Astronomy. 83 
he gives the formulas stipe by him for the pompression (e) of 
a planet resulting from superficial abrasion, and shows that this 
would be ree less than that resulting from the ‘gucci of 
primitive fluidity. The ee gerbe of the formule to the planets 
whose times of estios an n density are most similar to the 
earth give the following resu 
or the planet Mercury, if we 6 kami 86700” for its time of rota- 
tion, ‘075 for the ratio of its mass to that of the earth, and °378 | 
for the ratio of its diameter to the earth’s mean diameter, we 
find Q= ae a3 and if the planet were homogeneous, 
Hee 
~ 325 
With the same law of density as in the earth, on the fluid theory, 
1 
| Om ai8? 
and on the theory of abrasion, 
e= 586° 
These three results na that for Mercury no sensible compres- 
sion is likely to be e 
or Venus, if we pepe the values of the mass M, time of rota- 
tion T, and diameter a, generally admitted, namely 
1 
M=——_., T= 23" 21™ 22° = "954 
M = 779150" ajute Hoan toe 
I find for the compression, on bec ee of fluidity and a law 
of density like that for the ea 
ae Yc 
and by the hypothesis of abrasion at surface, 
oe 
é= 351° 
The first of these values approaches closely to the compression 
recently observed by Colonel Tennant—namely, e==—. So 
far, peed the figure of Venus is more consistent with the 
theory of fluidit ty than with the theory of as pte abrasion. 
Sines communicated my note on Mars to the Academy, I 
have become acquainted witht the new ty Sekaristbacin of the planet’s 
mass obtained from the motions of its satellites. The astronomers 
of the Washington Observatory have ata especial attention 
to the satellites of this planet. Professor Asaph Hall has pub- 
lished results* which lead to the Sscalanicn that the mass of Mars 
is probably about 3093500" ‘ 
With this value, and the values of other elements remaining 
* Washington Astronomical Observations, xxii, Appendix. 
