172 AN EMPIRICAL STUDY OF GYRATING BODIES. 



till the weight, the point of attachment, and the centre 

 of the body, are all in a straight line and then both 

 movements will cease. 



We are now brought to the question, Why do not these 

 results really occur % Why does P describe a circle (or 

 ellipse) which, instead of crossing the ecliptic, is parallel 

 to it i What has the motion of the earth around the sun 

 to do with it ? 



To answer this question we will, for the present, con- 

 sider only that part of the action which occurs at the 

 time of the solstices. These being 90° from the equi- 

 noxes, must move with them to the westward 50" a year ; 

 consequent]}'' the direction of their pull is constantly 

 changing to the same extent, and in the same sense. 

 What is true of the direction of the sun's influence at 

 the solstices, is true of it at any distance therefrom, and 

 consequently everywhere. It is constantly moving west- 

 ward. 



Pig. 42£ may make this movement more easily under- 

 stood. Suppose the sun at winter solstice to be at 4, 

 and the pole (P) to be at a. P will move towards b, 

 and the solstitial point will in a year move to 3, when 

 the sun will meet it as it comes around from the east. 

 The next year a similar movement will bring P to c, and 

 the solstitial point will meet the sun at 2, and so on. The 

 movement is always perpendicular to the line from P to 

 sun, and hence, in a circle, having for its axis the axis 

 of the eclijrtic. 1 



N. B. Evidently P may move either way in the line 

 of force ; i. e., it may approach or recede from Z without 

 interfering with this process. 



To verify these results experimentally, take the gyro- 

 scope (fig. 2), and adjust the weight till the axle is in- 

 clined about 23i° below the horizon. Attach a cord at 



1 Any line perpendicular to an infinite plane, may be called its axis. In this case the 

 •axis " passes through the centre of the earth. 



156 



