CH. V] MECHANICAL RECREATIONS 103 



The following experiment is explicable by the same argu- 

 ment. On the top of a vertical axis balance a thin horizontal 

 rod. At each end of this rod fasten a small vertical square or 

 sail of thin cardboard — the two sails being in the same plane. 

 If anyone blows close to one of these squares and in a direction 

 parallel to its plane, the square will move towards the side on 

 which one is blowing, and the rod with the two sails will rotate 

 about the axis. 



The experiments above described can be performed so as 

 to illustrate Hauksbee's Law; but unless care is taken other 

 causes will be also introduced which affect the phenomena : it 

 is however unnecessary for my purpose to go into these details. 



Gut on a Tennis-Ball. Racquet and court-tennia players 

 know that if a strong cut is given to a ball it can be made 

 to rebound off a vertical wall and then (without striking the 

 floor or any other wall) return and hit the wall again. This 

 affords another illustration of Hauksbee's Law The effect had 

 been noticed by Isaac Newton, who, in his letter to Oldenburg, 

 February, 1672, N.S.,says "I remembered that I had often seen 

 "a tennis-ball struck with an oblique racket describe such 

 " a curve line. For, a circular as well as a progressive motion 

 " being communicated to it by that stroke, its parts on that side 

 " where the motions conspire, must press and beat the contiguous 

 "air more violently than on the other; and there excite a 

 "reluctancy and re-action of the air proportionably greater. 

 " And. . .globular bodies," thus acquiring " a circulating motion. . . 

 " ought to feel the greater resistance from the ambient aether 

 " on that side where the motions conspire, and thence be con- 

 " tinually bowed to the other." 



The question was discussed by Magnus in 1837 and by Tait 

 in various papers from 1887 to 1896. The explanation* is that 

 the cut causes the ball to rotate rapidly about an axis through 



* See Magnus on 'Die Abweichung der Geschosse' in the Abhandlungen der 

 Akademie der Wissenschaften, Berlin, 1852, pp. 1 — 23; Lord Rayleigh, ' On the 

 irregular flight of a tennis ball,' Messenger of Mathematics, Cambridge, 1878, 

 vol. vii, pp. 14 — 16 ; and P. G. Tait, Transactions Royal Society, Edinburgh, 

 vol. xxvn, 1893 ; or Collected Scientific Papers, Cambridge, vol. II, 1900, 

 pp. 356—387, and references therein. 



