36 HEAT OF THE EARTH. 



by the slow burning of many compounds of 

 sulphur in coal-mines, and to vast fires thus 

 kindled in the earth. Some have even enlisted 

 into their schemes of explanation the great heat 

 which many salts containing no water give out in 

 taking up that liquid. But even if there be room 

 for supposing that, in some special instances, such 

 local sources of heat have any share in producing 

 the effect, yet the distribution of volcanoes, and 

 hot springs over all parts of the earth, their occur- 

 rence among the most different rock-formations, 

 and the often unmistakeable connection between 

 burning mountains lying far apart from each 

 other, are evidences of a far more general and 

 more deeply-seated cause of these phenomena. 



But the most decisive proof of the existence of 

 a general store of heat within the earth, is this — 

 that at all places on the globe, starting from a 

 certain limit below the surface, the temperature is 

 found to increase as we descend. This fact, which 

 has but recently been discovered, is now established 

 beyond any doubt. 



The influence of the heat coming from without 

 does not penetrate very deep into the earth. You 

 are well aware, that in good cellars it does not 

 freeze in the winter, and that in the summer they 

 remain cool. In many parts of the continent the 

 farmers bury their potatoes and turnips, only three 

 or four feet underground, to protect them from the 



