ASTRONOMY. 
principle?, the disjointed and incoherent fyftem of Tycho 
Brahe, though it was embraced heartily and completely 
by icarcely any body, was yet conftantly talked of by all 
the learned, as, in point of probability, upon a level with 
that of Copernicus. They took notice, indeed, of its 
inferiority with regard to coherence and connexion, ex- 
pieffing hopes however that thefe defeCls might be re¬ 
medied by fome future improvements. But when the 
world beheld that complete and almoft perfect coherence, 
which the philofophy of Des Cartes beftowe-d upon the 
fyllern of Copernicus, the imaginations of mankind could 
ho longer refufe tliemfelves the pleafure of going along 
■with fo harmonious an account of things. The fyftem of 
Tycho Braiie was every day lefs and lei's talked of, till at 
la if it was forgotten altogether. 
The iyftem of Des Cartes, however, though it con¬ 
nected together the real motions of the heavenly bodies 
according to the fyftem of Copernicus, more happily than 
had beeifdone before, did fo only when they were confi- 
dered in the grofs; but did not apply to them, when they 
were regarded in the detail. Des Cartes, as was faid before, 
had never himfelf obferved the heavens-with any particu¬ 
lar application. Though he was not ignorant of any of 
the obfervations which had been made before his time, 
yet lie feems to have paid them no great degree of atten¬ 
tion; which, probably proceeded from his own inexpe- 
perience in the fludy of altronomy. So far therefore from 
accommodating his fyftem to all the minute irregularities, 
which Kepler had afcertained in the movements of the 
planets ; or from (hewing particularly how thefe irregu¬ 
larities, and no other, (hould arife from it, he contented 
himfelf with obferving that perfect uniformity could not 
be expeCted in their motions, from the nature of the caufes 
which produced them ; that certain irregularities might 
take place in them, for a great number of fticcefljve re¬ 
volutions, and afterwards give way to others of a different 
kind : a remark which happily relieved him from the ne- 
ceffity of applying h is iyftem to the obfervations of Kepler, 
and the other aftronomers. 
But when the obfervations of Caftini had eftabiiftied the 
authority of thofe laws, which Kepler hadfirft difcovered 
in the fyftem, the philofophy of Des Cartes, which could 
afford no reafon wdiy fucli particular laws fliould be ob¬ 
served, might continue to amufe the learned in other 
Sciences, but could no longer Satisfy thofe that w ere (killed 
in aftronomy. Sir Ifaac Newton firft attempted to give a 
phylical account of the motions of the planets, which 
(hould accommodate itfelf to all the conftant irregularities 
which aftronomers had ever obferved in their motions. 
The phylical connection by which Des Cartes had endea¬ 
voured to bind together the movements of the planets, 
was the laws of itnpulfe ; of all the orders of fucceflion, 
thofe which are moft familiar to tiie imagination ; as they 
all flow from the inertnefs of matter. After this quality, 
there is no oilier with which we are fo w ell acquainted, as 
that of gravity. We never ad upon matter, but we have 
occafion to obferve it. The fupericr genius and fagacity 
of Sir Ifaac Newton, therefore, made the moft happy, 
and, we may now fay, the greateft and moft admirable, 
improvement that was ever made in philofophy, when lie 
difcovered, that lie could join together the movements of 
the planets by fo familiar a principle of connection, which 
completely removed all the difficulties the imagination had 
hitherto felt in attending to them. He demonftrated, that 
if the planets were fuppofed to gravitate towards the Sun, 
and to one another, and at the fame time to have a pro¬ 
jecting force originally itnpreffed upon them, the primary 
ones might all defcribe elliples in one of the foci of which 
that great luminary was placed ; and the fecondary ones 
might defcribe figures of tiie fame kind round their ref- 
pective primaries, without being difturbed by the con¬ 
tinual motion ot tiie centres of their revolutions. That 
if the force which retained each of them in their orbits, 
was like that of gravity, and directed towards tiie Sun, 
they would each of them defcribe equal areas m equal 
Vol. II. No. 75. 
