C. B. WARRING. 171 



The earth's diurnal rotation, being at right angles to 

 this tilting movement, neither increases nor diminishes it ; 

 consequently, therefore, when A (fig. 42) comes to X, or, 

 more accurately speaking, to X'— for the lateral move- 

 ment of P is in a plane at right angles to S — it pulls 

 the pole westward, i. e., backward in reference to the 

 orbital motion. 1 At the same time that part of the 

 equator is depressed, and consequently, crosses the 

 ecliptic sooner than it otherwise would. In other words. 

 the node X moves in a direction opposite to that of the 

 earth. It goes from east to west. 



We have thus far taken no account of the sun's ap- 

 parent annual motion (the earth's real motion), and for 

 a moment longer will continue to regard their centres as 

 at rest. 



Of course, there would be no orbit, but we may sup- 

 pose the orbital plane — the ecliptic — to remain. There 

 would be nodes, of course, and the points between them, 

 90° from each, may still be called, as a matter of con- 

 venience, solstitial points. 



The continuous pull of the sun on A, as in each daily 

 revolution it comes around into the plane PAS, would 

 cause P to continue to descend towards X', and X (the 

 node) to move westward. The final effect would be that 

 P would pass below the ecliptic and up the other side, 

 and so around and around. In the meanwhile, P C would 

 be slowly but constantly yielding to the pulling force, 

 and thus become more and more nearly perpendicular to 

 the line C S, until, at last, having attained that condition, 

 the movement of the node, which had all along been 

 growing slower, would cease. It will be readily seen 

 that this is the history of a Bohnenbergher, as experi- 

 mentally developed. If a weight be applied at B, tig. 39, 

 precession and tilting will at once begin, and continue 



1 Of course A pulls the earth towards the ecliptic not only at X but all along. We arc 

 however, considering the total effect as concentrated at the moment A crosses the ecliptic 



155 



