410 



INTF.KNAL HEAT. 



Internal 

 Eource of 

 heat. 



CUAPTER and motion of wliich may alike affect the external opera- 

 ■^•^^'^ tions that fall under our observation, while it is probably 

 also itself affected by the same laws which operate in 

 producing the ebb and flow of the sea. The facts which 

 prove the existence of an internal source of heat, alto- 

 gether apart from that which arises from the influence of 

 the solar rays, are familiar to us all. But its true influ- 

 ence or the degrees of heat which are manifested at any 

 regular depth, or under other defined circumstances, are 

 much more difficult to guage. " According to tolerably 

 uniform experiments in Artesian wells," our author re- 

 marks in his Cosmos, " it has been shown that the heat 

 increases on an average about 1° for every 54'5 feet. If 

 this increase can be reduced to arithmetical relations, it 

 will follow, as I have alreadj' observed, that a stratum 

 of granite would be in a state of fusion at a depth of 

 nearly twenty-one geographical miles, or between four 

 and five times the elevation of thp iiighest summit of the 

 Ilimalaya. 



" We must distinguish in our globe three different 

 modes for the transmission of heat. The first is periodic, 

 and affects the temperature of the terrestrial strata ac- 

 cording as the heat penetrates from above downwards, or 

 from below upwards, being influenced by the different 

 positions of the sun and the seasons of the year. The 

 second is likewise an effect of the sun, although extremely 

 slow : a portion of the heat that has penetrated into the 

 equatorial regions moves in the interior of the globe 

 towards the poles, where it escapes into the atmosphere 

 and the remoter regions of space. The third mode of 

 transmission is the slowest of all, and is derived from the 

 secular cooling of the globe, and from the small portion 

 of the primitive heat which is still being disengaged from 

 the surfiice. This loss experienced by the central heat 

 must have been very considerable in the earliest epochs 

 of the earth's revolutions, but within historical periods it 

 lias hardly been appreciable by our instruments. The 

 Burface of the earth is therefore situated between the 



TranrmissioD 

 of lieiit. 



Cooling of 

 tjie globe. 



