166 Prof. J. Prestwich. Underground Temperatures, [Feb. 12, 



which in tlie hilly district of Freiberg and Hungary introduces an 

 element of great uncertainty, it is impossible to arrive at any safe 

 conclusion. 



Artesian Wells and Borings. — This class of observations presents 

 results much more uniform, and whereas the mines observations 

 were made, the one in crystalline, and the other in unaltered palaeo- 

 zoic rocks, the wells are, with few exceptions, in the softer and less 

 coherent rocks either of Cretaceous, Jurassic, and Triassic age, which 

 are much more permeable, and, as a rule, much less disturbed. 



The causes of interference are mainly reduced to pressure on the 

 instruments and convection currents. The early experiments, where 

 no precautions were taken against these, are, with few exceptions, 

 unreliable, and must be rejected. The larger the bore-hole the greater 

 the risk of convection currents, and Professor Everett has shown 

 that in many cases of deep and large artesian borings, the water 

 which lodges in them is reduced to a nearly uniform temperature 

 throughout the whole depth by the action of these currents. In the 

 deep boring at Sperenberg, before the introduction of plugs to stop 

 these currents, it was found that the temperature near the top of the 

 bore was rendered 4*5° F. too high, and at the bottom at a depth of 

 3390 feet, 4-6°, if not 6*7°, too high by the currents. 



Taking the bore-holes in which the water does not overflow, and 

 where, owing to the precautions against these sources, such as those 

 of Kentish Town, Richmond, Grenelle, Sperenberg, Pregny, and Ostend, 

 we get a mean gradient of 51 "9 feet per degree. 



Overflowing artesian wells should, if we were sure of all the con- 

 ditions, give the best and most certain results. Taking those where 

 the volume of water is large, and the observations made by competent 

 observers, as in the case of the wells of Grenelle, Tours, Rochefort, 

 Mondorff, Minden, and others, we obtain a mean of 50'2 feet, or 

 taking the two sets of wells, of 51 feet per degree. 



The author, however, points out a source of possible error in those 

 wells, arising from a peculiarity of tubage which requires investiga- 

 tion, and owing to which he thinks the water may suffer a loss of heat 

 in ascending to the surface. 



With respect to the extra- European wells, more particulars are 

 required. It may be observed, however, that the wells in the Sahara 

 Desert, which were made by an experienced engineer accustomed to 

 such observations, the mean of eleven overflowing wells, at depths of 

 from 200 to 400 feet, gave 36 feet per degree. 



Tunnels. — For the Mont Cenis Tunnel, allowing for the convexity 

 of the surface, Professor Everett estimates the gradient at 79 feet, 

 and for the St. Gothard, 82 feet per degree. But Dr. Stapff found in 

 the granite at the north end of the tunnel a much greater heat and 

 more rapid gradient, for which there seemed no obvious explanation. 



