C. B. WARRING. 173 



c, and pass it over a fixed pulley in the ceiling, vertically 

 above the point of the screw S. Set the wheel revolving, 

 and hook to the free end of the cord a weight — for my 

 instrument two or three pounds, — and then leave the ap- 

 paratus to itself . The pole, c, will slowly yield to the 

 pull of the cord, and, at the same time, revolve with great 

 comparative rapidity around the fixed point. The axis 

 will soon rise to a horizontal — i. e., it becomes jierpen- 

 dicular to the line reaching to the pulley, or, if we trans- 

 fer this all to the solar system, perpendicular to a line 

 from the rotating body to the sun. 



This illustrates the first and simplest kind of precession, 

 to wit, when the earth and sun do not change their rela- 

 tive positions. In case of our earth, however, the pulling 

 force becomes zero so soon as the axis becomes perpen- 

 dicular to the line from centre to the sun, and therefore 

 the process stops ; but in our experiment, the pull does 

 not become zero, and consequently the tilting process 

 would continue till c comes into the line between the 

 pulley overhead and the point of the screw, S. 



To show what occurs when the direction of the sun is 

 constantly changing— as if it really revolved around the 

 earth, — use the same instrument, adjusted as before — 

 axis inclined 23|-° below the horizon. As it would be in- 

 convenient to have the cord as long as in the previous 

 experiment, and, as its length is immaterial, since it is 

 direction and not distance that produces the result, I 

 use a cord only ten or twelve inches long. I first jduII c 

 with a moderate force vertically upward ; it goes hori- 

 zontally from me. I then pull the cord horizontally 

 from me ; c drops vertically downward. I pull c ver- 

 tically downward ; it moves horizontally towards me. I 

 then pull c horizontally towards me ; and it rises. It 

 has made a circuit of 360° around a horizontal line 

 passing through the fixed point at bottom of s. This 

 line corresponds to the line from centre of the earth to 



157 



