174 AN EMPIRICAL STUDY OF GYRATING BODIES. 



that point which we call the pole of the ecliptic, and the 

 change of direction in the pull of the sun corresponds to 

 the different directions in which I pulled the string. 



Since the revolving body always tilts in the direction 

 of the applied force, — which, in this case, is our string — 

 c, steadily, although very slowly, moves towards the 

 axis about which it is revolving. In case of our earth 

 the same thing occurs, and the obliquity of the axis 

 tends to grow less — in fact, does grow less for nine years 

 out of eighteen. What happens in the other nine years 

 I shall consider hereafter. 



What I have said of the sun applies, mutatis mu- 

 tandis, to the moon. 



The precessionai movement is exceedingly like the 

 gyration of a top, the only difference being that the 

 latter gyrates in the direction in which it is revolving on 

 its axis, while the former gyrates the opposite way. 

 The reason for this is as follows : 



The "tilting force," i. e., gravity, tends to make the 

 upper end of the top recede from the vertical axis about 

 which it is gyrating ; while in case of the earth the 

 "tilting force" tends to make its pole approach the 

 vertical axis (i. e., a perpendicular to the ecliptic) about 

 which it gyrates. Hence the resulting lateral move- 

 ments are in opposite directions. 



The causes which affect the rate of precession, and 

 consequently the form of the path described by the 

 terrestrial pole, are numerous, but all may be reduced 

 to one principle, already demonstrated, to wit, "An in- 

 crease in the tilting force causes a more rapid gyration," 

 and the converse. 



1st. There is the variation in the distance of the sun. 

 This, with present eccentricity, makes a difference of 

 about one-tenth between summer and winter effects. ' The 

 eccentricity itself varies in the course of ages even to 



1 The tilting force varies inversely as the cube of the distance. 



15S 



