Physical Structure of the Earth. 249 



or 



d ~ C ' 



Consequently the precessional motion of such a shell would 

 be the same as that of a homogeneous spheroid of the same 



ellipticity. If e= ofvTy it appears that the value of precession 



for such a spheroid would be 57", while its observed value is 

 50"* 1*. Now, as it is impossible to admit such a difference 

 where the result of observation is so well established, we must 

 conclude that the solid shell of the Earth, composed of nearly 

 equielliptic strata, cannot extend to its centre — in other words, 

 that the Earth cannot be altogether a solid from its surface to 

 its centre. On the other hand, the fluid nucleus contained 

 within the shell cannot be devoid of friction and viscidity, but 

 must possess these properties in common with all fluids that 

 have ever been observed on the Earth's surface. These pro- 

 perties of the liquid may, as I have long since announced, 

 cause the shell and fluid nucleus to rotate together as one solid 

 mass. The same conclusion has been afterwards put forward 

 by M. Delauney ; and experiments made under his direction, 

 and afterwards, at the instance of the Royal Irish Academy, 

 by me, show that, in rotating glass vessels filled with water, 

 the amount of friction and viscidity is such as to render any 

 difference of slow motion between the liquid and its contain- 

 ing vessel insensible. With liquids so viscid that water is in 

 comparison limpid, such as pitch, honey, and especially vol- 

 canic lava in a fused state, the results would be absolutely 

 decisive. To this class of liquids the fluid matter of the Earth's 

 interior, so far as it has come under observation, undoubtedly 

 belongs ; and hence the overwhelming certainty of our general 

 conclusions as to the connexion between the Earth's structure 

 and its rotation. 



( 8) If the tendency of the solid crust is to become more 

 elliptical at its inner surface as it increases in thickness, some 

 interesting consequences appear to follow. If the shell were 

 unaccompanied by the nucleus, or if no friction existed at 

 their surfaces, the changes in the relations of the principal 

 moments of inertia of the shell might be supposed to cause 



* A revision of the numerical data from recent astronomical results 

 leads me to conclude that the precession for the solid spheroid would be a 

 little less and about 55" instead of 57". This I propose to prove in a short 

 paper, entitled " Note on the Annual Precession calculated on the hypo- 

 thesis of the Earth's Solidity." This note leaves the general conclusions 

 of the present paper unaltered. 



Phil. Mag. S. 5. Vol. 22. No. 136. Sept. 1886. S 



