ASTRONOMY. 
aflumed its form, it was in a fiate of liquefaction by fire. 
But that could not be the method of producing the pla¬ 
nets; for, if they were (truck of!' from the body of the 
Sun, they would move in orbits that would always pafs 
through the Sun, infiead of having the Sun for their fo¬ 
cus, or centre, as they are now found; fo that having been 
(truck off they would fall down into the Sun again, termi¬ 
nating their career as it.were after one revolution only. 
Although the relative denfities of the Earth and mod of 
the other planets have been known a conliderable time, it 
is but very lately that we have come to the knowledge of 
the abfolute gravity or denfity of the whole mafs of the 
Earth. This was calculated by Dr. Hutton, and deduced 
from the obfervations made by Dr. Mafkelyne, aftronomer 
royal, at the mountain Schehallein, in the years 1774, 5, 
and 6. The attraction of that mountain on a plummet, 
being obferved on both (ides of it, and its mafs being com¬ 
puted from a number of feCtions in all directions, and con¬ 
fiding of done ; thefe data, being then compared with the 
known attraction and magnitude of the Earth, gave by 
proportion its mean denlity, which is to that of water as 
9 to 2, and to common done as 9 to 5 : from which very con- 
(iderable mean denfity, it may be prefumed that the inter¬ 
nal parts contain great quantities of metal's. 
From the denfity now found its quantity of matter be¬ 
comes known, being equal to the product of its denfity by 
its magnitude. From various experiments too we know, 
that its attractive force, at the (itrface, is fuch, that bodies 
fall there through a (pace of feet in the fird fecond 
of time: from whence the force at any other place, either 
within or without it, becomes known; for the force at any 
part within it, is direCtly as its didance from the centre; 
but the forces of any part without it, reciprocally as the 
fquare of its did.mce from the centre. 
As to tlie motion of the Earth, it is now univerfally 
agreed, that, befides the final! motion which caufes the pre- 
cedion of the equinoxes, the Earth has two great and in¬ 
dependent motions; the one diurnal round its own axis 
in the fpace of twenty-four hours, which conditutes the 
natural day or nyCthemeron ; the other annual, about the 
Sun, in an elliptical orbit or track, in 365 days and nearly 
fix hours, conftituting the year. From the former arife 
the diverfities of night and day ; and, from the latter, the 
viciffititdes of feafons, fpring, Jummer, autumn, and winter. 
Of the Diurnal Motion of the Earth. 
Though it is this motion which gives 11s the grateful 
vicifiitudes of day and night, adjufted to the times of la¬ 
bour and red; yet many people find a difficulty in con¬ 
ceiving that the Earth moves ; and the more fo, becaufe, 
in order to allow it, they mud give up, in a great meafure, 
the evidence of their exterior fenfes, of which the im- 
predions are exceeding drong and lively. It will there¬ 
fore be neceffary to prove, that we ought by no means to 
infer that the Earth is at red, becaufe it appears fo; but, 
by a variety of faCts, to fiiew, that reafon was given to 
correCt all fuch fallacies of the fenfes. To this end we 
fhall point out fome indances, where apparent motion is 
produced in a body at red, by the real motion of the fpec- 
tator. Let 11s fuppofe a man in a (hip to be carried along 
by a bride gale, in a direction parallel to a fhore, at no 
great didance from him; while he keeps his eye on the 
deck, the mad, the fails, or any thing about the (hip, that 
is to fay, while he fees nothing but fome part of the velfel 
on-board of which he is, and confequently every part of 
which moves with him, he will not perceive the (hip moves 
at all. Let him, after this, look tathe fliore, and he will 
fee the houfes, trees, and hills, run from him in a direction 
contrary to the motion of the velfel; and, fuppofing him 
to have received no previous information on the 1 'ubjeCt, 
he might naturally conclude, that the apparent motion of 
thefe bodies was real. In a bmilar (ituation to this we may 
conceive the inhabitants of the Earth ; w ho, in early times, 
knew nothing of the true druChire or laws of the univerfe, 
faw the Sun, the ftars, and the planets, rife and let, and 
perform an apparent revolution about the Earth. They 
had no idea of the motion of the Earth, and therefore all 
this appearance feemed reality. But as it is highly rea- 
fonable to fuppofe, that as foon as the flighted hint fhould 
be given to the man, of the motion of the vedcl, he would 
begin to form a new opinion, and conceive it to be more 
rational, that fo fmall a thing as the fliip (hould move, ra¬ 
ther than all that part of the Earth which was open to his- 
view ; fo, in the fame manner, no fooner was an idea form¬ 
ed of the vafl extent and greatnefs of the univerfe, with 
refpeft to this Earth, than mankind began to conceive it 
would be more rational that the Earth fliould move thaa 
the whole fabric of the heavens. 
