Sfiii Prof. Bischof on the Temperature of 



thermal springs — as we have ssen in the foregoing chapter — of 

 variable temperature, it cannot be expected that one of the cold- 

 est should have a constant temperature. It may, therefore, be 

 assumed with safety, that the mean temperature of a place is 

 always below that of a spring whose temperature is constant at 

 different seasons of the year. 



At the first view, it would seem that deep welis were particu^ 

 larly suitable for determining the temperature of the soil. Many 

 objections may, however, be called to mind against this. Von 

 Buch remarks, that only such wells are proper for this purpose 

 as are in constant use, by which their waters are kept continually 

 in circulation ; but not such as remain undisturbed, for in them 

 the cold air sinks down from the atmosphere, and cools their 

 walls below to a greater extent than the propagation of heat 

 would allow. Farther, it must be presupposed that such wells 

 are only supplied by waters filtered through the superficial 

 strata of the earth, and that they are so far distant from any 

 mountains of importance, that it can no longer be considered 

 possible for waters to filter down into them from higher and 

 colder regions. But it must also be supposed that they do not 

 receive any tribute from warmer springs rising from a greater 

 depth. If, in sinking a well, a stratum be met with which is 

 impervious to water, and which has no where been broken 

 through, one may be tolerably certain that the latter case has 

 been avoided. A communication with clefts, by which they 

 might be supphed with waters from below, in the manner of 

 artesian wells, is most to be apprehended when a well is cut in 

 the solid rock. Deep wells, which communicate with neighbour- 

 ing deep rivers, are very likely to hold thermal waters, especial- 

 ly if there be any rising ground between the springs and the 

 river, although it be but a few hundred feet above the bottom 

 of the valley. Thus the temperature of a well of fifty-eight feet 

 under my laboratory, which is in connexion with the Rhine, is 

 almost constant, and is about 1°.35 higher than the mean tem- 

 perature of the soil here. This spring is consequently thermal. 

 The mean temperature of the soil cannot, therefore, be deduced 

 with certainty from observations on the temperature of wells. 

 An approximate result can only be obtained by extending the 

 observations to many different wells. 



