174 AXNL'AL OF SCIENTIFIC DISCOVERY. 



the attractions of the sun and moon tend to produce on a solid mass in one 

 case, and a fluid mass on the other. It has been recently stated, as an ob- 

 jection to this investigation, that the interior fluid mass of the earth may 

 move in the same manner as if it were solid. The only reply which could 

 be given to such an objection was, Mr. Hopkins conceived, that it was 

 mechanically impossible that these motions should be the same, though the 

 resulting- processional motion for the solid crust, under certain conditions, 

 to be determined only by the complete mathematical solution of the prob- 

 lem, might be the same as if the whole mass were solid. The effect of the 

 attractions of the sun and moon also depends on the ellipticity of the inner 

 surface of the solid shell; and it has been said that since that ellipticity 

 depends on the law of the earth's density, which can only be imperfectly 

 known, no result can be depended on which involves that ellipticity. This 

 was not a correct statement of the problem. It was assumed, in the solution 

 referred to, that the ellipticity of the inner surface would depend partly on 

 the law of density, and partly on the forms of the isothermal surfaces. Mr. 

 Hopkins had supposed it possible, at the time he was engaged in this inves- 

 tigation, that a surface of equal solidity might approximate to a surface of 

 equal pressure ; he has now experimental reasons for believing that it mu>t 

 approximate much more nearly to an internal surface of equal temperature. 

 Now for depths greater, probably much greater, than those which have 

 often been supposed to correspond to the thickness of the earth's solid crust, 

 there is no doubt that the inner isothermal surfaces have a greater ellipticity 

 than the external surface itself; a conclusion which is independent of the 

 law of density. Hence, a like conclusion will hold with reference to the 

 internal surface of the shell, if it approximate sufficiently to the surface of 

 equal temperature; and this is the conclusion most unfavorable to the thin 

 shell supposed by some geologists. Restricting the interpretation, then, of 

 Mr. Hopkins's results to the question, whether the earth's solid shell be as 

 thin as some geologists have supposed, or at least several hundred miles in 

 thickness, and this is the only question of geological importance, Mr. 

 Hopkins denied the validity of either of the objections above stated. Thus, 

 both the modes of investigation which had been described lead to like con- 

 clusions respecting the least thickness which can be assigned to the solid 

 envelope of our globe. It must be much greater than geologists have fre- 

 quently imagined it to be. 



MOTION PRODUCED DIRECTLY BY HEAT. 



A new apparatus for producing motion in metals directly, by means of 

 heat, has recently been devised by Mr. C. Gore, of Birmingham, England. 

 It consists of a massive circular railway of copper, the rails of which are 

 made red-hot, and balls of German silver placed upon them, and so arranged 

 as not to run off. Whenever this is effected, the balls roll on the rails, mak- 

 ing revolution after revolution on the track, as long as the rails remain 

 sufficiently hot. 



OX EFFECTS OF HEAT ON DIFFERENT GASES. 



Dr. Tyndall, in a recent lecture before the Royal Institution, London, 

 stated that he had been engaged in a series of experiments to ascertain the 

 correctness of the views of M. Pouillet, respecting the effect of aeriform 



