ON THE PHYSICAL STRUCTURE OF THE EARTH. 215 



Consequently the processional motion of such a shell would be the same 

 as that of a homegeneous spheroid of the same ellipticity. U e = 3^^, 

 it appears that the value of precession for such a splieroid would be 

 57", while its observed value is 50"- 1.* JTow, as it is impossible to 

 admit such a difference where the result of observation is so well estab- 

 lished, we must conclude that the solid shell of the Earth, composed of 

 nearly equielliptic strata, can not extend to its center— in other words, 

 that the Earth can not be altogether a solid from its surface to its center. 

 On the other hand, the tluid nucleus contained within the shell can not 

 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 properties of the liquid may, as I have long since an- 

 nounced, cause the shell and liquid nucleus to rotate together as one 

 solid mass. The same conclusion was afterward put forward by M, De- 

 lauuey; and experiments made under his direction, and afterward, 

 at the instance of the Koyal Irish Academy, by me, show that in rota- 

 ting 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 containing vessel insensible. With liquids so viscid that water 

 is in comparison limpid, such as pitch, honey, and especially volcanic 

 lava in a fused state, the results would be absolutely decisive. To 

 this class of li(|uids 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 connection 

 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 conse- 

 quences 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 sup- 

 posed to cause its rotation to become unstable, so as to bring about 

 conditions which might result in a change of the axis of rotation. It 

 is easy to show on the most favorable sui)positions that this could not 

 occur. The increasing ellipticity of the inner surface of the shell would 

 be due to the increasing oblateness of the surface of the fluid nucleus, 

 and this would be at its maximum if the neucleus approached a state 

 of homogeneity; but the fluid can not approach this state unless the 

 radius of the nucleus is so small that the variation in density due to 

 pressure becomes insensible, whence all its strata would possess the 

 same density. This condition with a certain thickness of the solid shell 



" A revision of the numerical data from recent astronomical results leads me to con- 

 clude that the precession for the solid spheroid vould be a little less, and about 55'' 

 instead of 57". This I propose to prove in a short paper, entitled "Note on the an- 

 nual precession calculated on the hypothesis of the earth's solidity." This note 

 [appended to this article] leaves the general conclusions of the present T>aper un- 

 altered. 



