•yS BULLETIN OP THE 



pressure sometimes exhibited in volcanoes not very far distant 

 from each other, he did not see why obstructions or even irregular 

 "cellular" conditions were not quite as reconcilable with a thin 

 crust, as with a very thick or solid one. He considered the 

 chemical and hydrothermal theories as supported by very slender 

 probabilities, and as having altogether the character of pro re 

 nata speculations. 



Secondly, with regard to the "precession" argument, he had 

 gradually come to the conviction that we have not at present the 

 physical data to permit a true mathematical solution of the prob- 

 lem ; and that, however refined and ingenious these attempted 

 mathematical investigations, they are entirely inconclusive. This 

 appears to be the judgment of Gen. Barnard in his IMemoir on 

 the subject, published in the Smithsonian Contributions (vol. 19). 

 The argument, in short, wheu analyzed, is found to be both irrel- 

 evant and fatally supererogatory ; it proves entirely too much. 

 Thus Sir William Thomson, in reenforcing the view of Mr. Hop- 

 kins, requiring as a minimum thickness 1000 miles, has gone 

 very much further, and has reached the conclusion that if the 

 earth were a globe of solid steel, it would be subject to solar and 

 lunar tides, at least half as large as if formed entirely of water ; 

 and that, accordingly, wath even this rigidity, the precession and 

 nutation would not be more than three-fifths of the amount re- 

 quired by observation, which is that due to perfect rigidity. Is 

 not this manifestly an impossible condition ? 



Now it must not be forgotten here that the conclusion reached 

 by Prof. Thomson is absolutely incompatible with the same emi- 

 nent geometer's discussion of the present interior heat of our 

 planet, in his most able Memoir " On the Secular Cooling of the 

 Earth," published only a year earlier. The truth is that in any 

 very large mass of matter, "solidity" becomes a merely relative 

 term — cohesion exerting its bond only between adjacent molecules 

 with a fixed limit of force — while gravity, the weakest of natural 

 forces, by pervading all thicknesses, becomes indefinitely prepon- 

 derant by mere accumulation. The strongest steel wire has not 

 a tensile modulus of eighteen miles The dynamic rigidity of a 

 rapidly rotating planet therefore, combined with even a very small 

 amount of internal friction, must be vastly greater than any static 

 rigidity of cohesion as known to us could possibly be. An irreg- 

 ular mass of granite as large as our earth would by "solid flow'^ 



