﻿136 Level of no Strain within a Solid Earth. 



will consequently be slightly diminished instead of being 

 increased by the more accurate hypothesis of sphericity. 



The present investigation is a sufficient reply to Professor 

 Blake's objection that u English writers have hitherto adopted 

 Lord Kelvin's assumption that the earth may be regarded as 

 of infinite radius, and have thus followed him in using linear 

 equations for the conduction of heat. But when later writers 

 go further, and at the same time introduce the radius of the 

 earth, already assumed infinite, into the calculation, they are 

 obviously inconsistent, and it may well be that the whole 

 of the results are derived from this inconsistency itself." 

 Plausible as this criticism at first sight appears, I have now 

 shown that it is altogether unimportant, seeing that the 

 position of the level of no strain is not appreciably altered 

 when the inconsistency complained of is avoided. At the 

 same time, such alteration as its removal introduces tends 

 in the opposite direction to that which Mr. Blake appears to 

 expect, bringing, as it does, the level of no strain nearer to 

 the surface, and therefore making the resulting elevatory 

 corrugations smaller than before. It may be as well to 

 remind the reader that the depth of the level of no strain 

 in a solid globe does not follow the same law as the radial 

 contraction. The former varies as the time, and therefore 

 the rate at which it descends is constant. The latter varies 

 as the square root of the time, so that the rate at which the 

 surface sinks diminishes as the time increases. 



Professor Blake concludes his critique with a formidable 

 list of difficulties to be overcome before " we can really face 

 the problem of a level of no strain/' If it is required to 

 find what its situation would be with accuracy, even within a 

 mile or so, no doubt we are not, and probably even our 

 remote descendants will never be, in a position to do so. 

 But this is obscuring the real issue. The reference to " the 

 positive teachings of geological facts " is beside the question. 

 In this connexion these merely tell us that the superficial 

 strata have again and again been ridged up by lateral pressure. 

 But some minds will not rest satisfied with this "positive 

 teaching" without seeking the cause of this phenomenon. 

 That it is due to the contraction of the globe through cooling 

 is an hypothesis seemingly simple and by prescriptive right 

 orthodox, but not necessarily true ; while the arrangement 

 of the axes of elevation in lines following undeviating direc- 

 tions for immense distances, instead of forming a network of 

 polygons, by no means favours it. But the discovery of a level 

 of no strain affords perhaps the strongest argument against this 

 theory, more especially if the earth is assumed to be solid, as 



