EXPERIMENTS IN BORINGS. 41 



the depths for which the thermometer rises 

 one degree of the Centrigade scale. You will at 

 once understand that there must be a difference 

 in the results, according as the borings are made 

 in high land, in valleys, or in plains. Besides it 

 must not be supposed that the shell of the earth 

 is everywhere equally conductive of heat; and 

 there may be many cases, especially in the higher 

 parts of the bore-holes, in which it may have been 

 impossible to shut out entirely all atmospheric 

 influences ; such for. instance as the oozing in of 

 cold surface-water. Lastly there is only one con- 

 dition in which we can conclude with certainty, 

 that the temperature of the water, with which the 

 shaft is always filled, is that of the ground at the 

 depth which the shaft has reached : this is when 

 this water has remained at the same point for a long 

 time, and cannot be mixed, ever so slightly, during 

 the observation, with the waters rising up from 

 below, or with those coming through from above. 



"We may assume, taking an average deduced 

 from the best observations, that the temperature 

 rises about one degree of the Centigrade scale 

 (1 0, 8 Fahr.) for every hundred feet of depth. 



But how is it possible, you will ask, to mark 

 the height of a thermometer in the depths of these 

 borings ? There are many means of doing this. 

 The simplest is the following: to let down into 

 the shaft a correct thermometer, clothed with some 



