136 Prof. BischofFwt the Temperature of 



cl, d) d, are wanting, and when the waters which sink through 

 the clefts to a certain depth, t, (PI. II, Fig. 8) are forced again 

 to the surface at h by hydrostatic pressure. 



2d, Let Fig 4, PI. II, represent a mountain. The springs in 

 the upper regions will most commonly descend through the 

 sloping channels ah, cd, ef. Here, then, will be an example of 

 case 1*^, namely, when the interior strata can only be acted on 

 by the external temperature. As the temperature decreases 

 from below upwards, the strata situated nearer to the foot of the 

 mountain will besides be cooled by the waterswhich descend from 

 the higher regions. Should any part of the meteoric waters be 

 enabled, by entering' extensive fissures in the mountain, to de- 

 scend far below its foot, and reappear in the plain, it would 

 bring up heat with it to the surface. So that, according as the 

 observations be made in the mountain or in the plain, all other 

 circumstances being alike in both cases, the results must shew 

 an unequal increase of temperature towards the centre of- the 

 earth. 



If we consider all the cases above investigated, we see that 

 the increase of temperature towards the centre of the earth, 

 even if supposed in itself to follow one fixed law, must be 

 found to be very different, according as the wells in which the 

 observations are made happen to be sunk at the foot of a moun- 

 tain, or more towards its summit, on an extensive plateau or 

 on the slope of a hill *, whether they be encompassed on all 

 sides by impervious strata, and whether they be immediately 

 surrounded by high mountains or not. What innumerable 

 combinations of these different cases may not be imagined by 

 which the increase of temperature towards the centre of the 

 eartl) might be influenced .? 



Chap. XIV — What influeiwe has the air on observations of tempe- 

 rature in mines ? 



That this influence must tend materially to disturb the ob- 

 servations is easily to be foreseen. When shafts communicate 

 with adits, or when several shafts of unequal depth are in commu- 



• On the influence which the configuration of the surface of the earth has 

 upon the increase of temperature towards its centre, see Chap. XVII. 



