Prof. H. Hennessy on the Figures of the Planets. 285 

 rotation T, and diameter a, generally admitted, namely 

 M= 412T50' T = 23" 21*> 22s, a=-954;> 



I find for the compression, on the hypothesis of fluidity and 

 a law of density like that for the earth, 



1 

 e ~247 J 

 and by the hypothesis of abrasion at surface, 



_ J_ 

 e ~351' 



The first of these values approaches closely to the com- 

 pression recently observed by Colonel Tennant — namely, 



e = 9fin* ^° ^ ar > therefore, the figure of Venus is more con- 

 sistent with the theory of fluidity than with the theory of 

 superficial abrasion. 



Since I communicated my note on Mars to the Academy, 

 I have become acquainted with the new determination of the 

 planet's mass obtained from the motions of its satellites. The 

 astronomers of the Washington Observatory have devoted 

 especial attention to the satellites of this planet. Professor 

 Asaph Hall has published results * which lead to the conclu- 

 sion that the mass of Mars is probably about o nQ or n(r 



With this value, and the values of other elements remain- 

 ing the same as in my previous note, Q becomes Q , or 

 I J(Jo'74 



^-r nearly. The compression on the fluid theory becomes 

 nr\c.<\a or ^r=. On the theory of abrasion the compression is 



^t-h. The first is much nearer to the observed compression ^r- 



than the last. 



It thus appears that, for the earth and the planets nearest 

 to it, and whose mean density and general appearance make 

 it probable that their materials resemble those of the earth in 

 physical and mechanical properties, the compressions deduced 

 from the theory of fluidity agree much better with observation 

 than the compressions deduced from the theory of superficial 

 abrasion. 



* Washington Astronomical Observations, xxii., Appendix, 



