PLA 
and Mercury, are opaque bodies, illuminated 
with the borrowed light of the Sun. And 
the same appears of Jupiter, from its being 
void of light in that part to which the sha- 
dow of the satellites reaches, as well as in 
that part turned from the Sun ; and that his 
satellites are opaque, and reflect the Sun’s 
light, is abundantly shown. Wherefore, since 
Saturn, with his ring and satellites, only 
yield a faint light, fainter considerably than 
that of the fixed stars, though these be 
vastly more remote, and than that of the 
rest of the planets : it is past donbt, he too, 
with his attendants, are opaque bodies. 2. 
Since the Sun’s light is not transmitted 
through Mercury and Venus, when placed 
against him, it is plain they are dense 
opaque bodies ; which is likewise evident of 
Jupiter, from his biding the satellites in his 
shadow; and therefore, by analogy, the 
same may be concluded by Saturn. 3. Fi om 
the variable spots in Venus, Mars, and 
Jupiter, it is evident these planets have a 
changeable atmosphere ; wliich changeable 
atmosphere may, by a like argument, be 
inferred of the satellites of Jupiter, and 
therefore by similitude the same may be 
concluded of tlie other planets. 4. In like 
manner, from the mountains observed in 
Venus, the same may be supposed in the 
other planets. 5. Since then, Saturn, Ju- 
piter, both their satellites. Mars, Venus, 
and Mercury, are opaque bodies, sliining 
with the Sun’s borrowed light, are furnish- 
ed with mountains, and encompassed with 
a changeable atmosphere; they have, of 
consequence, waters, seas, &c. as well as 
dry land, and are bodies like the Moon, and 
tlierefore like the Earth, And lienee, it 
seems highly probable that the other planets 
have their animal inhabitants, as well as our 
Earth. 
Planets, masses of. It would appear, 
at first view, impossible to ascertain the 
respective masses of the Sun and planets, 
and to calculate the velocity with which 
heavy bodies fall towards each wlien at a 
given distance from their centres ; yet these 
points may be determined from the theory 
of gravitation without much difficidty. It 
follows, however, from certain theorems 
relative to centrifugal forces, that the gra- 
vitation of a satellite towards its planet is 
to the gravitation of the Earth towards the 
Sun, as the mean distance of the satellite 
from its primary, divided by the square of 
the time of its sidereal revolution, or the 
mean distance of the Earth from tlie Sun 
divided by the square of a sidereal year. 
PLA 
To bring these gravitations to the same dis- 
tance from the bodies which produce them, 
we must multiply them respectively by the 
squares of the radii of the orbits which are 
described : and, as at equal distances the 
masses are proportional to the attractions, 
the mass of the Earth is to that of the Sun 
as the cube of the mean radius of tlie orbit 
of the satellite, divided by the square of the 
time of its sidereal motion, is to the cube of 
the mean distance of the Earth from the Sun, 
divided by the square of the sidereal year. 
Let us apply this result to Jupiter. The 
mean distance of his fourth satellite .sub- 
tends an angle of 1530".86 decimal seconds. 
Seen at the mean distance of the Earth 
from the Sun, it would appear under an 
angle of 7964 " .75 decimal seconds. The 
radius of the circle contains 636,619" .8 de- 
cimal seconds. Therefore the mean radii of 
the orbit of Jupiter’s fourth satellite, and of 
the Earth's orbit are to each other as these 
two numbers. The time of the sidereal 
revolution of the fourth satellite is 16^6890 
days; the sidereal year is 365,2564 days. 
These data give us — — — for the mass of 
® l06b.08 
Jupiter, that of the Sun being represented 
by I. It is necessary to add unity to the 
denomination of this fraction, because the 
force which retains Jupiter in his orbit is 
the sum of the attractions of Jupiter and 
the Sun. The mass of Jupiter is then 
. The mass of Saturn and Herschel 
1067.08 
may be calculated in llie same manner. 
That of the Eartli is best determined by the 
following method ; If we take the mean 
distance of the Earth from the Sun for_ 
unity, the arch described by the Earth in 
a second of time will be the ratio of the 
circumference to the radius divided by the 
number of seconds in a sidereal year. If 
we divide the square of that arch by the 
1479565 
diameter, we obtain — — for its versed 
sine, which is the deflection of the Earth 
towards the Sun in a second. But on that 
parallel of the Eqrth’s surface, the square of 
the sine of whose latitude is i, a body falls 
in a second 16i feet. To reduce this attrac- 
tion to the mean distance of the Earth from 
the Sun, we must divide the number by the 
feet contained in that distance ; but the 
radius of the Earth at the above-mentioned 
parallel is 19,614,648 French feet. If we 
divide this number by the tangent of the 
solar parallax, we obtain the mean radius of 
the Earth’s orbit expressed in feet. The 
