temperature of the interior of the earth. 1 23 



" 16. M. de la Place, regarding the astronomical facts observed 

 in the time of Hipparchus, as sufficiently exact to conclude that 

 the duration of a day has not diminished 1-300 of a centesimal 

 second in two thousand years, thought, that the contraction ac- 

 tually produced by the secular cooling of the globe was not suf- 

 ficient to increase the velocity of its rotation. This opinion sug* 

 gests an useful limit to the actual effect of general cooling. 



" 17. But if we consider the effects of contraction from the 

 commencement of the cooling process, we must admit that some 

 influence in this last respect has actually been exerted. On the 

 one hand, the duration of the day has very slightly been dimin- 

 ished, and, on the other hand, the figure of the earth has been 

 slightly altered in consequence of the increased velocity of its 

 rotation ; provided the flexibility of the earth's ci'ust has been 

 sufficient to endure this alteration of figure ; which we admit. 

 The day therefore is actually shorter, and the flattening at the 

 poles greater than at the first origin of things. If these data be 

 conceded, the two effects just mentioned are still going on. It 

 remains only to find some better means of calculating this weak 

 intensity; which is not impossible, as we shall see by and bye. 



" 18. Another consequence, not less probable or curious, to 

 which we are led by the theory of the incandescence and igneous 

 fluidity of the central mass, is this. If the flexibility of the 

 earth's crust be such as we have supposed, the phenomena of 

 tides take place in the crust of the earth itself. We shall not be 

 surprised at this effect, very feeble though it be, if we pay atten- 

 tion to the fact that it certainly did take place at the origin of 

 our globe, and while it was in the perfectly fluid state which all 

 hypotheses admit. It would be easy to show that the highest of 

 these ancient tides did not rise less than four or five metres. 



"19. The secular refiguration, continually increasing the 

 thickness of the earth's crust, gives room to inquire, whether the 

 incandescent matter submitted to this action passes entirely into 

 a state of solidity, or whether the production of gaseous substan- 

 ces does not accompany the process. So far from this being im- 

 possible, the daily coagulation of lavas offers a striking example. 

 This supposition explains very naturally the origin of the first 

 material cause of earthquakes. An excessive temperature keeps 

 this first material cause in a gaseous state, notwithstanding the 

 influence of immense pressure at the depth we are now consid- 

 ering. The capricious phenomena, earthquakes, may depend 

 also on the great inequality of the interior surface of the crust of 

 the earth. 



" 20. These data lead to a new explanation of volcanic phe- 

 nomena, which, to the very few persons who have a just notion 

 of the elements of this question, may appear more satisfactory 



