562 Chronicles of Science. [Oct., 



as they have gone, the results are re-assuring. I concur in the 

 probaLle accuracy of the announcement lately made by two of my 

 fellow commissioners, that the total quantity of coal in this island 

 will prove to be practically inexhaustible ; but until the complicated 

 details of quantities collected by the commission have been put 

 together, and expressed in totals, it is difficult to judge with cer- 

 tainty or accuracy on the subject. Although the duration of our 

 coal may, geologically speaking, be practically unhmited, we have 

 still to consider the important question. How long will England be 

 supplied with coal as good and as cheap as at present ? We have 

 unquestionably made greater inroads into our best and most acces- 

 sible coal-beds than other nations have done into theirs; and if 

 foreign coal should grow better and cheaper, and ours dearer and 

 v^orse, the balance may turn against us as a manufacturing country 

 long before our coal is exhausted in quantity. It is clear that our 

 stock of good coal is very large, but most of it lies at gi'eat depths ; 

 and one of the most important questions the Eoyal Commission has 

 to investigate is the depth at which coal can be worked with com- 

 mercial advantage. The chief obstacle to reachmg extreme depth 

 is the increase of temperature which is met as we descend. I am 

 justified, by ascertained facts, in saying that this rate of increase 

 will, as a rule, prove to be not less than 1° Fahi-enheit for every 

 20 yards in depth, and there is reason to expect that it will be even 

 more rapid at greater depths than have yet heen. attained. The 

 constant temperature of the earth, in this climate, at a depth of 

 50 feet, is 50° ; and the rate of increase, as we descend, is to be 

 calculated from this starting-point. Adopting these figures, you 

 will find that the temperature of the earth will be equal to blood- 

 heat at a depth of about 980 yards, and at a further depth of 500 

 yards, mineral substances will be too hot for the naked skin to 

 touch with impunity. It is extremely difficult to form an opinion 

 as to the maximum temperature in which human labour is practi- 

 cable in the damp atmosphere of a mine, and it is almost equally 

 difficult to determine how much the temperature of the air, in the 

 distant part of an extremely deep mine, can be reduced below that 

 of the strata with which it is brought in contact. It is certain, 

 however, that the limit of practicable depth will chiefly depend 

 upon the mechanical means which can be provided for reheving 

 the miners of the severest part of their labour ; for maintaining a 

 supply of sufficiently cool air at the working faces of the coal ; and 

 for superseding the use of horses, which sufter even more than men 

 from highly-heated air. For the relief of labour we must look to 

 coal-cutting machines ; for improvement of ventilation to exhaust- 

 ing fans; and for the superseding of horses, to hauling engines 

 driven by transmitted power. The employment of coal-cutting 

 machines, working by compressed air, convey(?d into the mine by 



