On Underground Temperatures. 



41 



surface for a distance of above 100 miles, these conditions are most 

 favourable. It is very interesting- to find, from information which M. 

 Daubree has just (Dec, 1884) obligingly obtained for me, that the 

 temperature of the water at the Grenelle well is now precisely the 

 same (27*8° 0.), if not 0*1° C. higher, than it was at the commence- 

 ment of the overflow forty-two years ago. In London, the distance of 

 10 miles at which the Lower Terbiaries and the Chalk out-crop is also 

 no doubt sufficient, but it is a question whether the excessive pumping 

 and constant passage of water may not have cooled the channels. 



2nd. A sufficient volume and consequently rapid upward flow of 

 the water in proportion to the depth, otherwise the water will part 

 with some of its heat, as it rises through the tube. This may possibly 

 be in some small degree the case at Grenelle, for in the carefully con- 

 ducted observations of Arago and Walferdin, when the well had 

 reached a depth of 400 metres, the temperature (23' 75° C.) gave a rate 

 of increase of 55 feet per degree, instead of that of 58 feet when it 

 overflowed. At the Ecole Militaire, the rate was, as ascertained by 

 Walferdin, 54 feet. It is possible, therefore, that the slower rate of 

 increase at Grenelle indicated by the overflow water at a greater 

 depth (548 metres) may arise from a reduction of temperature due 

 in part from the water overflowing with a velocity by no means great, 

 but mainly from the diameter of the tube at the top being considerably 

 greater than at the bottom. 



The conditions of depth and volume are generally satisfactory 

 in the case of the great saline wells in Germany, especially in 

 the instance of the well at Neu Salzwerk. Humboldt states the 

 supply of water was very abundant, the outflow being then at the 

 rate of 422,000 gallons daily. Its ascensional force was remarkable, 

 as was also the enormous discharge of carbonic acid. The deep 

 artesian wells of Minden, which derive their water from the same 

 source, give very similar results. 



The discharge of water at the thermal springs of MondorfF in 

 Luxembourg is very much less than at those of Neu Salzwerk. 

 It is situated further south, and the height of the ground is consider- 

 ably greater, being 673 feet above the sea-level ; the mean annual 

 surface temperature adopted by Walferdin was 48'4° F., but this was 

 only an estimate. The mean temperature of Luxembourg, according 

 to Arago, is only 8° C. (46*5° F.). 



The depth of the artesian well at Tours is much less than that of 

 the above, but the ascent of water was rapid and the discharge large. 

 The observations there were made by Walferdin, as were also those 

 at Rochefort. 



The following list is confined to those wells where the overflow is 

 abundant, and where the observations have been made by competent 

 observers. 



