A S T R O N O M Y. 
had oblerved' in the revolutions of the primary ones round 
the Sun, and that of the Moon round the Earth ; that each 
of them defcribed equal areas in equal times, and that the 
fquares of their periodic times were as the cubes of their 
diftances. When thefe two lad abftrufe analogies, which, 
when Kepler at fird obferved them, were but little re¬ 
garded, had been thus found to take place in the revolu¬ 
tions of the Four Satellites of Jupiter, and in thofe of the 
Five of Saturn, they were now thought not only to con¬ 
firm the doctrine of Kepler, but to add a new probability 
to the Copernican hypothefis. The obfervations of Caf- 
fini feem to eftabliflt it as a law of the fydem, that, when 
one body revolved round another, it defcribed equal areas 
in equal times; and that, when feveral revolved round 
the fame body, the fquares of their periodic times were as 
the cubes of their diftances. If the Eartli and the five 
planets were fuppofed to revolve round the Sun, thefe 
laws, it was faid, would take place univerfally. But if, 
according to the fydem of Ptolemy, the Sun, Moon, and 
five planets, were fuppofed to revolve round the Earth, the 
periodical motions of the Sun and Moon would, indeed, 
obferve the fird of thefe laws, would each of them defenbe 
equal areas in equal times; but they would not obferve 
the fecond, the fquares of their periodic times would not 
be as the cubes of their didances : and the revolutions of 
the five planets would obferve neither-the one law nor. the 
other. Or if, according to the fydem of Tycho Brahe, 
the five planets Were fuppofed to,revolve round the Sun, 
while the Sun and Moon revolved round the Earth,, the 
revolutions of the five planets round the Sun would, in- 
deed, obferve both thefe laws ; but thofe of the Sun and 
Moon round the Earth would obferve only the fird of 
them. The analogy of nature, therefore, could be pre- 
ferved completely, according to no other fydem but that 
of Copernicus, which,, upon that account, mud be. the 
true one. This argument is regarded by Voltaire, and the 
cardinal of Polignac, as an irrefragable demondration ; 
even M‘Laurin, who was more capable of judging, nay, 
Sir Ifaac Newton himfelf, feems to mention it as one of 
the principal evidences for the truth of that hypothefis. 
Yet, an analogy of this kind, it would feem, far from a 
demondration, could afford, at mod, but the diadow of a 
probability. It. is true, that, though Cadini fuppofed the 
planets to revolve in an oblong curve, it was in a curve 
fomewhat different from that of Kepler. In the ellipfe the 
fum of the two lines, which are drawn from any one point 
in the circumference to the two foci, is always equal to 
that of thofe which are drawn from any other point in the 
circumference to the fame foci. In the Curve of Cadini, 
it is not the fum 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 com¬ 
prehended than the other, the curve of Cadini has never 
had the vogue. 
Nothing now embarrafled the fydem of Corpernicus, 
but the difficulty which the imagination felt in conceiving 
bodies fo immenfely ponderous as the Earth, and the 
other planets, revolving round the Sun with fuch incre¬ 
dible rapidity. It was in vain that Copernicus pretended, 
that, notwithdanding the prejudices ot fenfe, this circular 
motion might be as natural to the planets as it is to a 
done to fall to the ground. The imagination had been 
accudomed to conceive fuch objects as tending rather to 
red than motion. This habitual idea of their natural inert- 
nefs was incompatible with that of their natural motion. It 
was in vain that Kepler, in order to affid the fancy in con¬ 
necting together this natural inertnefs with their adonifh- 
ing velocities, talked of fome vital and immaterial virtue, 
which was died by the Sun into the furrounding fpaces., which 
was whirled about with his revolution round his own axis, 
and which, taking hold of the planets, forced them, in fpite 
of their ponderoufnefs and drong propendty to red, thus, to 
whirl about the centre of the fydem. The imagination had 
no hold of this immaterial virtue, and could form no deter¬ 
minate idea of what it confided in. The imagination, felt a 
gap, or interval, betwixt the condant motion and the iup- 
pofed inertnefs of the planets, and had in this, as in all other 
cafes, fome general idea or apprehendon that there mud 
be a conneCling, chain of intermediate objeCts to link to¬ 
gether thefe difeordant qualities. Wherein this connect¬ 
ing chain confided, it was, indeed, at a lofs to conceive ; 
nor did the doCtrine of Kepler lend it any abidance in this 
refpeCt. That doCtrine, like alinod all thofe of the phi - 
lofophy in fadiion during his time, bedowed a name upon 
this invilible chain, called it an immaterial virtue, but af¬ 
forded no determinate idea of what was its nature. 