By another familiar indance it will be eafy to fliew, that, 
as the eye does not perceive its own motion, it alwaysjud- 
ges from appearances. Let a perfon go into a common 
windmill, and defire the miller to turn the mill round, 
while lie is fitting within it with his eyes fixed on the up¬ 
right pod in the centre ; this pod, though at red, will ap¬ 
pear to him to turn round with conliderable velocity; the 
real motion of the mill being the caufe of the apparent 
motion outlie fwivel-pofl. Sea-faring people are furniflied 
with various indances to illudrate this (ubjeft; thofewho 
are bufy in the hold of a (hip at anchor, cannot by any 
perception determine whether the (hip has fwung round or 
not by the turn of the tide. When a (hip fird gets under 
way with a light breeze, die may be going at a good rate 
before thofe who are between decks can perceive it. Hav¬ 
ing thus obviated the objections which arife from the tefti- 
mony of the fenfes, we may proceed to confider the argu¬ 
ments which tend more direCtly to prove the motion of the 
Earth. All the celedial motions will, on this fuppofition, 
be incomparably more fimple and moderate. This opinion 
is alfo much more agreeable to our notions of final caufes, 
and our knowledge ot the oeconomy of nature ; for, if the 
earth be at red, and the flars, &c. move round it once in 
twenty-four hours, their velocity mud be immenfe ; and 
it is certainly more agreeable to reafon, that one Angle bo¬ 
dy, and that one of the fmalled, (hould revolve on its own 
axis in twenty-four hours, than that the whole univerfe 
fhould be carried round it in the lame time, with incon¬ 
ceivable velocity. 
J he rotation ot the Earth round its axis is analogous to 
what is oblerved in the Sun, and mod of the planets; it 
being highly probable, that the Earth, which is itfelf one 
ot the planets, fliould have the fame motion as they have, 
for producing the fame eft'eCt: and it would be as abf'urd 
in 11s to contend (or the motion of the whole heavens round 
us in twenty-four hours, rather than allow a diurnal motion 
to our globe, as it would be for the inhabitants of Jupiter 
to infid that our globe and the whole heavens mud revolve 
round them in ten hours, that all its parts might fuccef- 
fively enjoy theiiglit, rather than grant a diurnal motion 
to their habitation. All the phenomena relative to this 
fubjeCl, areaseafily folved on the fuppofition of the Earth’s 
motion, as on the contrary hypothefis. Befides the fore¬ 
going conficierations, there are feveral arguments to be de¬ 
duced from the higher parts of aflronomy, which demon- 
flrably prove the diurnal motion of the Earth. 
I hough the globe of the Earth appears fo large to thofe 
who inhabit it, yet it is fo minute a fpeck, when compared 
to the immenfe fphere of the heavens, that at that didance 
the planes of the rational and fendble horizons coincide : 
or, in other words, the didance between them in the fphere 
ot the heavens, is too fniall for admeafurement. 
Io illudrate this, let ACBD, in the following figure, 
reprelent the Earth ; 2 hno, the fphere of the darry hea¬ 
ven. If an inhabitant of the Earth dand upon the point 
A, his fenfible horizon is se, his rational one ho ; the dis¬ 
tance between the planes of thefe two horizons is AF, the 
femidiameter ot the Earth, which is meafured in a great 
circle upon the fphere of the heaven, by the angle eFo, or 
or the arc co ; this arc in fo fmall a circle as zhno , would 
amount to feveral degrees, and confequently the difference 
between the fenfible and rational horizon would be great 
enough 
