20 



PRESIDENT S ADDRESS. 



to a crust not exceeding 45 miles in thickness and that the 

 temperature from that point to the earth's centre is not greater 

 than about 1500°C.(273^:°F.). 



Mr. 0. Fisher has examined the subject more in detail.* 

 Taking the two most commonly accepted values for the temper- 

 ature gradient of the earth's crust at the surface, that of Prestwich, 

 which is 1°F. for each 42-2 feet of descent, and the more modern 

 one of 60 feet for a similar rise of temperature,! he shows that 

 the amount of radium required is a diminishing quantity down- 

 wards, which is expressed in the following table : — 



The figures for °F. given by Mr. Fisher are in some cases not 

 in agreement with the °C.; these 1 have corrected in above table. 



Mr. Fisiier points out that Professor Bartoli ascertained the 

 temperature of lava flowing from Mount Etna to be lOBCC. 

 (1940°F.), corresponding to a depth of from 30 to 40 miles, 

 according to which of the above temperature gradients is adopted. 

 The 45 miles thickness of crust adopted by Mr. Strutt agrees 

 very well with that arrived at by Professor Milne from a study 

 of the propagation of earthquake waves. 



The values for radium content in Mr. Fisher's table come well 

 within the scope of the actual amounts ascertained by Mr. Strutt 

 to exist in accessible rocks. 



* Nature. Ixxiv., 11 Oct. 1906, p. 585. 

 t In my address, Proc. Linn. Soc. N. S.Wales, 1905, p.618,T took the 

 mean of these, viz., 51 ft. Prof. Gregory, Chem. News xciv., Sept. 21, 1906, 

 p. 143, adopts 55 feet. 