Des Cartes was the fird who attempted to afeertain, 
precifely, wherein this invifible chain confided, and to 
afford the imagination a train of intermediate events, which, 
fucceeding each other in an order that was of all others 
the mod familiar to it, fliould unite thofe incoherent qua¬ 
lities, the rapid motion, and the natural inertnefs, of the 
planets. Des Cartes was the fird who explained wherein 
confided the real inertnefs of matter ; that it was not in 
an averfion to motion, or in a propendty to red, but in a 
power of continuing indifferently either at reft or in mo¬ 
tion, and of redding with a certain force whatever endea¬ 
voured to change its date from the one to the other. Ac¬ 
cording to that ingenious and fanciful philofopher, the whole 
of infinite fpace was full of matter, for with him matter 
and extenfion were the fame, and confequently there could 
be. no void. This immenfity of matter, he fuppofed to 
be divided into an infinite number of very fmall cubes ; 
all of which, being whirled about upon their own centres, 
neceflarily gave occafion to the production of two diffe¬ 
rent elements. The fird confided of thofe angular parts, 
which, having been neceflarily .nibbed ob', and grinded 
yet fmaller by their mutual friCtion, condituted the molt 
fubtile and moveable part of matter. The fecond con¬ 
fided of thofe little globules that were formed by the rub¬ 
bing off of the fird. The interdices betwixt thefe globules 
of the fecond element was filled up by the particles of the 
fird. But in the infinite collifions, which mud,occur in 
an infinite fpace filled with matter, and all in motion, it 
mud neceflarily happen, that many of the globules of the 
fecond element fhould be broken and grinded down into 
the fird. The quantity of the fird element having thus 
been increafed beyond what was fufficient to fill up the 
interdices of the fecond, it mud, in many places, have 
been heaped up together without any mixture of the fe-. 
cond along with it. Such, according to Des Cartes, was 
the original divifion of. matter. Upon this infinitude of 
matter thus divided, a certain quantity of motion was ori¬ 
ginally impreffed by the Creator of all things, and the 
laws of motion were fo adjuded as always to preferve the 
fame quantity in it, without increafe, and without dimi¬ 
nution. Whatever motion was lod by one part of matter, 
was communicated to fome other ; and whatever was ac¬ 
quired by one part of matter, was derived from fome other: 
and tints, through an eternal revolution, from red to mo¬ 
tion, and from motion to red, in every part of the univerfe, 
the quantity of motion in the whole was always the fame. 
But, as there was no void, no one part of matter could 
be moved without thruding fome other out of its place, 
nor that without thruding fome other, and fo on. To 
avoid, therefore, an infinite progrefs, lie fuppofed, that 
the matter which any body pufhed before it rolled imme¬ 
diately backwards, to fupply the place of that matter which 
flowed in behind it; as we may obferve in the fwimming 
of a fifh, that the water, which it pudies before it, imme¬ 
diately rolls backwards, to fupply the place of what flows 
in behind it, and thus forms a fmall circle or vortex round 
the body of the fifh. It was, in the fame manner, that 
the motion originally imprefled by the Creator upon the 
infinitude of matter, neceflarily produced in it an infinity 
of greater and fmaller vortices, or circular dreams : anil 
the law of motion being fo adjuded as always to preferve 
the fame quantity of motion in the univerfe, thofe vortices 
either continued forever, or by their diflblution gave birth, 
to others of the fame kind. There was thus at all times 
an infinite number of greater and fmaller vortices, or cir¬ 
cular dreams, revolving in the univerfe. 
3 
