180 AN EMPIRICAL STUDY OF GYRATING BODIES. 



creased its precessional movement, and began to descend. 

 I opened the valve again ; the precessional motion began 

 to grow less, and the gyroscope to ascend. 



At another time I reversed the experiment. I closed 

 the valve of the empty pail, and set the wheel going as 

 before. There was no horizontal or vertical movement. 

 The wheel revolved, and the instrument stood still. I 

 then poured into the pail a little shot, and then more 

 and more. The gyroscope started off in both horizontal 

 and downward movements, increasing, as I poured in the 

 shot, until the rapidity of the motion compelled me to 

 desist. 



The precessional movement went on in the same direc- 

 tion, whether the shot were dropping out, or were being 

 poured in, but the tilting was first up and then down, 

 and the precession varied its rate. 



It may perhaps be questioned whether we can con- 

 fidently reason from this experiment to the movements 

 of our earth, because in the gyroscope, the fixed point 

 of support is at one end of the axis, and in our earth it 

 is at the centre. I therefore tried the same experiment 

 on my Bohnenbergher (fig. 39), in which the fixed point 

 is at the centre, and found the results the same. 



We may, therefore, formulate the following most ex- 

 traordinary law which, so far as I know, is new : 



THE LAW OF NUTATION. 



If a body revolving on its axis be acted upon by a 

 force tending to make it revolve about another axis per- 

 pendicular to the first, it will tilt in the direction of the 

 applied force, if this is constant or increasing ; and it 

 will tilt in the opposite direction, if it — the applied 

 force — is decreasing. 



In other words, pulling down will, in one case, send 

 the body down, and in the other will send it up, as the 

 pull is an increasing or diminishing one. 



16-4 



