ON GEOLOGICAL TIME. 61 



wood, several bores in the neighbourhood of his 

 house have been put at the disposal of the com- 

 mittee of the British Association, to which I have 

 referred. In one of these bores very accurate 

 observations have been made, showing an increase 

 of temperature downwards, but which is not ex- 

 actly the same in all the strata, the difference 

 being, no doubt, due to different thermal conduc- 

 tivities of their different substances. I need not 

 specify minutely the numbers, but I may say in a 

 general way, that the average increase is almost 

 exactly -2$ of a degree Fahrenheit per foot of de- 

 scent ; which agrees with the estimate generally 

 admitted as a rough average for the rate of increase 

 of underground temperature in other localities. 



Another bore has been put at the disposal of the 

 committee, and the investigation of it is to be com- 

 menced immediately, so that I hope in the course 

 of a few days some accurate results will be got. It 

 has been selected because the mining engineer 

 states in his report that the coal has been very 

 much burned or charred, showing the effect of 

 heat ; and it becomes an interesting question, Are 

 there any remains of that heat that charred the 



