Henxessy — Oil tlie 2Ioh'cuIar Influence of Fluids. 57 



"when such, oscillations arc purposely produced, as "was done in some of 

 the experiments to which reference has been made. 



The researches on the motions of liquids which I have com- 

 menced are intended to elucidate the phenomenon of fluids in rotation, 

 under varying conditions. I was led to study the matter, chiefly from 

 its bearing upon a question regarding the structure of our earth and 

 the other bodies of our planetary system. Among astronomers and 

 mathematicians the figure of the earth had been long required as a 

 standing proof that it was originally in a fluid state. The arguments 

 of Playfaii" and Sir John Herschel to account for this figure by the 

 abrasion of a soKd globe under the action of denuding forces have 

 been long since shown by me, in a communication to this Academy,^ to 

 lead to results entii'ely inconsistent Avith observation. The results of 

 my calculations on this point have been subsequently confirmed by 

 •other mathematicians, and the question seems therefore to be finally 

 at rest. But although the primitive fluidity of our planet seems now 

 to be universally admitted, doubts have been put forward as to 

 whether any great portion of its interior is yet in a fluid state. As 

 this is obviously a question of the highest importance in geological 

 theory, and in any attempts at accounting for the phenomenon of 

 earthquakes and volcanos, its solution has long been under discus- 

 sion. The idea occurred to the late Mr. Hopkins, of Cambridge, that 

 if the greater part of the earth's interior were fluid, the resulting 

 motions of the earth's axis known to astronomers as precession and 

 nutation might be diiferent from what would take place in a perfectly 

 solid spheroid. He investigated the problem mathematically, with the 

 aid of the usual equations of hydro-dynamics and the equations of 

 rotation. His conclusions were that the earth was chiefly, or almost 

 wholly, solid. His formulae were deduced upon the supposition that 

 no friction, or resistance, existed between the solid and fluid parts of 

 the earth, and that no internal friction existed in the fluid. In the 

 words of Archdeacon Pratt, "The success of the calculation depends 

 upon a remarkable result at which he has arrived, that the precession 

 caused by the disturbing forces in a homogeneous shell, filled with 

 homogeneous fluid, in which the ellipticities of the inner and outer 

 surfaces are the same, is the same, whatever the thickness of the 

 shell. It is, therefore, the same for a spheroid solid to the centre." I 

 am now enabled to say, that if Mr. Hopkins' hypothesis of a fluid 

 with particles entirely destitute of resistance to their own motions, 

 and without friction against its solid envelope, be adopted, the result 

 alluded to by Pratt can be obtained by a very simple analysis. I hope 

 soon to be able to submit such a proof in another paper. The result is 

 one of some importance, for, combined with another to which I had 

 been led, it shows that the earth cannot be entirely solid, and that it 

 is, for the most part, a mass of fluid contained within a shell of undc- 



' Proceedings of tlie Royal Irish Academy," vol. iv., seiics i., p. 



333. 



