THE EARTH S INTERIOR HEAT. 313 



length of a day, however, has remained constant, the cooling 

 effect of the earth during the same period of time must have 

 shortened the day ^th of a second. A diminution of the 

 earth s radius to the amount of 4 metres in 2500 years, and 

 a daily loss of 200 cubic miles of heat, correspond to this 

 effect. Hence, in the course of the last 25 centuries, the 

 temperature of the whole mass of the earth must have de 

 creased ii. 



The not inconsiderable contraction of the earth resulting 

 from such a loss of heat, agrees with the continual transfor 

 mations of the earth s surface by earthquakes and volcanic 

 eruptions ; and we agree with Cordier, the industrious ob 

 server of volcanic processes, in considering these phenomena 

 a necessary consequence of the continual cooling of an earth 

 which is still in a molten state in its interior. 



When our earth was in its youth, its velocity of rotation 

 must have increased to a very sensible degree, on account of 

 the rapid cooling of its then very hot mass. This accelera 

 ting cause gradually diminished, and as the retarding pressure 

 of the tidal wave remains nearly constant, the latter must 

 finally preponderate, and the velocity of rotation therefore 

 continually decrease. Between these two states we have a 

 period of equilibrium, a period when the influence of the 

 cooling and that of the tidal pressure counterbalance each 

 other ; the whole life of the earth therefore may be divided 

 into three periods youth with increasing, middle age with 

 uniform, and old age with decreasing velocity of rotation. 



The time during which the two opposed influences on the 

 rotation of the earth are in equilibrium can, strictly speak 

 ing, only be very short, inasmuch as in one moment the cool 

 ing, and in the next moment the pressure of the tides must 

 prevail. In a physical sense, however, when measured by 

 human standards, the influence of the cooling, and still more 

 so that of the tidal wave, may for ages be considered con 

 stant, and there must consequently exist a period of many 

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