286 MAN AND NATURE. 



perature in coming nearer the central heat of the 

 globe. Here, indeed, we must note one of the most 

 serious obstacles to farther penetration downwards. 

 It is now well known, from observations in mines and 

 artesian wells, that the increase of heat below what 

 may be called the stationary line of temperature is at 

 the rate of l e of Fahrenheit for every 60 or 65 feet of 

 increasing depth. In several deep copper and coal 

 mines the average heat of the lowest workings reaches 

 80 to 85; and one instance, in the Poldice Mine, is 

 noted by Mr. Fox, an eminent authority on this sub- 

 ject, where the thermometer rose to nearly 100, a 

 temperature incompatible with any form of profitable 

 or even possible human labour. 



It may seem a small matter to speculate upon, and 

 yet when speaking of changes effected on the earth by 

 human action we cannot discard the effects of forty or 

 fifty million tons of coal burnt every year upon the 

 English soil on which we are living. It is the transla- 

 tion, from within the earth to without, of this enormous 

 amount of carbonaceous matter, with its various che- 

 mical adjuncts. Though not well able to say how all 

 this is disposed of in its various later combinations, we 

 may at least affirm that a substance like carbon, so 

 large a constituent of life in all its forms, and .having 

 such endless relations to other chemical elements, can- 

 not be wholly inert in the addition it thus makes to the 

 surface without. This is one of the cases where 

 eventual effects may differ from, and go beyond, those 

 more directly obvious to the eye. 



Such is a mere outline of the changes, taking our 



