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. 
