DEEP WELL-BORINGS. 39 



that in the borings for deep "wells, of which such a 

 surprising number have been carried out of late 

 years, the temperature has always been found to 

 increase gradually as the depth increased. Some 

 of these wells have been sunk to a depth of fifteen 

 and eighteen hundred feet, one at Neusalzwerk, 

 in Westphalia, even to two thousand two hun- 

 dred feet. In this latter the thermometer rose 

 to 32°*75 C. (90°-95 P.), a height which in this 

 place it never reaches at the surface. 



The results of the observations hitherto made 

 in mines, have not yet led us to any certain con- 

 clusions as to the law of this gradual increase 

 of temperature. However, from several experi- 

 ments made, under peculiarly favourable cir- 

 cumstances, in some of these borings, it is quite 

 evident that the temperature increases in direct 

 proportion to the depth. Here I may instance two 

 sets of observations, which were made by different 

 inquirers, under very different conditions. The one 

 was made by De la Eive and Marcet, in a 

 mountainous district, at Pregny near Geneva, 

 more than fourteen thousand feet above the sea 

 (the mouth of the boring was two hundred and 

 ninety-nine feet above the Lake) ; the other 

 by Magnus in a boring (near Riidersdorf in 

 the Margravate of Brandenburg), which reaches 

 to seven hundred and thirty feet below the sea- 

 level. 



