ARTICLES 59; 



body, so that we have here another basis of investigation besides 

 the pull of the sun and moon and planets on the equatorial 

 protuberance. The present centre of polar motion may be 

 the resultant, for all we know, of both sets of forces. Newton 

 having found a cause for this time-honoured theory, the prestige 

 of his name made it readily adopted ; hence the curve traced 

 by the earth's axis in the heavens was accepted as a circle, 

 and the centre of the circle must, if Newton was right, be the 

 pole of the ecliptic. 



The important point, however, is that if the earth's centre 

 of gravity is not the centre of its figure, the centre of the pre- 

 cession curve cannot well be the pole of the ecliptic ; and that 

 the centre of gravity is eccentric is supported by the fact that 

 pendulum experiments disclose a want of symmetry in the shape 

 of the earth. If to this is added the disparity in density and 

 distribution of continent and ocean it will be recognised how 

 improbable it is that the pole of the ecliptic and the centre of 

 the precession movement should exactly coincide. 1 



In Herschel's Outlines of Astronomy we can now under- 

 stand the theory on which Art. 316 is based, where he says : 

 " It is found that in virtue of the uniform part of the motion 

 of the pole it describes a circle in the heavens round the pole 

 of the ecliptic as a centre, keeping constantly at the same 

 distance of 23 28' from it." Later on he tells us that it does 

 vary its distance at the rate of about 48" per century. It is 

 now 23°26' 59"'36. 



So far the only explanation offered to meet this paradox 

 of a circle round a point with a varying radius has been by 

 ascribing it to a movement of the earth's orbital plane with 

 respect to the invariable plane, investigated by Laplace with 

 a view to establishing the stability of the planetary system. 



Drayson has dealt with this matter fully and has shown 

 that this movement, if it occurs, can by no means affect the 

 obliquity of the ecliptic. As a matter of fact the distance 

 between the pole of the heavens and the pole of the ecliptic, 



1 Robert Grant in his History of Physical Astronomy (Bohn), writing of 

 Newton's solution of the problem relating to the attraction of spheroids, said that 

 Newton "found that the polar axis was to the equatorial as 229 to 230. The 

 ellipticity of the earth is considerably greater, whence it may be inferred that the 

 density is not homogeneous." Recent seismological research has led to the 

 opinion that below a distance of T V of the earth's radius the earth's substance is 

 fluid {English Mechanic, February 21st, 191 9). 



39 



