Masterpieces of Science 



chemical and some to mechanical causes; while 

 others assume that it is merely the residue of the 

 heat which the earth originally possessed, if, as 

 seems likely, it existed at one time in a state of 

 fusion. Dismissing, however, these vexed ques- 

 tions, it is sufficient to remark that some source 

 of heat unquestionably does exist in the earth 

 beneath our feet. 



If a thermometer be buried in the ground at a 

 depth of only a few inches below the surface, it is 

 found to be affected by all superficial changes 

 of temperature, and its indications show that 

 it is cool at night and warm in the day, cold in 

 the winter and hot in the summer. But plunged 

 deep into the ground, or placed in a deep cellar 

 or cavern, these variations disappear, and one 

 uniform temperature is registered under all cir- 

 cumstances. What that temperature is will 

 depend principally on the climate of the locality, 

 the constant temperature being nearly the mean 

 temperature of the surface. 



On going still deeper, the heat is found to in- 

 crease; and, at the bottom of a deep mine, it is 

 generally so warm that the miners are glad to 

 discard most of their clothing. At present, the 

 deepest mine in this country is the Rosebridge 

 Colliery, at Ince, near Wigan, which has reached 

 a depth of 2,445 f eet - Experiments on the tem- 

 perature at different depths, while sinking this 

 pit, showed that the average increase is about 

 i Fahr. for every fifty-four feet. In other 

 sinkings, somewhat different results have been 

 188 



