DENSITY. 
denfity of the fun and planets, is a problem in phylical af- 
ronomy, not more interefting from the information we 
i s refult, than from the nature of the method 
the folation is obtain 
antient eat eat sas. who certainly were not 
deficient . their powers of reafoning, nothing 
od by which 
» as may be intelligible ad fatisfattory to 
thofe who may not be familiar with the agate n of the 
Newtonian philofophy, to queftions of this nature. 
The hypothefis, which it is neceflary to admit before we 
can attempt the folution of this problem, is that of univerfal 
‘his fuppof-s that all bodies (at leaft thofe in 
n} attract ck other, in proportion diredétly to their 
ma{les or quanie of matter, and inver{ely as the {quares of 
es difta ing but the 
rd is by n 
the objc€tions that have been made to at it endows 
matter with metaphyfical and occult qualities. When we 
Say that it is the nature of iron to be attracted hy the mag- 
et, we mean nothing more, than that every piece of iron 
on which the experiment has been made, has, without ex- 
ception, obeyed the magnetic impulfe. The Newtonian 
pel aia on which all aS —— is built, afferts 
um, ies, in 
tree del nae appa ach other, becaufe it 
reater effort in them to remain at reft, or to mo 
any. oe direCtion. When we fee a faicr forcibly drawn to 
a piece of excited fealing wax, we attribute the phenome- 
non to the effe& of the eleGiric fluid, a name we he given, 
by analogy, to this extraordinary agent, whofe nature we 
es as little of, as of the nature of gravity, and which moft 
probably neither refembles a fluid, nor any other form of 
matter with which we are “acquainted. But to return more 
immediately to the fubje 
Weare now to fhew by bat train of reafoning we arrive at 
the knowledge of the comparative quantity of matter which 
the fun and planets contain, relatively to that in the 
earth. 
If aheavy body, for inftance a cannon ball, be fuffered 
vy 
to defcend from a a flat te of reft, 
attractive power of the earth. 
It is demonftrated by mathematicians, and we will here 
take it for granted, that the attraCtive force of a large {phe- 
rical mafs of matter will be the fame, let the dimenfions of 
the globe, into which that matter is compreffed, be what they 
‘rye 
may. So that if the whole mafs of the earth could be come 
preffed into a central a its attractive force would remain 
the fame, and would at 4c0o miles diftance, that is at the 
fame diftance as before, (4000 miies being equal to the 
earth’s radius) caufe a heavy body to move towards it with 
an faa velocity of 16 feet in the firft fecond of time 
é fuppofe the bail, inftead of defcending fou a fate 
of relt, to be projected horizontally from a cannon, it will 
ft:ll equally obey the attraGtive power of the eart 
defcend exa@l i 
rth were really comprefled 
into a {mall central fpace, as we eer now fuppofed, the ball 
would circulate round it, and would de 
angent 16 et in in 
e firft fecond of time. If ie ball be taken up 60 t 
as far frem the centre of the earth, Seman to the diane 
of the moon) the attraGtive power of the earth will th std 
diminifhed 3600 times, ( 3600 being i fquare of 60) b 
caufe the force naiety as the fquare of the diftstce 
increafes ; 3 an e ball will defcend from a ftate. of 
16 feet ; e projected as before, f 
feribe an or pee round . earth, that oa wou defleé& 
from its aie xh And in i moon itfelf 
curious coinciden s the firft piematos Newton ob- 
tained of the ark ei his hypothefi 
f then we could find in our planetary hea a fatellite 
or fecondary body, revolving round its prin 
equally diftant from its centre, as the moon ta the céntre 
of the earth, we fhould cafily pirsce whether that planet 
“pipes more or lefs matter than the earth, by obferving 
much the orbit of the fatellite defleéted from the tar 
gent in ove fecond of time. If the deflection was equal 
o that of the moon we fhould conclude, the mafs of the 
bias to be equal to that of the earth; if we found it greater 
or lefs, it would indicate a mafs of the planet to be greater 
or lefs in the fame propor 
e planet Jupiter sffor ie an obvious example to illuftrate 
this a ee its firft fatellite revolves round it at a difs 
arly equal 
tance ne to that of t oon from the earth, 
but in one fecond it defleéts from its tangent 256 times as 
much as the moon doe f Jupiter is therefore 
256 times greater than that of thee The principles of 
the calculation are not materially different for fatellites at dif- 
ferent diftances, it is only neceflary to compute what the at- 
traGive power would be at equal diftances. The mafs of a 
planet being thus oka. and its magnitude determined by 
obfervation, its relative denfity may be computed accord- 
ing to the pence aeeated in the former part of this 
article. 
We fhall now proceed to explain the praGtical methods 
a are ufually employed for the folution of this pro- 
a has been before ftated, - : a hey pee things, 
de, the 
namely, magonitu the den 
ody, any two be given, the bara ne be found. The 
magnitudes of the fun and planets are here fuppofed to be 
determined by obfervation ; to afcertain their denfities, we 
begin by computing their matfes, 
The mafs of a planet may be computed by comparing the 
velocity i in its orbit round the fun, either with the velocity 
of its fatellite, or with the force of gravity at its furface. 
If the planet has no fatellite, aftronomers have recourfe to a 
method much lefs accurate, depending on the cfeé& “a 
y 
