72 AXIS OF ROTATION INVARIABLE. Sfccr. X. 



responding variations ; and the geographical latitudes of 

 all places being estimated from the equator, assumed to 

 be fixed, would be perpetually changing. A displace- 

 ment in the position of the poles of only two hundred 

 miles would be sufficient to produce these effects, and 

 would immediately be detected. But as the latitudes 

 are found to be invariable, it may be concluded that the 

 terrestrial spheroid must have revolved about the same 

 axis for ages. The earth and planets differ so little 

 from ellipsoids of revolution, that in all probability any 

 libration from one axis to another produced by the 

 primitive impulse which put them in motion, must have 

 ceased soon after their creation from the friction of the 

 fluids at their surface. 



Theory also proves that neither nutation, precession, 

 nor any of the disturbing forces that affect the system, 

 have the smallest influence on the axis of rotation, which 

 maintains a permanent position on the surface, if the 

 earth be not disturbed in its rotation by a foreign cause, 

 as the collision of a comet, which might have happened 

 in the immensity of time. But had that been the case, 

 its effects would still have been perceptible in .the varia- 

 tions of the geographical latitudes. If we suppose that 

 such an event had taken place, and that the disturbance 

 had been very great, equilibrium could then only have 

 been restored with regard to a new axis of rotation by 

 the rushing of the seas to the new equator, which they 

 must have continued to do till the surface was every- 

 where perpendicular to the direction of gravity. But it 

 is probable that such an accumulation of the waters 

 would not be sufficient to restore equilibrium if the de- 

 rangement had been great, for the mean density of the 

 sea is only about a fifth part of the mean density of the 

 earth, and the mean depth of the Pacific Ocean is sup- 

 posed not to be more than four or five miles, whereas 

 the equatorial diameter of the earth exceeds the polar 

 diameter by about 26i miles. Consequently the influ- 

 ence of the sea on the direction of gravity is veiy small. 

 And as it thus appears that a great change in the posi- 

 tion of the axis is incompatible with the law of equilib- 

 rium, the geological phenomena in question must be 

 ascribed to an internal cause. Indeed it is now demon- 



