the Interior ofilie Earth. 



made in the time of Hipparchus to be sufficiently accurate to af- 

 ford evidence that the duration of the day has not diminished 

 3 Jg of a centesimal second for twenty centuries, thought that 

 the contraction which is actually produced by the secular cool- 

 ing of the globe, is not sufficiently great to increase the velocity 

 of rotation in a sensible degree. This opinion gives us a useful 

 limit of the actual effect of the g-eneral refrigeration. 



17. But if the effects of contraction since the commence- 

 ment of the cooling are considered, one cannot help admitting 

 that it has exercised a certain influence in the above point of 

 view. On the one hand, the duration of the day has succes- 

 sively diminished a small quantity ; and, on the other, the figure 

 of the earth must have undergone a slight alteration, in conse- 

 quence of the acceleration of the velocity of rotation, provided 

 the flexibility of the consolidated crust has been sufficient to per- 

 mit the change of figure, which we admit as being the case. 

 Thus at present the day is a little shorter, and the spheroid a 

 little flatter toward the poles, than at the commencement of 

 things. If these data are correct, it is evident that the two ef- 

 fects continue. All that is to be done is to find a better means 

 than that mentioned above for appreciating the feeble intensity ; 

 which is not impossible, as we shall presently see. 



18. Another consequence, not less probable, and not less cu- 

 rious, to which we are led by the hypothesis of central incan- 

 descence and fluidity, is the following. However little the crust 

 of the earth may be influenced by the flexibility which, accord- 

 ing to our ideas, must be attributed to it, it follows that the 

 phenomenon of the tides is exercised, without its having been 

 hitherto suspected, upon the terrestrial mass itself. This effect, 

 which, besides, must be excessively feeble, will not excite asto- 

 nishment, when we reflect that it certainly existed at the com- 

 mencement of things, that is to say, when the surface of the 

 globe possessed the perfect fluidity which is admitted in all the 

 theories. It is easy to demonstrate that the greatest of these 

 ancient land-tides could not have been less than from four to 

 five metres. 



19. As the secular refrigeration of the earth is continually in- 

 creasing the thickness of its crust, it may be asked if the incan« 

 descent matter which is subjected to this action, passes entire- 



