86 AXIS OF EOTATION INVARIABLE. [SECT.X. 



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, provided it be not dis- 

 turbed by a foreign cause, but that the rotation about any 

 other axis will only be for an instant, and consequently 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 per- 

 manent axes of rotation (N. 142). Hence, if the ellip- 

 soid 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 corre- 

 sponding 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 ellipsoids of revolution, that in all pro- 

 bability 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 



