LIBRATION OF THE MOON. 339 



hours, correspond always to the same material points of 

 the terrestrial spheroid ? In other words, do the poles 

 of rotation, which from year to year correspond to dif- 

 ferent stars, undergo also a displacement at the surface 

 of the earth ? 



In the case of the affirmative, the equator is movable 

 as well as the poles ; the terrestrial latitudes are vari- 

 able ; no country during the lapse of ages will enjoy, 

 even on an average, a constant climate ; regions the 

 most different will, in their turn, become circumpolar. 

 Adopt the contrary supposition, and every thing assumes 

 the character of an admirable permanence. 



The question which I have just suggested, one of the 

 most important in Astronomy, cannot be solved by the 

 aid of mere observation on account of the uncertainty of 

 the early determinations of terrestrial latitude. Laplace 

 has supplied this defect by analysis. The great geom- 

 eter has demonstrated that no circumstance depending 

 on universal gravitation can sensibly displace the poles 

 of the earth's axis relatively to the surface of the ter- 

 restrial spheroid. The sea, far from being an obstacle 

 to the invariable rotation of the earth upon its axis, 

 would, on the contrary, reduce the axis to a permanent 

 condition in consequence of the mobility of the waters 

 and the resistance which their oscillations experience. 



The remarks which I have just made with respect to 

 the position of the terrestrial axis are equally applicable 

 to the time of the earth's rotation which is the unit, the 

 true standard of time. The importance of this element 

 induced Laplace to examine whether its numerical value 

 might not be liable to vary from internal causes such 

 as earthquakes and volcanoes. It is hardly necessary 

 for me to state that the result obtained was negative. 



