SECT, xxvi.] HEAT IN MINES AND WELLS. 879 



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 increasing 

 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 peculiar method of raising 

 water first resorted to in Artois, and since become very 

 general. An Artesian well consists of a shaft of a few inches 

 in diameter, bored into the earth till a spring is found. To 

 prevent the water being carried 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 evi- 

 dent that the law of increase cannot be obtained in this man- 

 ner. Perhaps the most satisfactory experiments on record 

 are those made by MM. August de la Rive and F. Marcet 

 during the year 1833, in a boring for water about a league 

 from Geneva, at a place 318 feet above the level of the lake. 

 The depth of the bore was 727 feet, and the diameter only 

 between four and five inches. No spring was ever found ; 

 but the shaft filled with mud, from the moisture of the ground 

 mixing with the earth displaced in boring, which was pecu- 

 liarly favourable for the experiments, as the temperature at 

 each depth may be considered to be that of the particular 

 stratum. In this case, where none of the ordinary causes of 

 disturbance could exist, and where every precaution was 

 employed by scientific and experienced observers, the tem- 



