THE POSITION OF THE EARTH^S AXIS. 123 



sun and moon on the mass of the equatorial bulge. It is 

 from no sufl&cient change having taken place that we have 

 been told the figure of rotation has not altered in 3000 

 years (the limit of known observation); but this has been 

 based upon a preternatural rigidity of the earth. Now, 

 although we see that the earth does not instantaneously 

 take its new figure after any change, or else every moun- 

 tain would be levelled in twenty-four hours, yet the changes 

 must be taking place so gradually and the earth readjust- 

 ing itself so imperceptibly, that I do not know if ob- 

 servation ought to be able to discover any such small 

 changes in the precession and nutation. Further, we must 

 not forget that in many conditions of configuration the 

 forces working to move the axis in one direction would be 

 balanced by those in the other ; and this may have been the 

 condition of the earth during the last 3000 years. Geolo- 

 gical evidences show, from the smallest scale to the largest, 

 that the earth is not an unyielding body. This the plica- 

 tions and foldings of the hardest rocks show, and the manner 

 of elevation of continental areas : this has not always been 

 allowed by physicists. But now Sir William Thomson, in 

 his address to the Mathematical Section of the British 

 Association, 18 76, said, " were it of continuous steel and 500 

 kilometres thick, it would yield as much as if it were india 

 rubber to the deforming influence of centrifugal force, and 

 of the sun- and moon-attraction "'^. 



* It will be seen that it is not the rigidity, as bearing upon the question of 

 tides or of a iluid interior, that is under consideration, but how far the earth 

 is a yielding body — whether it may, "as a whole, be stiff enough to practi- 

 cally fulfil the condition of unyieldingness." To which Sir William Thomson 

 answers, "No, decidedly it would not." It is true that, with a solid globe, the 

 deforming influence which a fluid interior would give by the outward pres- 

 sure of this fluid, caused by the sun and moon's attraction, would be wanting ; 

 yet the centrifugal force, which is always acting in the same direction, and 

 not intermittently, as tidal periods, would alone, with a yielding body, seem 

 suflicient to change the figure. 



