84 LIGHT SCIENCE FOR LEISURE HOURS. 



colliery in Belgium has nearly approached that depth, and 

 no inconvenience is experienced by the miners.' But 

 the Commissioners state that at a depth of only 2,419 

 feet in the Eosebridge mine (the deepest in England), 

 the temperature is 94 degrees of Fahrenheit, or within 

 four degrees of blood heat. ' The depth at which the 

 temperature of the earth would amount to blood heat/ 

 they add, is about 3,000 feet.' They express a belief 

 that by the ' long wall system ' of working (a system 

 as yet seldom adopted in the chief northern mines) it 

 will be possible to reach a depth of 3,420 feet before this 

 heat is attained ; but it is by no means certain that this 

 will prove to be the case. 



On the other hand, it will be well to regard the 

 more promising aspect of the question. 



We must not forget, in the first place, that in all 

 matters of statistical research there is room for mis- 

 apprehension unless careful attention be paid, not 

 merely to the observed facts, but to the circumstances 

 with which those facts are more or less intimately asso- 

 ciated. If we consider, for example, the progress of the 

 consumption of our coal during the past fifteen years, we 

 find that a law of increase exists, which is, as we have 

 seen, easily expressed, and which, after being tested 

 by a process resembling prediction, has been singularly 

 confirmed by the result. But if we inquire into the 

 various causes of the great increase in the consumption 

 of coals, we find that while those causes have been 

 increasing in activity so to speak to a degree quite 

 sufficient to explain the observed consumption, they are 



