230 HEAT IN MINES AND WELLS. SECT. XXV. 



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 con- 

 sists of a shaft 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 pre- 

 viously descended from high ground through the interior of the 

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

 ture 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 increase cannot be obtained in this 

 manner. Perhaps the most satisfactory experiments on record 

 are those made by MM. Auguste de la Pave 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 peculiarly 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 expe- 

 rienced observers, the temperature was found to increase regu- 

 larly and uniformly with the depth at the rate of about 1 of 

 Fahrenheit for every 52 feet. Professor Eeich of Freyberg has 

 found that the mean of a great number of observations both in 

 mines and wells is 1 of Fahrenheit for every 55 feet of depth ; 

 and from M. Arago's observations in the Grenelle Artesian well at 

 Paris, the increase is 1 of Fahrenheit for every 45 feet. Though 

 there can be no doubt as to the increase of temperature in pene- 

 trating the crust of the earth, there is still much uncertainty as 

 to the law of increase, which varies with the nature of the soil 

 and other local circumstances ; but, on an average, it has been 



