THE EARTH'S INTEEIOK HEAT. 305 



As long as the surface of our globe was in an incandes- 

 cent state, it must have lost heat at a very rapid rate ; grad- 

 ually this process became slower ; and although it has not yet 

 entirely ceased, the rate of cooling must have diminished to a 

 comparatively small magnitude. 



Two phenomena are caused by the cooling of the earth, 

 which, on account of their common origin, are intimately re- 

 lated. The decrease of temperature, and consequent contrac- 

 tion of the earth's crust, must have caused frequent distur- 

 bances and revolutions on its surface, accompanied by the 

 ejection of molten masses and the formation of protuberances ; 

 on the other hand, according to the laws of mechanics, the 

 velocity of rotation must have increased with the diminution 

 of the volume of the sphere, or, in other words, the cooling 

 of the earth must have shortened the length of the day. 



As the intensity of such disturbances and the velocity of 

 rotation are closely connected, it is clear that the youth of our 

 planet must have been distinguished by continual violent 

 transformations of its crust, and a perceptible acceleration of 

 the velocity of its axial rotation ; whilst in the present time 

 the metamorphoses of its surface are much slower, and the 

 acceleration of its axial revolution diminished to a very small 

 amount. 



If we imagine the times when the Alps, the chain of the 

 Andes, and the Peak of Teneriffe were upheaved from the 

 deep, and compare with such changes the earthquakes and 

 volcanic eruptions of historic times, we perceive in these 

 modern transformations but weak images of the analogous 

 processes of bygone ages. 



Whilst we are surrounded on every side by the monu- 

 ments of violent volcanic convulsions, we possess no record 

 of the velocity of the axial rotation of our planet in antedi- 

 luvian times. It is of the greatest importance that we should 

 have an exact knowledge of a change in this velocity, or in 

 the length of the day during historic times. The investiga- 



