338 THE STORY-HOOK OF SCIENCE 



name of artesian well is ^ivcn to a cylindrical hole 

 which by means of strong iron bars, fitted end to 

 end, is made in the ground until some reservoir of 

 subterranean water, fed by the infiltrations of neigh- 

 boring streams or lakes, is reached. The water that 

 comes up from far under ground as the result of 

 such a boring reaches the surface at a temperature 

 equal to that of those depths; and thus we learn 

 about the distribution of heat in the bowels of the 

 earth. One of the most remarkable of these wells 

 is that of Grenelle, at Paris. It is 547 meters deep, 

 and the water in it is constantly at 28 degrees, a 

 temperature almost as high as that of the hottest 

 summer days. The water of the artesian well of 

 Mondorf, on the frontier of France and Luxemburg, 

 comes from a far greater depth, 700 meters. Its 

 temperature is 35 degrees. Artesian wells, of which 

 there are at present a considerable number, illustrate 

 the same principle as mines : for every thirty meters 

 of depth the heat increases one degree." 



* ' Then by digging wells deep enough we should at 

 last come to boiling water 1" 



"Certainly. The difficulty is to attain the desired 

 depth. To reach the temperature of boiling water 

 it would be necessary to bore about three quarters 

 of a league, which is impossible. However, a num- 

 ber of natural springs are known which, as they 

 come from the ground, possess a high temperature, 

 sometimes reaching the boiling point. They are 

 called thermal springs, which means hot springs. 

 There prevails, then, at the depth from which they 

 come, a heat sufficient to make them tepid, or even 



