, and of the

gravity of the Water, and which was elevated by the first into the

Air, but retained by the last from ascending into the region of Fire.

The relative cold, which they supposed prevailed in the middle region

of the Air, upon account of its equal distance, both from the region

of Fire, and from the rays that are reflected by the surface of the

Earth, condensed this vapour into Water; the Fire escaped it, and flew

upwards, and the Water fell down in rain, or, according to the

different degrees of cold that prevailed in the different seasons, was

sometimes congealed into snow, and sometimes into hail. In the same

manner, Fire, when mixed with Earth, produced sometimes a fiery

exhalation, whose qualities {390} were heat and dryness, which being

elevated by the levity of the first into the Air condensed by the

cold, so as to take fire, and being at the same time surrounded by

watery vapours, burst forth into thunder and lightning, and other

fiery meteors. Thus they connected together the different appearances

in the Air, by the qualities of their Four Elements; and from them,

too, in the same manner, they endeavoured to deduce all the other

qualities in the other homogeneous bodies, that are near the surface

of the Earth. Thus, to give an example, with regard to the hardness

and softness of bodies; heat and moisture, they observed, were the

great softeners of matter. Whatever was hard, therefore, owed that

quality either to the absence of heat, or to the absence of moisture.

Ice, crystal, lead, gold, and almost all metals, owed their hardness

to the absence of heat, and were, therefore, dissolvable by Fire.

Rock-salt, nitre, alum, and hard clay, owed that quality to the

absence of moisture, and were therefore, dissolvable in water. And, in

the same manner, they endeavoured to connect together most of the

other tangible qualities of matter. Their principles of union, indeed,

were often such as had no real existence, and were always vague and

undetermined in the highest degree; they were such, however, as might

be expected in the beginnings of science, and such as, with all their

imperfections, could enable mankind both to think and to talk, with

more coherence, concerning those general subjects, than without them

they would have been capable of doing. Neither was their system

entirely devoid either of beauty or magnificence. Each of the Four

Elements having a particular region allotted to it, had a place of

rest, to which it naturally tended, by its motion, either up or down,

in a straight line, and where, when it had arrived, it naturally

ceased to move. Earth descended, till it arrived at the place of

Earth; Water, till it arrived at that of Water; and Air, till it

arrived at that of Air; and there each of them tended to a state of

eternal repose and inaction. The Spheres consisted of a Fifth Element,

which was neither light nor heavy, and whose natural motion made it

tend, neither to the centre, nor from the centre, but revolve round it

in a circle. As, by this motion, they could never change their

situation with regard to the centre, they had no place of repose, no

place to which they naturally tended more than to any other, but

revolved round and round for ever. This Fifth Element was subject

neither to generation nor corruption, nor alteration of any kind; for

whatever changes may happen in the Heavens, the senses can scarce

perceive them, and their appearance is the same in one age as in

another. The beauty, too, of their supposed crystalline spheres seemed

still more to entitle them to this distinction of unchangeable

immortality. It was the motion of those Spheres, which occasioned the

mixtures of the Elements, and from hence, the production of all the

forms and species, that diversify the world. It was the approach of

the Sun and of the {391} other Planets, to the different parts of the

Earth, which, by forcing down the element of Fire, occasioned the

generation of those forms. It was the recess of those bodies, which,

by allowing each Element to escape to its proper sphere, brought

about, in an equal time, their corruption. It was the periods of those

great lights of Heaven, which measured out to all sublunary things,

the term of their duration, of their growth, and of their decay,

either in one, or in a number of seasons, according as the Elements of

which they were composed, were either imperfectly or accurately

blended and mixed with one another. Immortality, they could bestow

upon no individual form, because the principles out of which it was

formed, all tending to disengage themselves, and to return to their

proper spheres, necessarily, at last, brought about its dissolution.

But, though all individuals were thus perishable, and constantly

decaying, every species was immortal, because the subject-matter out

of which they were made, and the revolution of the Heavens, the cause

of their successive generations, continued to be always the same.

