IS THE EARTH A SOLID BODY^ 3 



deposits ; moreover it would seem that they must come from a portion that 

 has either never solidified, or which through some cause has been reliquefied. 

 Here, then, it will be desirable tliat some examination should be made of 

 the evidence derived from physical and mathematical laws on which is based 

 the opinion held by many that the earth is solid. 



This evidence may be considered under two divisions. 1°. That derived 

 from the phenomena of precession and nutation, and of the tides. 2°. That 

 derived from the action of matter under the combined influence of heat and 

 pi'essure. 



In the first case, the conclusions which have been reached have been 

 obtained by assuming certain hypothetical globes with a certain definite 

 structure, substituting for these the name earth, and then claiming that the 

 conclusions applied to tlie actual earth instead of to the Ivipothctical globes, for 

 which the name earth was used just as the algebraist uses x and i/. Hop- 

 kins assumed for his globes : 1°, a homogeneous fiuid mass enclosed in a 

 homogeneous solid shell ; 2", a heterogeneous fluid mass enclosed in a hetero- 

 geneous solid shell. The transition between tlie entire solidity of the shell 

 and the perfect fluidity of the interior mass was assumed by him as being 

 an abrupt one. He further assumed that the circulation would go on in 

 the mass until it lost its perfect fluidity in every part at nearly the same 

 moment.* 



Sir William Thomson, in the same way, drew his conclusions from globes 

 assumed to have a thin shell, passing abruptly either into a homogeneous 

 incompressible fluid, mobile like water; or into a heterogeneous viscid 

 fluid interior.! 



Likewise Professor George H. Darwin has taken as the basis for his dis- 

 cussions, if he is not misunderstood, homogeneous spheroids which are vis- 

 cous and non-el;istic, also those which are elastico-viscous, and those whicli 

 are either elastic, plastic, or viscous.^ 



The view that the phenomena of precession and nutation prove the 

 earth to be solid was opposed by Hennessy,§ Delaunay, || Newcomb and 



* Pliilos. Trans., 1839, pp. 381-423; 1840, pp. 103-208 ; 1842, pp. 43-55. 



t Trans. Roval Soc. Edin., 1864, xxiii. 157-169; Phil. Mag.,lS63 (4), xxv. 1-14,149-151; Pliil. 

 Trans., 1863, pp. 573-582; Trans. Geol. Soc. Glas., 1878, vi. 38-49; Nat. Phil., 1867, I. 670-727; 

 Kature, 1872, v. 223-224, 257-259. 



X Phil Tians., ISSO, elxx. 1-35,447-593; 1SS2, clxxii. 187-230. 



§ Phil. Trans., 1851, pp. 495-547; Katuvc, lb71, iii. 420; 1872, v. 288, 289 ; Geol. Mag., 1871 (1), 

 viii. 216-218. 



II Geol. Mag., 1868 (1), v. 507-511. 



