182 TENTH ANNUAL REPORT OF 



it annually. And so it will do, as often as I cliange tlie order of the 

 circular motion. If I press very gently, to produce the orbit motion, 

 without actually moving, the spheroid reverses its axis slowly ; but if 

 I begin to move rapidly, it tlirows. itself over v/ith such energy that 

 it nearly jerks the frame out of my hands. 



Now we cannot infer from this experiment, that the axial and or- 

 bital revolutions of a planet are so connected, that one must be in the 

 same direction as the other. If the earth were to be stopped in its 

 orbit, and sent backward through the signs of the ecliptic, that would 

 be no reason for its throwing itself over vyith its north pole to the 

 south, and its south pole to the north. The diurnal rotation would 

 go on undisturbed ; for we have already seen that the earth or any 

 revolving body might be projected in any way whatever through 

 space, without causing the least displacement of its axis. This ex- 

 periment is exactly in point for illustrating the composition of two- 

 revolutions, which is the topic now in hand. I make the spheroid to 

 rotate from west to east ; I then begin to carry it round me irom east 

 to west. This is in fact nothing else than turning it on its oivn axis 

 from east to west ; for^ when I commence, the side of.the frame nearest 

 to me (and of the ring, coniined to the frame) faces noHli; after a 

 quarter revolution, the same side faces east; after a half revolution, 

 west ; and so through all jioints of the compass. So far as the sphe- 

 roid is concerned, it is the same as though I take hold of the frame, 

 and turn it round in its place on the table. I repeat the experiment 

 in that manner ; and you perceive that the instant I turn the fra.me 

 and coniined ring from east to west, the spheroid reverses its poles ; 

 and on my. turning it back, from west to east, it reverses again, tluis 

 resuming its original position. Now here is no orbit-motion ; the 

 body stays in its place, and exhibits the resultant effect of two rota- 

 tions. Let us examine this case of composition. Please to notice 

 that the axis is not/ree to place itself in any position when I move the 

 frame ; the spheroid cannot, therefore, maintain a parallel position ; but 

 is, on the contrary, constrained to receive a second revolution, which 

 I impress upon it. This second revolution is round a vertical axis, 

 whether I carry the frame about me, or turn it on the table. So long as 

 the spheroid keeps its own axis precisely vertical, although revolving 

 in the opposite direction, it does not tend to turn over^ but revolves 

 with the difference of the two motions, which are in the same plane. 

 Fig. 7. But the axis of the spheroid will inevitably be jarred 



■^ gg ^ slightly from its vertical position ; and if so, it 



cannot recover it. If, for example, the upper pole 

 is jarred towards me, each particle on the right hand 

 will, by the first rotation, be moving from me in a 

 line slightly ascending ; and, by the second, horizon- 

 tally towards me ; thus the two forces v/iil act at a 

 large obtuse angle, within which the particle will 

 direct itself, throwing the upper pole farther towards 

 ine.* The angle of the forces is thus diminished a 



» In figure 7, the particles at R ascend from the observer in the line of the arrow A, by 

 the revolution of the spheroid ; and move hormmtalhj lowards him in the line of the arrow 

 B, by pressure on the frame ; they, therefore, move Itiuxtn, as shown by the arrow C ; 

 that is, the upper end of the axis N moves towards him. 



