IS THK EAIiTlf A SOLID 1',m1jV ; 3 



deposits; moreover it would seein that they riiust como from a portion that 

 has either never soHdified, or wijich through some cause has been rehquefied. 

 Here, then, it will be desirable that 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 inlluence of heat and 

 pressure. 



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 the actual earth instead of to the hiJixjlJuticnl t/lohrx, for 

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

 kins assumed for his globes : 1°, a homogeneous fluid ma.ss enclosed in a 

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

 geneous solid shell. The transition between the 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.t 



Likewise Professor George H. Dar^vin has taken as the basis for his dis- 

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

 cous and non-elastic, also those which are elastico-viscous, and those which 

 are either elastic, plastic, or viscous.J 



The view that the phenomena of precession and nutation prove the 

 earth to be solid was opposed by Hennessy,§ Delaunay.|| Newcon\b and 



* Philos. Trans., 1S39, pp. 381-423; 18W, pp. 193-208 ; 18-42, pp. 43-55. 



t Trans. Koval Soc. Ediu., 18G4, xxiii. 157-109; Phil. Mag., lSr.3 (t), xxv. 1-14,119-151; Phil. 

 Trans., 1803, pp. 573-582; Trans. Gcol. Soc. Glas., 1S78, vi. 38-19; Nat. Phil., 1S67. I. 670-727; 

 Nature, 1872, v. 223-221, 257-259. 



+ Phil. Trans., 1880, clxx. 1-35,447-593; 1882, clxxii. 187-230. 



§ Phil. Trans., 1851, pp. 195-547; Nature, 1871, iii. 420; 1S72, v. 288, 289; Geol. Ma?., lS/1 (1). 

 viii. 216-218. 



II Geol. Mag., 1808 (1), v. 507-511. 



