THE DOCTRINE OF ISOSTATIC COMPENSATION 695 



Hayford's categorical statements reach their culmination in the 

 following : 



These are the facts, established by abundant geodetic evidence. These 

 facts may not be removed or altered by showing that difficulties are encountered 

 when one attempts to make them fit existing theories, geological or otherwise. 

 The theories must be tested by the facts and modified if necessary [p. 201]. 



Methods and assumptions within the field of the exact sciences. — 

 Inasmuch as Dr. Hayford's statement last cited raises a question 

 concerning the comparative reliability of the data obtained from 

 geodetic and from geological observations, it may perhaps be 

 pointed out that geologists have before been warned of the fallacy 

 of their conclusions by workers in the field of the so-called exact 

 sciences. Two fairly recent instances will suffice, though it would 

 be easy to cite others. 



With a degree of assurance which was perhaps warranted by 

 his pre-eminence in research, the late Lord Kelvin served notice 

 upon geologists that the longest period that could by any possi- 

 bility be allowed them as representing time since the beginning of 

 life upon the globe was a hundred million years, with a probability 

 that it could not exceed twenty million years ; r and this figure was 

 soon reduced by Professor Tait to ten million years. 2 With the 

 question of whether this allowance is adequate we are not at the 

 moment concerned, since the developments in the realm of physics 

 have destroyed the value of Kelvin's argument. At the meeting of 

 the British Association held at Winnipeg in 1909, Sir J. J. Thomson, 

 referring in his presidential address before that body to studies of 

 radium by Rutherford and others, showed how Kelvin's argument 

 was based upon incomplete evidence and must now be abandoned 

 in view of the new light which has been shed upon the problem. 3 



It was Helmholtz, another conspicuous champion of the exact 

 sciences, who stated that the atmospheric envelope of the earth 

 could not extend above an altitude of 27 or 28 km., since the tempera- 

 ture gradient required that the absolute zero of temperature 



1 Sir William Thomson, "The Age of the Earth," annual address to Victoria 

 Institute, Phil. Mag., 1899, p. 66; also, Popular Lectures and Addresses, II, 64. 



2 Recent Advances in Physical Science, p. 174. See rejoinder by A. Geikie, Land- 

 scape in History and other Essays (1905), pp. 206-8. 



3 Rept. Brit. Ass'n Adv. Sci., Winnipeg Meeting, igog (19 10), pp. 27-28. 



