WHAT KEEPS THE BICYCLER UPRIGHT? 767 



it stays up as you see ; the wheel of the bicycle revolves on its 

 axis, and therefore it stays up, was his theory and demonstration, 

 and the whole of it, and, so far as one can judge from the report, 

 he was satisfied, however it may have been with his audience. 



Of all machines, none seem to be so little understood as the 

 top and its near relation, the gyroscope. Hence the best that can 

 be said is, that the lecturer availed himself of the tendency found 

 in most minds to " explain " an unfamiliar phenomenon by re- 

 ferring it to some other more familiar one, longer known, but 

 equally incomprehensible as if, as in grammar, two negatives 

 make an affirmative, so, in physics, two unknowns make a known. 



Without going into the theory of the top, or of the gyroscope, 

 it is easy to show experimentally that their stability and that of 

 the bicycle must be due to different principles. I spin on the 

 table before you a top with a somewhat blunt point (Fig. 1). 

 You notice it runs around in a circular or rather a spiral path, 

 and gradually rises to a perpendicular. I strike it quite a hard 

 blow, but do not upset it. I send it flying across the table, or off 

 to the floor, but still it maintains its upright position. You no- 

 tice that, when it is perpendicular, it stands still ; but, if it leans 

 ever so little, it immediately begins to swing or gyrate around a 

 vertical axis. I now change the top for one whose point is very 

 fine and well centered and sharp (Fig. 2). You see that it hardly 



Fig. 1. Blunt-pointed Top. Fig. 2. Sharp-pointed Top. 



travels at all. I now cause the point to fall into a slight pit in 

 the surface of the table : it ceases to travel, but continues for a 

 very considerable time to swing around a vertical axis, and is 

 remarkably stable, whatever the angle at which it leans. Stop- 

 ping its traveling has, as you see, no effect upon its stability ; but 

 now I put my pencil before the axle and stop the gyration or 

 swinging around. Immediately the power of staying up is gone, 

 and the top falls. I may vary the experiment in every possible 

 way : so long as the axis is inclined, the result is the same ; the 

 moment the gyration ceases, the top falls. 



In the case of the bicycle there is no gyrating around a verti- 

 cal axis. Whatever else it may do, it does not do that. Yet, as 



