ASTRONOMY. 
which would appear to be mod above the reach of human 
reafon, is that which computes the weights and denlities 
of the Sun, and of the feveral planets. This, however, 
was indil'penfably necelfary to complete the coherence of 
the Newtopian fydem. The power of attraction which, 
according to the theory of gravity, each body poirdfes, is 
in proportion to the quantity of matter contained in that 
body, but the periodic time in which one body, at a given 
didance, revolves round another that attracts it, is fliorter 
in proportion as this power is greater, and confequently as 
the quantity of matter in the attracting body. If the 
denlities of Jupiter and Saturn were the fame with that of 
the Earth, the periodic times of their feveral fatellites 
would be fhorter than by obfervation they are found to be, 
becaufe the quantity of matter, and confequently the at¬ 
tracting power, of each of them, would be as the cubes of 
their diameters. By comparing the bulks of thole planets, 
and the periodic times of their datellites, it is found that, 
upon the hypothefis of gravity, thedenlity of Jupiter mud 
b_* greater than that of Saturn, and the denfity of the Earth 
greater than that of Jupiter. This feems to edablifh it 
as a law in the fydem, that the nearer the feveral planets 
approach to the Sun, the denfity of their matter is the 
greater : a conditution of things which would feem to be 
the mod advantageous of any that could have been eftab- 
li filed ; as water of the fame denfity with that of our Earth, 
would freeze under the equator of Saturn, and boil under 
that of Mercury. 
Such is the folar fydem adltmed by Sir Ifaac Newton, a 
fydem vvliofe parts are all more driCtly connected together, 
than thofe of any other philofophical hypothefis. Allow 
his principle, the univerfality of gravity, and that it de- 
creafes as the fquares of the didance increafe, and all the 
appearances, which he joins together by it, necedarily fol¬ 
low. Neither is their connection merely a general and loofe 
connection, as that of mod other fydems, in which either 
thefe appearances, or fome fuch like appearances, might 
indid'erently have been expeCted, It is every where the 
mod precife and particular that can be imagined, and af- 
certains the time, the place, the quantity, the duration, 
of each individual phenomenon, to beexaCtly fuch as, by 
obfervation, they had been determined to be. Neither 
are the principles of union, which it employs, fuch as the 
imagination can find any difficulty in going along with. 
The gravity of matter is, of all its qualities, after its inert- 
nefs, that which is mod familiar to us. We never aCt 
upon it without having occafion to obferve this property. 
The law too, by which it is fuppoled to diminifh as it re¬ 
cedes from its centre, is the fame which takes place in all 
other qualities which are propagated in rays from a centre, 
in light, and in every thing elfe of the fame kind. It is 
fuch, that we not only find that it does take place in all 
fuch qualities, but we are necedarily determined to con¬ 
ceive that, from the nature of the thing, it mud take 
place. The oppofition which was made in France, and in 
fome other foreign nations, to the prevalence of this fyl¬ 
tem, did not arife from any difficulty which mankind na¬ 
turally felt in conceiving gravity as an original and pri¬ 
mary mover in the conditution of the univerfe. The 
Cartefian fydem, which had prevailed fo generally before 
it, had accudomed mankind to conceive motion as never 
beginning, but in confequence of impulfe, and had con¬ 
nected the defcent of heavy bodies, near the furface of 
the Earth and the other planets, by this more general bond 
of union ; and it was the attachment the world had con¬ 
ceived for this account of things, which indifpofed them to 
that of Sir Ifaac Newton. His fydem, however, now pre¬ 
vails overall oppofition, and has advanced to the acquifi- 
tion of the mod univerfal empire that was ever edablidied 
in philofophy. His principles, it mud be acknowledged, 
have a degree of firmnefs and folidity that we Ihould in 
vain look for in any other fydem. The mod fceptical can¬ 
not avoid feeling this. They not only connedt together 
mod perfectly all the phenomena of the heavens which 
had been oblerved before his time, but thole alio which 
the perfevering indudry and more perfect indruments of 
later adronomers have made known to us; have been either 
eafily and immediately explained by the application of his 
principles, or have been explained in confequence of more- 
laborious and accurate calculations from thefe principle.;, 
than had been indituted before. Can we wonder then, 
that it Ihould have gained the general and complete appro¬ 
bation of mankind, and that it Ihould now be conddered, 
not as an attempt to conned! in the imagination the pheno-. 
