C. B. WARRING. L75 



three or four times its present amount. Hence, at such 

 times, the difference between summer and winter rate of 

 precession would be far greater than it now is. 



2d. As the earth moves from the solstices to the 

 nodes, the tilting- influence of the sun grows smaller, 

 and, hence, the precessional rate less, until, at the node, 

 it becomes zero. 



3d. The moon constantly changes its distance from 

 the earth as well as its declination, and these movements 

 result in increasing or decreasing the tilting force, and, 

 therefore, in variation in the rate of precession. 



As the result of all the forces with their varying in- 

 tensities, the path of the pole is an ellipse of ever-chang- 

 ing eccentricity. 



NUTATION. 



As has been said, the sun and moon tend to tilt the earth 

 always in one direction — towards a vertical. The planets 

 exert their influence in the same sense. It would seem, 

 therefore, that the axis of the earth must have been 

 more oblique in the past than at present, and that, 

 eventually, it will become perpendicular to the ecliptic. 



Unwilling to accept such a conclusion, I enquired 

 whether the motion of the earth in its orbit around the 

 sun, did not in some way, introduce a compensation by 

 which the tilting influences were prevented from pro- 

 ducing their legitimate results. But a very little thought 

 sufficed to show that the effect of the annual revolution 

 is to reduce by one day in the year, the relative 

 motion of the earth on its axis. It is as if the velocity 

 of axial rotation was diminished, or the length of the 

 day increased. Experiment, as well as theory, proves 

 that a decrease of axial velocity increases the rate of 

 tilting ; consequently the effect, and so far as I could 

 then see, the only effect of the earth's orbital motion, 

 was an increase of the movement for which I was seeking 

 a compensation. 



159 



