. XXVI. HEAT IN MINES AND WELLS. 243 



5 or 6, and that if the heat could be attributed to this 

 cause, the seasons would sensibly affect the temperature 

 of mines, which it appears they do not where the deptk 

 is great. Besides, the Cornish mines are generally 

 ventilated by numerous shafts opening into the galleries 

 from the surface or from a higher level. The air circu- 

 lates freely in these, descending in some shafts and as- 

 cending in others. In all cases, Mr. Fox found that the 

 upward currents are of a higher temperature than the 

 descending currents ; so much so, that in winter the 

 moisture is often frozen in the latter to a considerable 

 depth ; the circulation of air, therefore, tends to cool 

 the mine instead of increasing the heat. Mr. Fox has 

 also removed the objections arising from the compara- 

 tively low temperature of the water in the shafts of 

 abandoned mines, by showing that observations in them, 

 from a variety of circumstances which he enumerates, 

 are too discordant to furnish any conclusion as to the 

 actual heat of the earth. The high temperature of 

 mines might be attributed to the effects of the fires, 

 candles, and gunpowder used by the miners, did not a 

 similar increase obtain in deep wells, and in borings to 

 great depths in search of water, where no such causes 

 of disturbance occur. In a well dug with a view to 

 discover salt in the canton of Berne, and long deserted, 

 M. de Saussure had the most complete evidence of in- 

 creasing heat. The same has been confirmed by the 

 temperature of many wells, both in France and England, 

 especially by the Artesian wells, so named from a pecu- 

 liar method of raising water first resorted to in Artois, 

 and since become very general. An Artesian well con- 

 sists of a shaft of a few inches in diameter, bored into 

 the earth till a spring is found. To prevent the water 

 being earned off by the adjacent strata, a tube is let 

 down which exactly fills the bore from top to bottom, in 

 which the water rises pure to the surface. It is clear 

 the water could not rise unless it had previously de- 

 scended from high ground through the interior of the 

 earth to the bottom of the well. It partakes of the 

 temperature of the strata through which it passes, and 

 in every instance has been warmer in proportion to the 

 depth of the well ; but it is evident that the law of in- 



