754 



SCIENCE, 



[N. S. Vol. XII. No. 307. 



the ground.' It is doubtful whether we 

 have yet discovered a rate that in any use- 

 ful sense can be spoken of as ' average.' 

 The widely divergent views of different 

 authorities as to the presumed value of this 

 rate may well lead to reflection. The late 

 Professor Prestwich thought a rise of 1° F. 



recorded measurements would, I believe, 

 lead to a rate of 1° F. in 80 or 90 feet as 

 more closely approaching the mean. This 

 would raise Lord Kelvin's estimate to 

 nearly fifty millions of years. 



When from these various averages we 

 turn to the observations on which they are 

 based, we encounter a surprising di- 

 vergence of extremes from the mean ; 

 thus in the British Isles alone the rate 

 varies from 1° F. in 34 feet to 1° F. 

 in 92 feet, or in one case to 1° F. in 

 130 feet. It has been suggested, and 

 to some extent shown, that these ir- 

 regularities may be connected with 

 differences in conductivity of the rocks 

 in which the observations were made, 

 or to the circulation of underground 

 water ; but many cases exist which 

 cannot be explained away in such a 

 manner, but are suggestive of some 

 deep-seated cause, such as the distri- 

 bution of molten matter below the 

 ground. Inspection of the accom- 

 panying map of the British Isles, on 

 which the rates of increase in different 

 localities have been plotted, will af- 

 ford some evidence of the truth of this 

 view. Comparatively low rates of in- 

 crease are found over Wales and in 

 the province of Leinster, districts of 

 relatively great stability, the remnants 

 FIG. l.-Map of the British Isles, showing the distribution ^^ ^J' inland that have in all prob- 

 of rates of increase of temperature with descent. The rates ability StOod above the sea ever 

 are taken from the 'British Association Eeport,' except in since the close of the Silurian period. 



To the north of this, as we enter a 

 region which was subject to volcanic dis- 



the case of those in the south of Ireland. 



for every 45 feet of descent below the zone 

 of constant temperature best represented 

 the average ; Lord Kelvin in his earliest es- 

 timates has adopted a value of 1° F. for 

 every 51 feet ; the committee of this asso- 

 ciation appointed to investigate this ques- 

 tion arrived at a rate of 1° F. for every 60 

 feet of descent; Mr. Clarence King has 

 made calculations in which a rate of 1° F. 

 for 72 feet is adopted ; a re- investigation of 



turbances during the Tertiary period, the 

 rate increases. 



It is obvious that in any attempt to esti- 

 mate the rate at which the earth is cooling 

 as a solid body the disturbing influence of 

 subterranean lakes of molten rock must as 

 far as possible be eliminated ; but this will 

 not be effected by taking the accepted mean 

 of observed rates of increase of tempera- 