mena of the heavens, but as the greated difeovery that 
ever was- made by man, the difeovery of an immenle chain 
ot the moll important andfublime truths, all clofely con- 
nedted together by one capital fadl, of the reality of which 
we have daily experience. See Phil. Elf. (edt. iv. 
From the date of thefe improvements of Sir ifaac New¬ 
ton, to the era of the adronomical fame of Dr. Herfchel, 
though the intervening fpace was long, though the num¬ 
ber of adronomers was increafed, and though alfiduity in 
obferving was allifted by accuracy and perfection in the in- 
druments of obfervation, yet we hear of no further addi¬ 
tion to the Copernican fydem, nor of any new difeoveries 
in the phenomena of the heavens, by which the fcienceof 
adronomy could be improved or enlarged, other than the 
celebrated difquilitions of La Place and La Grange, who, 
guided by a molt refined calculus, have anticipated the re- 
fearches of future centuries, by difclofing thofe beautiful 
cycles which bind and perpetuate the revolutions of nature. 
Beyond all this, however, the penetrating eye of Dr. Herf¬ 
chel, on March 13, 1781, difeovered a new planet, confi- 
derably more remote in the folar fydem than that of Sa¬ 
turn. The attention of this able adronomer was fird 
drawn towards it by the deadinefs of its light; this indu¬ 
ced him to apply greater magnifying powers to his telef- 
cope, by which its diameter w as increafed, and its general 
appearances brought more into view. In two days he ob- 
felved that its place was.changed, and he then concluded 
that it was a comet; but in a little time, and, after a con- 
fultation with other adronomers, he determined it to be a 
planet, from its vicinity to the ecliptic, and the direction 
of its motion ; being dationary in the time and in fuch cir- 
cumdances as correfpond with fimilar appearances in other 
planets. With a telefcope which magnifies about 300 
times, it appears to have a very well-defined vifibie difk; 
but with indruments of a fmaller power it can hardly be 
didinguilhed from a fixed dar between the fixth and fe- 
venth magnitude. In a clear night, and when the Moot 
is abfent, it may be feen with the naked eye. Dr. Herfchel 
has fince fhewn that it is attended by two fatellites; a dif¬ 
eovery that afforded him confiderable pleafure, as the lit¬ 
tle fecondary planets feemed to give a dignity to the pri¬ 
mary one, and to raife it into a more confpicuotis fituation 
among the great bodies of our folar fydem. For this dif¬ 
eovery Dr. Herfchel obtained from the Royal Society the 
honorary recompence of Sir Godfrey Copley’s medal ; and 
his majedy has taken him under his protection, and grant¬ 
ed him an annual lalary. By this princely munificence, 
the king wifely gave (cope to a very uncommon genius, 
which has enabled hint to profecute his adronomical Un¬ 
dies with unremitted ardour. 
That the mind may readily embrace a clear comprehen- 
fion of the three mod popular fydems of which we have 
been fpeaking, namely, the Ptolemaic, Tychonic, and Co¬ 
pernican, we have in the Adronomical-Piate I. projected 
figures of each of them, and of which the following is an 
explanation : 
Fig. r, reprefents the Sydem introduced by Ptolemy, in 
which he fuppofed the Earth to be at red in the centre, 
and the Sun and planets to move in circles round it. The 
]) Moon he placed fird and neared, to enable her to per¬ 
form her monthly revolutions. Next to the Moon was 
placed ^ Mercury ; next above him 9 Venus ; and then 
the 0 Sun, occupying the fourth circle, whole complete 
revolution was taken to be the meafure of the year. Next 
above the Sun was $ Mars, then Jupiter, and next 
Tp Saturn, in their leparate and proper orbits. The fir¬ 
mament, 
