170 AN EMPIRICAL STUDY OF GYRATING BODIES. 



speak, eacli its greatest possible leverage ; moreover, in 

 this position, the tilting forces of the two bodies act so 

 as to aid each other ; or, in other words, the total effect 

 is the sum of the individual effects. 



From this maximum the tilting force runs down to 

 nothing, when the declination of sun and moon is zero. 



As already said, two movements are produced, one at 

 right angles to a plane, through P, c, and S, and the 

 other in the direction of the pulling force, or towards S. 



Of course these 1 combine into one, but it is more con- 

 venient to consider them separately. 



We will study the lateral motion first. This move- 

 ment being affected by the earth's orbital revolution, re- 

 sults in that peculiar change in the position of the 

 nodes, or equinoxes, which astronomers call 



PRECESSION. 



While studying this phenomenon we must constantly 

 remember that we have here only a particular case of the 

 effects of changing momentum from one side of a fixed 

 point to the opposite, as we have seen them developed 

 in our reversing tee-square, and illustrated in our Bohn- 

 enbergher and top. 



Let fig. 42 represent the earth at time of winter solstice, 

 as seen from some remote point in the direction of the 

 pole of the ecliptic, and let the paper represent the 

 plane of this circle. Let the large arrow denote the di- 

 rection of the earth in its orbit. 



The solar attraction causes the protuberance, A, to 

 move towards the ecliptic with a certain small but real 

 velocity, as indicated by the small arrows, m and n, fig. 

 41. The effective component of the dowmward pull is 

 not the tangents, m and n, but perpendiculars from A 

 and B to the ecliptic. 



1 In case of the earth, the lateral movement is much the larger, and this is generally so 

 in laboratory work also ; but, sometimes, it is the smaller, e. g.. when the velocity of rota- 

 tion is small and the downward, or tilting force, great. 



154 



