THE SMITHSONIAN INSTITUTION. 187 



F'g-i<>- itself at that angle, and revolve as before. 



The direction of the horizontal revolution is the 

 reverse of that which the spheroid has when 

 broufijht down to hang beneath the wire. For 

 example, in the present experiment, I made 

 the spheroid rotate from W. to E. ; and you see 

 the system going from E. to W. 1 will now 

 revolve the spheroid from E. to W. ; and having 

 dropped it again, you observe the revolution 

 C _ ''I; ;; to be reversed ; the system revolves from W. to 



Unaccountable as these phenomena appear at first, they are found 

 to be very obvious cases of compound rotations. Gravity, at first 

 sight, appears to have no effect on the weight, since it is not at all 

 depressed. But it is, in truth, exerting its full energy upon it every 

 moment, producing, in conjunction with the rotation of the spheroid, 

 the horizontal revolution. Let me stop the latter motion for a few 

 moments, that we may examine the manner in which the two forces 

 are com])ounded. As I hold the spheroid up on the right of the wire, 

 the particles on the top are coming toiuards me ; if I should abandon 

 it to the action of gravity, that force would urge the same particles to 

 the rigid., in the arc of a vertical circle, described about the hook as a 

 centre ; consequently they assume a direction between these two direc- 

 tions, which can be done only by the system moving, not downward, 

 but horizontally from me. This composition of forces is momentarily 

 repeated, in exactly the same circumstances, and hence the rotation is 

 continued uniformly so long as the spheroid maintains its speed. 



There is another species of disturbance in the planetary motions, 

 easily illustrated by experiments, and which will demand but a few 

 moments' attention. The orbits of the planets and satellites, though 

 nearly circular, are really ellipses ; and, if no attraction operated on 

 a given revolving body except that of the central body, the elli[)se 

 would always present its longest axis in the same direction. But this 

 is not true in fact. The remote end of the longest diameter of the 

 earth's orbit, called its aphelion, which now points to the constella- 

 tion Gemini, ten thousand years ago was directed to Taurus, and ten 

 thousand years hence will be advanced in the order of the signs to 

 Cancer. This motion is so exceedingly slow that sixty thousand years 

 will be required for the aphelion and perihelion to change places, and 

 one hundred and twenty thousand years to make a full revolution. 

 The extremities of the moon's orbit, in like manner, are advancing ; 

 but the disturbance in this case is rapid, since they pass entirely round 

 the heavens in about nine years. 



Tou will observe that this line (called the line of the apsides) travels 

 in the same direction as the revolving body, while the line of the 

 nodes moves, as we have seen, the opposite way. 



This eiiect is produced by the action of a body, or of bodies, lying 



■*■ Fig. 10. The particles at A are moving in the line D, by the rotation of the spheroid, 

 and are nrged l>y gravity towards B, in the plane of a vertical circle around the centre, H. 

 The resultant is towards E, vvdiich direction can be attained only by tlie rotation of the 

 centre of gravity G, in the order of the arrows F, horizontally around the centre C. 



