I907.] AND CONTRACTION OF THE EARTH. 219 



through I ° Fahr. ; and A is the latent heat measured in thermal units 

 centigrade, water being the standard substance. In the appendix, p. 

 20, Fisher shows that Riicker's experiments make A =49.60, the 

 corresponding value for water at zero centigrade being 79.25. Taking 

 the mean specific heat of average rock at 0.22, he makes A = 406° 

 Fahr., and F= 1688° Fahr. This is the temperature at which basalt 

 is entirely melted. Finally, the quantity 3; is a function representing 

 the activity of the substratum, and therefore zero when the layer is 

 inert (cf. Chapter vl., p. 73). 



Using these values Fisher finds for an inert substratum, that the 

 true values are : 

 ^=1.007; M = 0.7493736; and the thickness of the crust 



51 X 1688 X 1.007 



5280 X 0.74937 



21.91 miles. (25) 



He concludes that the least thickness of the crust will be -7.^ 16.30 



miles, and that the true thickness of the crust will lie between 16.30 

 and 21.91 miles. If we take k^= 17.5 miles, which is very near the 

 thickness of the crust indicated by the great earthquake which de- 

 vastated San Francisco, and apply formula (24) we find for the age 

 of the earth 



k\S2^oY 



Using ^ = 21.91 miles, in the same formula, Fisher finds 



/= 8,248,380 years. (26) 



This age for the encrustation of the earth seemed to him surprisingly 



small, and he therefore remarked : 



"This is a far shorter period than geological phenomena appear to 

 require, for although it is not possible for them to assign any definite limit 

 to the world's age, we can form some idea of an inferior limit which it must 

 have exceeded. Sir A. Geikie thinks that the stratified rocks alone, which 

 contain organic remains, can not have taken much less than 100 million 

 years for their formation." 



The Rev. O. Fisher then proceeds to examine the hypothesis of 



an energetic substratum, and by this process reaches a greater age 



for the world. But for reasons pointed out in the previous paper on 



the cause of earthquakes, the hypothesis of an inert substratum is 



