36 



J. F. TWISDEN ON POSSIBLE DISPLACEMENTS 



earth's axis of rotation coincides sensibly with the axis of figure. 

 In the diagram suppose that is the centre, and C the axis 



about which the earth is at any instant revolving ; and let us suppose 

 that in consequence of elevations and depressions the axis of figure 

 is shifted into the position C, C being on the surface of the earth. 

 With centre C and radius C'C describe the small circle C P Q K, D. 

 The lines which join points in the circumference of this circle to 

 will be successively axes of the earth's rotation ; and when any one 

 of them is actually the axis of rotation, it will come into a posi- 

 tion nearly coinciding with the original direction of OC, i. e. with Oz, 

 which continues nearly fixed in space. Thus OP, OQ, OR, OD will 

 come successively into a position nearly coinciding with Oz, and when 

 there will be at that instant the axis of rotation. If COC were 

 an angle of 20° there would be a small diurnal variation in the 

 direction of Oz ; but the main effect would be to bring OD round 

 into coincidence with Oz, i. e. to bring a point in latitude 50° N". up 

 to the north pole, and then to bring it round again to its original 

 position. This interchange of positions would go on continuously, 



