GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



405 



tion is made from one level to another, by means of a 

 shaft or ^' winze," or from one shaft to another by a level, 

 even in the deepest parts, there is no want of air under 

 ground. The current of air is frequently so strong, even 

 towards the bottom of the mine, as sometimes to blow out 

 a candle ; in fact, no deficiency of air is found under 

 such circumstances, except when a level is extended a 

 considerable way from the shaft without having a com- 

 munication with any other part; or in a shaft sunk far 

 below a level ; in which case recourse is had to some ar- 

 tificial means to procure air in such a position. In gal- 

 leries of great length and where there is not very frequent 

 communication with other levels or with more than shaft 

 and a level, the current of air is often scarcely percepti- 

 ble, although the air may be good. 



In sinking deep shafts or driving long galleries, where 

 the air becomes vitiated by respiration and combustion 

 and the gases generated by the explosion of powder, 

 artificial currents are produced by air pipes, &c. In 

 galleries which have been long abandoned and whose 

 communication has been partially stopped, bad air often 

 accumulates; except in coal mines, no explosion from 

 contact with fire occurs, owing to the absence of hydro- 

 gen gas. 



It was stated in the preceding page that there is 

 a progressive increase in the temperature of mines as 

 we descend in depth. It is not to be pretended that 

 this can be accounted for by animal heat of the miner, 

 combustion of candies, explosion of gunpowder, friction 

 and percussion. These, no doubt, have some influence, 

 but are decidedly unable to account for those startling 

 facts that are about to be stated. 



The most conclusive proof of the increase of tempera- 

 ture, as we recede from the surface of the earth, is af- 

 forded by the relative temperature of the pump water of 

 the same mines at diiferent times. In one instance in 



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