156 AN EMPIRICAL STUDY OF GYRATING BODIES. 



zontal rotation, and the instrument falls. In the second 

 case, they slide on the glass, thus permitting horizontal 

 rotation, and the instrument stands. 



Instead of tilting the gyrostat, we may attach a weight 

 to one side, say, at T, fig. 32. The effect is the same. 



A very pretty and instructive experiment is as follows : 



Set the instrument, fig. 33, going, and place it on a plate 

 of glass. After a time it will begin to lean, say to the 

 right, and to gyrate more and more rapidly as the incli- 

 nation increases. Now apply with the finger, or in any 

 other way, to the side of the foot-piece, a very slight 

 pressure, in the direction in which it is moving (i. e., 

 hurry it) — the instrument will at once straighten up. 

 Retard the foot-piece by gently pressing the other way, 

 and the fall will be increased. It requires some effort 

 to do this aright, for when a thing appears to be about 

 to fall we instinctively put our hand in front of it to 

 stop it; but in this case, to put our hand in front of 

 the instrument, would accelerate its fall, while pushing 

 from behind is the way to stop its falling ; and more 

 than that, if it is almost down, pushing from behind 

 will make it become upright. From this contradiction 

 of our experience, we are very apt to put the pressure 

 on the wrong side. 



Rationale : The mass q will be pushed from the ob- 

 server, and hence, when it arrives at top — remember it 

 loses no energy in this motion — it pushes the top from 

 the observer. On the other hand, p expends its energy 

 in trying to make the supporting edge slide towards the 

 observer. The friction almost wholly prevents this. 



Experiment : Remove B A, and substitute for it a 

 similar knife blade considerably longer than B A, with, 

 however, the lower side concave, so that the instrument 

 rests on the two extremities. Set the wheel to rotating 

 as before, and place it on the glass plate. It will fall at 

 once, if the foot is long enough. 



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