3 4i 
times.- That if this attractive power of the Sun, like all 
other qualities which are diffufed in rays from a centre, 
dhniniftied in the fame proportion as tiie fquares of the 
diftances increafed, their motions would be fwifteft when 
neareft the Sun, and ftoweft when fartheft oft' from him, 
iii tiie fame proportion in which, by obfervation, they are 
difcovered to be ; and that, upon the fame fuppolition, of 
this gradual diminution of their refpeCtive gravities, their 
periodic times would bear tiie fame proportion to their dif- 
tances which Kepler and Caftini had eftabli-fhed betwixt 
diem. Having thus (hewn, that gravity might be tiie 
connecting principle which joined together the movements 
of the planets, he endeavoured next to prove that it really 
was fo. Experience (hews us what is the power of gra¬ 
vity near the furface of the Earth. That it is l'ucli as to 
make a body fail, in the firft fecond of its defeent, through 
about fifteen Parifian feet. The Moon is about iixty fe- 
midiameters of the Earth diftant from its furface. If gra¬ 
vity therefore was (uppofed to diminifti, as tiie fquares of 
tliediftance increafe, a body, at the Moon, would fall to¬ 
wards the Earth in a minute, tiiat is, in (ixty feconds, 
through the fame (pace which it falls near its furface in 
one fecond. But the arch which the Moon deferibes in a 
minute, falls, by obfervation, about fifteen Parifian feet 
below the tangent drawn at the beginning of it. So far, 
therefore, tiie Moon may be conceived as conftantly fall¬ 
ing towards the Earth. 
The fyftem of Sir Ifaac Newton correfponded to many’ 
other irregularities which aftronomers had obferved in the 
heavens. It afligned a reafon, why the centres of the re¬ 
volutions of the planets were not precifely in the centre 
of tiie Sun, but in the common centre of gravity of the 
Sun and the planets. From the mutual attraction of tiie 
planets, it gave a reafon for fome other irregularities in 
their motions; irregularities, which are quite fenfible in 
thofe of Jupiter and Saturn, when thofe planets are nearly 
in conjunction with one another. But of all the irregu¬ 
larities in the heavens, thofe of the Moon had hitherto 
given the greateft perplexity to aftronomers; and the fyl¬ 
tem of Sir I(aac Newton correfponded, if poflible, yet 
more accurately with them than with any of the oilier pla¬ 
nets. The Moon, when either in conjunction oroppofition 
to the Sun, appears fartheft from the Earth, and neareft to 
it when in her quarters. According to the fyftem of that 
pliilofopher, when (lie is in conjunction with the Sun, (lie 
is nearer tiie Sun than the Earth is ; confequently, more 
attracted to him, and therefore more feparated from the 
Earth. On the contrary, when in oppofition to tiie Sun, 
(he is farther from the Sun than the Earth. The Earth, 
therefore, is more atiraCled to the Sun ; and confequently, 
in this cafe, too, farther feparated from the Moon. But, 
on tiie other hand, when the Moon is in her quarters, die 
Earth and tiie Moon, being botli at equal diftances 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, blit in lines which meet 
in liis centre, they are thereby (till further approached to 
one another. Sir Ifaac Newton computed the difference 
of the forces, with which tiie Moon and tiie Eartli ought, 
in all thofe different (filiations, according to his theory, to 
be impelled towards one another ; and found, that the dif¬ 
ferent degrees of their approaches, as they had been ob¬ 
ferved by aftronomers, correfponded exaCliy to his com¬ 
putations. As the attraction of the Sun, in the conjunc¬ 
tions and oppolitions, diminiflies the gravity of tiie Moon 
towards the Earth, and confequently, makes her neceffarily 
extend her orbit, and therefore require a longer periodical 
time to finifh it. But, when the Moon and the Earth are 
in that part of the orbit which is neareft the Sun, this at¬ 
traction of the Sun will be the greateft ; confequently tiie 
gravity of the Moon towards the Earth vvill there be moft 
diminifiied ; her orbit be moft extended ; and her peri¬ 
odic time be therefore tiie longeft. This is alfo agreeable 
to experience, and in the very fame proportion, in which 
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