THE EARTH. 3? 



grows hotter as we go deeper, till, at last, the labourers can scarce 

 bear any covering as they continue working. 



This difference in the air was supposed by Boyle to proceed from 

 magazines of fire that lay nearer the centre, and that diffused theii 

 heat to the adjacent regions. But we now know that it may be as- 

 cribed to more obvious causes. In some mines, the composition of the 

 earth all around is of such a nature, that upon the admission of water 

 or air, it frequently becomes hot, and often bursts out into eruptions. 

 Besides this, as the external air cannot readily reach the bottom, or 

 be renewed there, an observable heat is perceived below, without t he- 

 necessity of recurring to the central heat for an explanation. 



Hence, therefore, there are two principal causes of the warmth at 

 the bottom of mines : the heat of the substances of which the sides 

 are composed ; and the want of renovation in the air below. Any 

 sulphureous substance, mixed with iron, produces a very great heat, 

 by the admission of water. If, for instance, a quantity of sulphur be 

 mixed with a proportionable share of iron filings, and both kneaded 

 together into a soft paste, with water, they will soon grow hot, and at 

 last produce a flame. This experiment, produced by art, is very com- 

 monly effected within the bowels of the earth by nature. Sulphurs and 

 irons are intimately blended together, and want only the mixture 

 of water or air to excite their heat ; and this, when once raised, is 

 communicated to all bodies that lie within the sphere of their opera- 

 tion. Those beautiful minerals called marcasites and pyrites, are often 

 of this composition ; and wherever they are found, either by imbibing 

 the moisture of the air, or having been by any means combined with 

 water, they render the mine considerably hot.* 



The want of fresh air also, at these depths, is, as we have said, 

 another reason for their being found much hotter. Indeed, without 

 the assistance of art, the bottom of most mines would, from this cause, 

 be insupportable. To remedy this inconvenience, the miners are 

 often obliged to sink, at some convenient distance from the mouth 

 of the pit, where they are at work, another pit, which joins the former 

 below, and which, in Derbyshire, is called an air-shaft. Through 

 this the air circulates; and thus the workmen are enabled to breathe 

 freely at the bottom of the place ; which becomes, as Mr. Boyle 

 affirms, very commodious for respiration, and also very temperate 

 as to heat and cold.t Mr. Locke, however, who has left us an 

 account of the Mendip mines, seems to present a different picture. 

 " The descent into these is exceedingly difficult and dangerous ; for 

 they are not sunk like wells, perpendicularly, but as the crannies of 

 the rocks happen to run. The constant method is to swing down 

 by a rope placed under the arms, and clamber along by applying both 

 feet and hands to the sides of the narrow passage. The air is con- 

 veyed into them through a little passage that runs along the sides from 

 the top, where they set up some turfs, on the lea-side of the hole, to 

 catch and force it down. These turfs being removed to the windy 

 side, or laid over the mouth of the hole, the miners below presently 

 want breath, and faint ; and if sweet-smelling flowers chance to be 



Kircher Mand. Subt. vol. ii. p. 216. t Boyle, vol. iii. p. 258. 



