On Underground Temperatures. 



35 



mines are rarely more than 100 to 300 or 400 feet above the sea 

 level ; but the mines of Freiberg are on higher ground, whilst those of 

 Prizbam and Chemnitz are situated amongst high hills, and the tem- 

 perature at the end of a long gallery may be, in relation to depth 

 beneath the surface, very different to that given by the depth of the 

 shaft. 



The mines of Freiberg are those in which the greatest number of 

 observations have been made, though none of them are of very recent 

 date. The deepest mine, and one in which the observations were 

 made in holes in the rock (gneiss), gives, if we are right in our 

 estimate of the surface temperature,* a rate of increase with depth of 

 54 feet per degree. But in this instance, — as in the case of Mr. Fox's 

 observation (No. 73) in Dolcoath Mine, where the thermometer was left 

 in position for one and a half year, — it was here left two years, and 

 there must have been as before described, a cooling of the rock, such 

 as would reduce the temperature so far below the normal, as to 

 place it in equilibrium with the temperature of the cooler air in the 

 galleries. The temperature of 66° at Freiberg, and of 76° at Dolcoath, 

 with their thermometric gradients of 54 and 53 feet, I take to 

 represent the normal temperature of the rock, minus the loss of heat 

 due to ventilation ; and if that can be estimated at anything like the 

 loss shown to have taken place in a level in the Wheal Yor Mine, which, 

 after it had been opened some time and the mine had been deepened, 

 amounted to about 6°, or in a similar instance mentioned by Mr. Hen- 

 wood, where the difference amounted to 7°, it would indicate a normal 

 temperature of these mines more in accordance with the gradient 

 which we have adopted on other grounds. 



For the same reason the numerous observations made by Mr. Hen- 

 wood in the mines of Brazil and Chili, are, for the present, not 

 available, though it is possible that they may be rendered so at a 

 future period, should the other factors necessary for determining the 

 difference between the surface and the underground temperature be 

 ascertained. 



III. Artesian Wells and Borings. {Table IV, p. 106.) 



This class of observations presents results much more uniform than 

 those taken either in Coal or Mineral Mines, and whereas the obser- 

 vations in Mines were in Palaeozoic or crystalline rocks, those in Wells 

 are, with few exceptions, either in Cretaceous, Jurassic, or Triassic 



* This is based on the temperature of Dresden, the nearest place where we have 

 . recorded observations. The mean annual temperature of Dresden is 47°, and a* the 

 mine is situated at the height of about 1300 feet, or about 900 feet above Dresden, 

 if we allow 1° for every 300 feet of elevation, we shall have about 44° for the surface 

 temperature at the mine. 



D 2 



