On the Figure of the Planet Mars. 119 



as sharp as with a telescope. The function of the telescope is 

 in fact to allow the use of a wider, and therefore more easily 

 measurable, aperture. 



An interesting modification of the experiment is obtained 

 by using lights of various wave-lengths. For this purpose we 

 may have recourse to coloured glasses ; but the best results 

 would doubtless require the rays of the spectrum. 



Cavendish Laboratory, Cambridge, 

 July 9, 1880. 



XVIII. On the Figure of the Planet Mars. 

 By Henet Hennessy, F.R.S.* 



IN October 1878 I communicated a paper to the Academy 

 of Sciences in Paris, which was subsequently printed in 

 the Comptes Rendus f, in which I established my priority to 

 the discovery of formulas connecting the polar compression of 

 a planet with its mean density and surface-density; and I drew 

 some conclusions from these formulae as to the configuration 

 of the planet Mars. Very recently an American astronomer, 

 Professor C. A. Young J, has published a series of observations 

 made by him of the equatorial and polar diameters of Mars, 

 which seem to have been made under remarkably favourable 

 circumstances. The observations were carefully reduced and 

 corrected for different small disturbing influences ; and the 

 final value of £, or the polar compression after all reductions 



are completed, is e' = ^y^. It can be easily shown that this 



value is much more accordant with the theory of former 

 fluidity of the planet, than with the theory of superficial abra- 

 sion and denudation by the action of a liquid ocean having 

 the same density as water. If Mars was originally in a fluid 

 state from heat, the mass, in accordance with the properties 

 of fluids, would be arranged in spheroidal surfaces of equal 

 density with a law of increasing density in going from the 

 surface to the centre. The ellipticity or compression would 

 depend on this law and on the periodic time of rotation of the 

 planet, as in the case of the earth. In such a liquid spheroid, 



«'= 5 -f-'F(«'), 



where q f is the ratio of centrifugal force to gravity at the 



* Translation of a paper read to the Academie des Sciences, June 14, 

 1880. Communicated by the Author. 



t Comptes Rendus de V Academie des Sciences, October 22, 1878, p. 590 : 

 also Phil. Mag. January 1879. 



\ American Journal of Science, March 1880, p. 200, 



