204 Rev. J. Challis on the EUipticities of the Planets. 



V 



The following values of — , are calculated from the masses of 



the bodies of our system, as given by Laplace. For the Sun, 

 54-31 ; Jupiter, 5-33; Saturn, 3*06; Uranus, 1-97; the Earth, 

 ]; Venus, -981; Mercury, -651; Mars, -5; the Moon, -23. 

 It is observable that the order of magnitude of these quan- 

 tities is the same as the order of the bodies arranged accord- 

 ing to their masses. It follows, therefore, from our theory, 

 that because in the greater masses the velocity of propagation 

 is greater, the materials of which they are composed possess 

 greater elastic force. This may be owing, not to any differ- 

 ence in the constituent elements, but to a greater degree of 

 proper caloric, or of the force, whatever it be called, by which 

 the constituent atoms are held in their positions relatively to 

 each other. For it is not unreasonable to suppose that the 

 proper underived caloric of any mass, such as we know exists 

 in the earth and forms the principal part of its caloric, is some 

 function of the mass, and is specifically greater as the mass is 

 greater. The Sun, which is the largest, is the hottest body 

 of our system. According to this view, if there be granite at 

 the surface of the moon, it will be more compressible than 

 the granite of the earth ; it will possess both a density and a 

 compressibility depending on the mean density and compres- 

 sibility of the moon's nucleus. 



The theory we have been considering requires us to believe 

 that the interior of a planet is solid, and not fluid. On the 

 supposition of fluidity, it would be difficult to account for the 

 contradiction presented by Mars to Clairaut's Theorem. May 

 we not conjecture, that this planet is hollow about its centre, 

 or in the direction of its axis? Generally speaking, the least 

 bodies of the solar system are the densest, if we set aside the 

 satellites, which seem to partake of the density of their pri- 

 maries. But Mars is not so dense as Mercury, Venus, or the 

 Earth. This fact favours in some degree the conjecture. . 



Assuming the truth of our theory, we may readily conceive 

 that any change in the state of the internal heat of the earth, 

 would give rise to great changes at its stuface, and perhaps 

 produce effects like those exhibited by geological phaeno- 

 mena. 



Upon the whole, a review of the planets seems to favour 

 the idea, that any increase of density towards their centres, is 

 owing either wholly, or at least in part, to the compressibility 

 of the matter of which their nuclei are composed. 



Papworth, St. Evcrard, Aug. 10, 1831. 



XXVII. Analysis 



