286 Sir George Greenhill [May ;>,, 



WEEKLY EVEXING MEETING, 

 Friday, May 3, 1018. 



Sir James Crichtox-Browxe, J.P. M.D. LL.D. F.R.S.. 

 Treasurer and Vice-President, in the Chair. 



Sir George Greenhill. 

 The Spinning-Top in Harness. 



" To those who study the progress of exact science, the common 

 spinning-top is a symbol of the labours and perplexities of men wha 

 had threaded successfully the mazes of planetary motion. [A sly 

 dig at Xewton, I fear, in his struggle with Precession.] The mathe- 

 matician of the last century [the eighteenth], searching through 

 Nature for problems worthy of his analysis, found in the toy of 

 youth ample occupation for the highest mathematical powers." 



These are Maxwell's words, reprinted in his Collected "Works, 

 I. p'. '24:S, from his memoir on " A Dynamical Top," and here is the 

 top, designed to show off the results of his theory. The toy of 

 youth is shown in a collection here on the table. But to make the 

 motion visible to this audience, we have chosen some on a larger 

 scale, as this 5 2 -inch wheel of Mr. C. V. Boys's Otto bi-di-cycle. 

 The point of the stalk is placed in a small cup, and the wheel is spun 

 by hand. Dynamical similitude, as well as geometrical, is obtained 

 by making the rim speed proportioual to the square root of the 

 diameter ; so that here a slow rotation, about one revolution a second^ 

 will be enough, contrasted with the 20,000 revolutions a minute m a 

 gyro-compass, or 2,500 per second of the rifle bullet — tbat is, 150,000 

 per minute. 



The ordinary toy is too small to be seen at a distance, and a 

 string must be used to spin it, to give velocity enough. A glance at 

 the length of his string will show us when the operator will bungle 

 — foozle the spin. This will be apt to happen if the string is longer 

 than this, half the stretch of the arms — a fathom. Here's the cheap- 

 jack's secret, selling this toy gyroscope here. 



The attention to reach is the great secret in boxing. The master 

 buttons on the gloves of the novice, and standing up with hands 

 behind his back he invites attack : — " Hit me ! — as hard as you can. 

 Kill me, if it pleases you ; and it does'nt hurt me ! " How mortify- 

 ing to find that he is'always just out of reach, drawing his head back 

 quite a short way ! 



The master teases her by quoting the advice in his Latin 

 grammar, how to fight the girl as he was taught as a boy — " Rixa, 



