APPENDIX. 



383 



mese Oberland ; but the difference in the geological structure of the 

 two ranges will, I conceive, be sufficient to explain, why hot springs 

 are more rare in the latter than in the southern range. Most of the 

 highest mountains in the Bernese Alps are covered with secondary 

 strata ; and the valleys are chiefly excavated in these strata, or in 

 enormous beds of sandstone and conglomerate, that form a thick in- 

 tervening mass between the surface and the primary rocks, sufficient to 

 obstruct the rise of thermal waters ; for it has before been stated, that 

 all the thermal waters in the Pennine Alps, issue from the primary 

 rocks, or near their junction with the low'est calcareous strata. 



ON THE TEMPERATURE OF MINES AND WELLS. 



It was stated in Chap. XXIV. that the temperature of the water in 

 Artesian wells (or those wells formed by boring) had been found in 

 France to increase about 1° centigrade for 25 metres in depth. But 

 this increase of temperature is sometimes variable in different situa- 

 tions. France has been the seat of active volcanoes at no remote ge- 

 ological epoch ; and, in the volcanic districts, there are numerous hot 

 springs remaining : it is, therefore, not improbable that, in the south- 

 ern and central departments, the increase of temperature with the in- 

 crease of depth in Artesian wells, may be derived from the remains of 

 volcanic heat. In England, many borings for water have been exe- 

 cuted ; but I am not aware of any experiments having been made on 

 the water to ascertain the temperature. At Boston, in Lincolnshire, 

 water was bored for to the extraordinary depth of 600 feet : the bor- 

 ing, during the whole depth, was in clay ; and the experiment was un- 

 successful, no good water being obtained. It is to be regretted that 

 the temperature of the water at that depth had not been ascertained. 



Many experiments have been made on the temperature both of the 

 air, the water, and the rocks in mines, at different depths; and the 

 general results of each have indicated a considerable increase of heat 

 with the increase of depth. In Dolcoath copper mine, Mr. Fox found 

 the temperature of the water (at about 480 yards from the surface) to 

 be more than 30° of Fahrenheit above the mean temperature of the 

 country. A thermometer, plunged into the earthy matter, at the bot- 

 tom of another mine in the same county, 400 yards deep, and which 

 had been inundated for two days, was raised 38° above the mean tem- 

 perature. I apprehend that in these instances, and in many others 

 that have been stated, one source of error has not been sufficiently at- 

 tended to, viz. the increase of heat by chemical changes that are ta- 

 king place in the mineral substances in mines, from access to water or 

 the atmosphere. I was informed by working miners in Cornwall, 

 that they could generally tell when they were approaching to a cop- 

 per lode, by the increased warmth of the water ; but this was not the 

 case when they came to a lode of tin ore. The cause of this warmth 

 seems very intelligible : the copper ore of Cornwall is chiefly a mix- 

 ture of iron pyrites and copper pyrites ; and it is well known that iron 

 pyrites is more or less decomposed by the access of air and water, 



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