In the first ages of the world, the seeming incoherence of the

appearances of nature, so confounded mankind, that they despaired of

discovering in her operations any regular system. Their ignorance, and

confusion of thought, necessarily gave birth to that pusillanimous

superstition, which ascribes almost every unexpected event, to the

arbitrary will of some designing, though invisible beings, who

produced it for some private and particular purpose. The idea of an

universal mind, of a God of all, who originally formed the whole, and

who governs the whole by general laws, directed to the conservation

and prosperity of the whole, without regard to that of any private

individual, was a notion to which they were utterly strangers. Their

gods, though they were apprehended to interpose, upon some particular

occasions, were so far from being regarded as the creators of the

world, that their origin was apprehended to be posterior to that of

the world. The Earth, according to Hesiod, was the first production of

the chaos. The Heavens arose out of the Earth, and from both together,

all the gods, who afterwards inhabited them. Nor was this notion

confined to the vulgar, and to those poets who seem to have recorded

the vulgar theology. Of all the philosophers of the Ionian school,

Anaxagoras, it is well known, was the first who supposed that mind and

understanding were requisite to account for the first origin of the

world, and who, therefore, compared with the other philosophers of his

time, talked, as Aristotle observes, like a sober man among drunkards;

but whose opinion was, at the time, so remarkable, that he seems to

have got a sirname from it. The same notion, of the spontaneous origin

of the world, was embraced, too, as the same author tells, by the

early Pythagoreans, a sect, which, in the ancient world, was never

regarded as irreligious. Mind, and understanding, and consequently

Deity, being {392} the most perfect, were necessarily, according to

them, the last productions of Nature. For in all other things, what

was most perfect, they observed, always came last. As in plants and

animals, it is not the seed that is most perfect, but the complete

animal, with all its members, in the one; and the complete plant, with

all its branches, leaves, flowers, and fruits, in the other. This

notion, which could take place only while Nature was still considered

as, in some measure, disorderly and inconsistent in her operations,

was necessarily renounced by those philosophers, when, upon a more

attentive survey, they discovered, or imagined they had discovered,

more distinctly, the chain which bound all her different parts to one

another. As soon as the Universe was regarded as a complete machine,

as a coherent system, governed by general laws, and directed to

general ends, viz. its own preservation and prosperity, and that of

all the species that are in it; the resemblance which it evidently

bore to those machines which are produced by human art, necessarily

impressed those sages with a belief, that in the original formation of

the world there must have been employed an art resembling the human

art, but as much superior to it, as the world is superior to the

machines which that art produces. The unity of the system, which,

according to this ancient philosophy, is most perfect, suggested the

idea of the unity of that principle, by whose art it was formed; and

thus, as ignorance begot superstition, science gave birth to the first

theism that arose among those nations, who were not enlightened by

divine Revelation. According to Timæus, who was followed by Plato,

that intelligent Being who formed the world endowed it with a

principle of life and understanding, which extends from its centre to

its remotest circumference, which is conscious of all its changes, and

which governs and directs all its motions to the great end of its

formation. This soul of the world was itself a God, the greatest of

all the inferior, and created deities; of an essence that was

indissoluble, by any power but by that of him who made it, and which

was united to the body of the world, so as to be inseparable by every

force, but his who joined them, from the exertion of which his

goodness secured them. The beauty of the celestial spheres attracting

the admiration of mankind, the constancy and regularity of their

motions seeming to manifest peculiar wisdom and understanding, they

were each of them supposed to be animated by an Intelligence of a

nature that was, in the same manner, indissoluble and immortal, and

inseparably united to that sphere which it inhabited. All the mortal

and changeable beings which people the surface of the earth were

formed by those inferior deities; for the revolutions of the heavenly

bodies seemed plainly to influence the generation and growth of both

plants and animals, whose frail and fading forms bore the too evident

marks of the weakness of those inferior causes, which joined their

different parts to one another. According to Plato and Timæus, neither

the {393} Universe, nor even those inferior deities who govern the

Universe, were eternal, but were formed in time, by the great Author

of all things, out of that matter which had existed from all eternity.

This at least their words seemed to import, and thus they are

understood by Cicero, and by all the other writers of earlier

antiquity, though some of the later Platonists have interpreted them

differently.

According to Aristotle, who seems to have followed the doctrine of

Ocellus, the world was eternal; the eternal effect of an eternal

cause. He found it difficult, it would seem, to conceive what could

hinder the First Cause from exerting his divine energy from all

eternity. At whatever time he began to exert it, he must have been at

rest during all the infinite ages of that eternity which had passed

before it. To what obstruction, from within or from without, could

this be owing? or how could this obstruction, if it ever had

subsisted, have ever been removed? His idea of the nature and manner

of existence of this First Cause, as it is expressed in the last book

of his Physics, and the five last chapters of his Metaphysics, is

indeed obscure and unintelligible in the highest degree, and has

perplexed his commentators more than any other parts of his writings.

