76 AXIS OF ROTATION INVARIABLE. SECT. X. 



highest mountains and in almost every part of the globe, that 

 immense continents have been elevated above the ocean which 

 must have engulfed others. Such a catastrophe would be occa- 

 sioned by a variation in the position of the axis of rotation on 

 the surface of the earth ; for the seas tending to a new equator 

 would leave some portions of the globe and overwhelm others. 

 Now, it is found by the laws of mechanics that in every body, 

 be its form or density what it may, there are at least three axes 

 at right angles to each other, round any one of which, if the 

 solid begins to rotate, it will continue to revolve for ever, pro- 

 vided it be not disturbed by a foreign cause, but that the rota- 

 tion about any other axis will only be for an instant, and con- 

 sequently the poles or extremities of the instantaneous axis of 

 rotation would perpetually change their position on the surface 

 of the body. In an ellipsoid of revolution the polar diameter and 

 every diameter in the plane of the equator are the only permanent 

 axes of rotation (N. 145). Hence, if the ellipsoid were to begin 

 to revolve about any diameter between the pole and the equator, 

 the motion would be so unstable that the axis of rotation and the 

 position of the poles would change every instant. Therefore, as 

 the earth does not differ much from this figure, if it did not turn 

 round one of its principal axes, the position of the poles would 

 change daily ; the equator, which is 90 distant, would undergo 

 corresponding variations ; and the geographical latitudes of all 

 places, being estimated from the equator, assumed to be fixed, 

 would be perpetually changing. A displacement 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 ellip- 

 soids 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 rota- 

 tion by a foreign cause, as the collision of a comet, which might 



