352 Applications of Physics and Mathematics to Geology. 



Professor Prestwich, may be strongly urged against Roche's 

 investigations. 



Some remarks of M. Roche's, on his pp. 240, 24 1, throw con- 

 siderable light on his standpoint and that of many other 

 theorists : — " Les astronomes qui persistent a admettre la 

 fluidite .... cherchent a eluder les objections de Hopkins et 

 de Thomson, en attribuant . . . . au liquide central une vis- 

 cosite assez grande pour que .... P ensemble en arrive a 

 tonrner tout d'une piece .... La masse tournante offre une 

 telle rigidite qu'elle est assimilable sous ce rapport a un bloc 

 solidifie, mais admettre cette assimilation revient a depouiller 

 le milieu interne des proprietes ordinal res des liquides, et a 

 lui en conserver le nom tout en Pidentifiant a un corps solide." 

 He proceeds to point out that the mere question of a name is 

 of no account, considering our ignorance of what would be 

 the properties of matter under such pressures and at such a 

 temperature as the theory of fluidity would lead to. His line 

 of argument is not very clear, but there is no hesitation appa- 

 rent in his conclusion : — " En effet, la pression supportee 

 par les couches centrales, dans la supposition d'une complete 

 fluidite, depasserait deux millions et demi d'atmospheres. La 

 grandeur meme de ce nombre est a elle seule une objection 

 peremptoire a l'hypothese qui y conduit." 



Such a position as this may be all very well for a philo- 

 sopher who supposes the external world a mere idea, the 

 private property of his own mind and so necessarily obedient 

 to laws which his understanding can fully grasp, or for a 

 scientist who believes the earth created for the special purpose 

 of supplying problems of precisely that amount of difficulty 

 which be personally is able to solve, but from a common-sense 

 point of view it seems utterly irrational. No physicist or 

 geologist has any reason to suppose that there are not nume- 

 rous problems whose full comprehension requires more exten- 

 sive knowledge than is possessed by himself or any of his 

 contemporaries. 



The necessity for theories has been eloquently urged by 

 Professor Darwin*, who fays " A theory is, then, a necessity 

 for the advance of science, and we may regard it as the branch 

 of a living tree, of which facts are the nourishment." Employ- 

 ing this simile, I must confess tbat the subject treated in this 

 paper resembles, in my opinion, a tree which combines a sad 

 deficiency of sap with a great superfluity of branches. It will, 

 I dare say, be generally admitted that the premature craving 



* 'Nature/ vol, xxxiv. (1886) p. 420, Address to British Association, 

 Section A. 



