140 



changing outlets. From this would arise great and variable resist- 

 ances, retarding, diverting, and even arresting the movement pro- 

 pagated from below, just as when the ocean tides are transmitted 

 through narrow and ramifying passages, or when they reach open 

 spaces through different channels and in opposite phases, we find 

 the tidal phenomena greatly modified, and sometimes even 

 entirely destroyed. 



Recurring to M. Perrey's researches. Prof. Rogers remarked, 

 that the results, if confirmed by a fuller induction, would be of 

 the utmost importance to geological theory : first, by setting at 

 rest any doubts that may exist as to the igneous fluidity of the 

 interior of the globe ; and secondly, by demonstrating the great 

 thinness of the earth's crust, which alone could make it sensitive 

 to the tidal movements of the molten mass beneath. Such tenuity 

 of the crust had long since been urged by Prof. H. D. Rogers 

 and himself, as inferable from the arched and folded structure of 

 mountain chains, as well as from the wavelike motion in earth- 

 quakes ; but geologists are still far from being agreed on this, 

 and the allied points relating to internal heat. Even the general 

 fact of an increasing temperature, as we descend below the sur- 

 face, although leading directly to the inference of an intense heat 

 within the earth, has not been accepted by all, as proving the 

 existence of 3l fluid nucleus, and among those who admit the 

 latter conclusion as demonstrated, there are many who contend 

 that the solid crust, instead of being some thirty miles, cannot be 

 less than eight hundred miles in thickness. 



This last estimate of the thickness of the earth's crust, de- 

 duced by Prof. Hopkins, of Cambridge, Eng., from mathematical 

 considerations connected with the precession of the equinoxes, 

 had apparently been accepted by Sir Charles Lyell as a basis of 

 geological argument, but Prof. Rogers looked upon it as belong- 

 ing to a class of inferences, which are more of the nature of 

 ingenious mathematical exercises on physical problems, than 

 expressions of the facts or laws of nature. 



Such problems often involve mechanical conditions, too various 

 and complex to be amenable even to the most profound analysis ; 

 so that, to bring them within his grasp, the mathematician is 

 compelled to resort to simplifying hypotheses, and in doing so, 



