240 POPULAR LECTURES AND ADDRESSES. 



By a simple effort of geological calculus it has 

 been estimated that \ J per 30 metres gives 1000 

 per 30,000 metres, and 3,333 per 100 kilometres. 

 This arithmetical result is irrefragable ; fcut what 

 of the physical conclusion drawn from it with 

 marvellous frequency and pertinacity, that at 

 depths of from 30 to 100 kilometres the tem- 

 peratures are so high as to melt all substances 

 composing the earth's upper crust ? It has been 

 remarked, indeed, that if observation showed any 

 diminution or augmentation of the rate of in- 

 crease of underground temperature in great 

 depths, it would not be right to reckon on the 

 uniform rate of i per 30 metres or thereabouts 

 down to 30 or 60 or 100 kilometres. " Hut ob- 

 servation has shown nothing of the kind ; and 

 therefore surely it is most consonant with induc- 

 tive philosophy to admit no great deviation in 

 an}- part of the earth's solid crust from the rate 

 of increase proved by observation as far as the 

 greatest depths to which we have reached ! " 

 Now I have to remark upon this argument that 

 the greatest depth to which we have reached 



