INTERNAL HEAT OF THE EAETH. 163 



be applicable under considerable modifications. It seems 

 highly probable that the action of the Sun and Moon, which 

 produces the ebb and flow of the ocean, is also felt in these 

 subterranean depths. "We may suppose periodic heavings 

 and subsidings of the molten mass, and consequent varia- 

 tions in the pressure against the vaulted covering formed by 

 the solidification of the upper rocks. The amount and effects 

 of such oscillations must, however, be small ; and though the 

 relative position of the heavenly bodies may here also occa- 

 sion " spring tides," yet it is certainly not to these, but to 

 more powerful internal forces, that we must attribute the 

 movements which shake the surface of the Earth. There are 

 groups of phsenomena to the existence of which it may be 

 useful to refer, for the purpose of illustrating the universality 

 of the attraction of the Sun and Moon on the external and 

 internal condition of our globe, however little we may be 

 able to assign numerically the amount of such influence. 



Tolerably accordant experience has shewn that in Arte- 

 sian wells, the average increase of temperature in the strata 

 pierced through is 1 of the Centigrade thermometer for 

 92 Parisian feet of vertical depth (54'5 English feet for 1 

 of Fahrenheit) ; or if we suppose this increase to continue 

 in an arithmetical ratio, a stratum of granite would, as I have 

 already remarked ( 138 ), be in a state of fusion at a depth of 

 nearly 21 geographical miles, or between four and five times 

 the elevation of the highest summit of the Himalaya. 



In the globe of the Earth three varieties in the mode of the 

 propagation of heat are to be distinguished. The first is 

 periodical, and causes the temperature of the strata to vary, 

 as, according to the position of the Sun and the season of 

 the year, the warmth penetrates from above downwards, 



