696 WILLIAM HERBERT HOBBS 



should be reached at that level. 1 From actual sounding of the 

 atmosphere we now know that the convective zone within which 

 the temperature gradient is essentially adiabatic ends abruptly 

 at an altitude ranging from 9 to 18 km., and that the envelope 

 extends to at least 70 km., 2 with nearly isothermal conditions above 

 the convective zone. 



From these examples and others which might be cited, we should 

 learn that while the methods of " exact science" may not be lacking 

 in precision, the assumptions which enter into the solutions, whether 

 they are used consciously or not, possess the same measure of falli- 

 bility as those employed in other fields of science. It is therefore 

 with assumptions unconsciously made by advocates of isostatic 

 compensation that this paper will deal. 



Hayford' s negative argument for a failing earth, based upon data 

 now shown to be inapplicable. — In his paper entitled "The Earth 

 a Failing Structure," Hayford has shown us how his conclusions 

 and those of the late Sir George Darwin dealing with the same sub- 

 ject are diametrically opposed to each other, for the reason that 

 the basal assumptions differ so widely. In the same paper six 

 negative reasons are given why the earth must be a failing structure; 

 that is, be incapable of supporting its protuberances by virtue of 

 its rigidity. These reasons may be reduced to one and stated in 

 this form : Even if the earth throughout had the strength of granite, 

 it would upon the basis of known tests be incapable of supporting 

 without failure the loads upon it. Since this statement was made, 

 studies by Bridgman 3 and Adams 4 have shown that under hydro- 

 static conditions of compression such as must be conceived to exist 

 within the earth the crushing strengths of materials are enormously 

 enhanced over those derived from tests in which no lateral con- 

 straint is imposed — the data employed by Hayford. The studies 



1 Cf. A. Wegener, Thermodynamik der Atmosphdre (1911), pp. 109, 185. 

 3 W. S. Bruce, Polar Exploration (London, 1911), pp. 210-11. 



3 P. W. Bridgman, "The Collapse of Thick Cylinders under hydrostatic Pressure," 

 Physical Review, XXXIV (January, 191 2), 1-24. 



4 F. D. Adams, "An Experimental Contribution to the Question of the Depth 

 of the Zone of Flow in the Earth's Crust," Jour. Geol., XX (February-March, 1912), 

 97-118. See also L. V. King, "On the Limiting Strength of Rocks under Conditions 

 of Stress Existing in the Earth's Interior, ibid., pp. 119-38. 