Thus far, however, he seems to express himself plainly enough: that

the First Heavens, that of the Fixed Stars, from which are derived the

motions of all the rest, is revolved by an eternal, immovable,

unchangeable, unextended being, whose essence consists in

intelligence, as that of a body consists in solidity and extension;

and which is therefore necessarily and always intelligent, as a body

is necessarily and always extended: that this Being was the first and

supreme mover of the Universe: that the inferior Planetary Spheres

derived each of them its peculiar revolution from an inferior being of

the same kind; eternal, immovable, unextended, and necessarily

intelligent: that the sole object of the intelligences of those beings

was their own essence, and the revolution of their own spheres; all

other inferior things being unworthy of their consideration; and that

therefore whatever was below the Moon was abandoned by the gods to the

direction of Nature, and Chance, and Necessity. For though those

celestial beings were, by the revolutions of their several Spheres,

the original causes of the generation and corruption of all sublunary

forms, they were causes who neither knew nor intended the effects

which they produced. This renowned philosopher seems, in his

theological notions, to have been directed by prejudices which, though

extremely natural, are not very philosophical. The revolutions of the

Heavens, by their grandeur and constancy, excited his admiration, and

seemed, upon that account, to be effects not unworthy a Divine

Intelligence. Whereas the meanness of many things, the disorder and

confusion of all things below, exciting no such agreeable emotion,

seemed to have no marks of being directed by that Supreme

Understanding. Yet, though this opinion saps the foundations of human

worship, and must have the {394} same effects upon society as Atheism

itself, one may easily trace, in the Metaphysics upon which it is

grounded, the origin of many of the notions, or rather of many of the

expressions, in the scholastic theology, to which no notions can be

annexed.

The Stoics, the most religious of all the ancient sects of

philosophers, seem in this, as in most other things, to have altered

and refined upon the doctrine of Plato. The order, harmony, and

coherence which this philosophy bestowed upon the Universal System,

struck them with awe and veneration. As, in the rude ages of the

world, whatever particular part of Nature excited the admiration of

mankind, was apprehended to be animated by some particular divinity;

so the whole of Nature having, by their reasonings, become equally the

object of admiration, was equally apprehended to be animated by a

Universal Deity, to be itself a Divinity, an Animal; a term which to

our ears seems by no means synonymous with the foregoing; whose body

was the solid and sensible parts of Nature, and whose soul was that

etherial Fire, which penetrated and actuated the whole. For of all the

four elements, out of which all things were composed, Fire or Ether

seemed to be that which bore the greatest resemblance to the Vital

Principle which informs both plants and animals, and therefore most

likely to be the Vital Principle which animated the Universe. This

infinite and unbounded Ether, which extended itself from the centre

beyond the remotest circumference of Nature, and was endowed with the

most consummate reason and intelligence, or rather was itself the very

essence of reason and intelligence, had originally formed the world,

and had communicated a portion, or ray, of its own essence to whatever

was endowed with life and sensation, which, upon the dissolution of

those forms, either immediately or some time after, was again absorbed

into that ocean of Deity from whence it had originally been detached.

In this system the Sun, the Moon, the Planets, and the Fixed Stars,

were each of them also inferior divinities, animated by a detached

portion of that etherial essence which was the soul of the world. In

the system of Plato, the Intelligence which animated the world was

different from that which originally formed it. Neither were these

which animated the celestial spheres, nor those which informed

inferior terrestrial animals, regarded as portions of this plastic

soul of the world. Upon the dissolution of animals, therefore, their

souls were not absorbed in the soul of the world, but had a separate

and eternal existence, which gave birth to the notion of the

transmigration of souls. Neither did it seem unnatural, that, as the

same matter which had composed one animal body might be employed to

compose another, that the same intelligence which had animated one

such being should again animate another. But in the system of the

Stoics, the intelligence which originally formed, and that which

animated the world, were one and the same, all inferior intelligences

were detached portions {395} of the great one; and therefore, in a

longer, or in a shorter time, were all of them, even the gods

themselves, who animated the celestial bodies, to be at last resolved

into the infinite essence of this almighty Jupiter, who, at a distant

period, should, by an universal conflagration, wrap up all things, in

that etherial and fiery nature, out of which they had originally been

deduced, again to bring forth a new Heaven and a new Earth, new

animals, new men, new deities; all of which would again, at a fated

time, be swallowed up in a like conflagration, again to be

re-produced, and again to be re-destroyed, and so on without end.

----------

THE PRINCIPLES

WHICH LEAD AND DIRECT

PHILOSOPHICAL ENQUIRIES;

ILLUSTRATED BY THE HISTORY OF THE

ANCIENT LOGICS AND METAPHYSICS.

-----

IN every transmutation, either of one element into another, or of one

compound body either into the elements out of which it was composed,

or into another compound body, it seemed evident, that both in the old

and in the new species, there was something that was the same, and

something that was different. When Fire was changed into Air, or Water

into Earth, the Stuff, or Subject-matter of this Air and this Earth,

was evidently the same with that of the former Fire or Water; but the

Nature or Species of those new bodies was entirely different. When, in

the same manner, a number of fresh, green, and odoriferous flowers

were thrown together in a heap, they, in a short time, entirely

changed their nature, became putrid and loathsome, and dissolved into

a confused mass of ordure, which bore no resemblance, either in

sensible qualities or in its effects, to their former beautiful

appearance. But how different soever the species, the subject-matter

of the flowers, and of the ordure, was, in this case too, evidently

the same. In every body therefore, whether simple or mixed, there were

evidently two principles, whose combination constituted the whole

nature of that particular body. The first was the Stuff, or

Subject-matter, out of which it was made; the second was the Species,

the Specific Essence, the Essential, or, as the schoolmen have called

it, the Substantial Form of the Body. {396} The first seemed to be the

same in all bodies, and to have neither qualities nor powers of any

kind, but to be altogether inert and imperceptible by any of the

senses, till it was qualified and rendered sensible by its union with

some species or essential form. All the qualities and powers of bodies

seemed to depend upon their species or essential forms. It was not the

stuff or matter of Fire, or Air, or Earth, or Water, which enabled

those elements to produce their several effects, but that essential

form which was peculiar to each of them. For it seemed evident that

Fire must produce the effects of Fire, by that which rendered it Fire;

Air, by that which rendered it Air; and that in the same manner all

other simple and mixed bodies must produce their several effects, by

that which constituted them such or such bodies; that is, by their

Specific Essence or essential forms. But it is from the effects of

bodies upon one another, that all the changes and revolutions in the

material world arise. Since these, therefore, depend upon the specific

essences of those bodies, it must be the business of philosophy, that

science which endeavours to connect together all the different changes

that occur in the world, to determine wherein the Specific Essence of

each object consists, in order to foresee what changes or revolutions

may be expected from it. But the Specific Essence of each individual

object is not that which is peculiar to it as an individual, but that

which is common to it, with all other objects of the same kind. Thus

the Specific Essence of the Water, which now stands before me, does

not consist in its being heated by the Fire, or cooled by the Air, in

such a particular degree; in its being contained in a vessel of such a

form, or of such dimensions. These are all accidental circumstances,

which are altogether extraneous to its general nature, and upon which

none of its effects as Water depend. Philosophy, therefore, in

considering the general nature of Water, takes no notice of those

particularities which are peculiar to this water, but confines itself

to those things which are common to all Water. If, in the progress of

its inquiries, it should descend to consider the nature of Water that

is modified by such particular accidents, it still would not confine

its consideration to this water contained in this vessel, and thus

heated at this fire, but would extend its views to Water in general

contained in such kind of vessels, and heated to such a degree at such

a fire. In every case, therefore, Species, or Universals, and not

Individuals, are the objects of Philosophy. Because whatever effects

are produced by individuals, whatever changes can flow from them, must

all proceed from some universal nature that is contained in them. As

it was the business of Physics, or Natural Philosophy, to determine

wherein consisted the Nature and Essence of every particular Species

of things, in order to connect together all the different events that

occur in the material world; so there were two other sciences, which,

though they had originally arisen out of that system of Natural

Philosophy I have just {397} been describing, were, however,

apprehended to go before it, in the order in which the knowledge of

Nature ought to be communicated. The first of these, Metaphysics,

considered the general nature of Universals, and the different sorts

or species into which they might be divided. The second of these,

Logics, was built upon this doctrine of Metaphysics; and from the

general nature of Universals, and of the sorts into which they were

divided, endeavoured to ascertain the general rules by which we might

distribute all particular objects into general classes, and determine

to what class each individual object belonged; for in this, they

justly enough apprehended, consisted the whole art of philosophical

reasoning. As the first of these two sciences, Metaphysics, is

altogether subordinate to the second, Logic, they seem, before the

time of Aristotle, to have been regarded as one, and to have made up

between them that ancient Dialectic of which we hear so much, and of

which we understand so little: neither does this separation seem to

have been much attended to, either by his own followers, the ancient

Peripatetics, or by any other of the old sects of philosophers. The

later schoolmen, indeed, have distinguished between Ontology and

Logic; but their Ontology contains but a small part of what is the

subject of the metaphysical books of Aristotle, the greater part of

which, the doctrines of Universals, and everything that is preparatory

to the arts of defining and dividing, has, since the days of Porphery,

been inserted into their Logic.