i 9 o THE POPULAR SCIENCE MONTHLY 



move the rough ball into the place previously occupied by the smooth 

 one, and you see that the difference of the levels is more than doubled, 

 showing that with the same spin and speed of air blast the difference 

 of pressure for the rough ball is more than twice that for the smooth. 

 We must now go on to consider why the pressure of the air on 

 the two sides of the rotating ball should be different. The gist 

 of the explanation was given by Newton nearly 250 years ago. Writing 

 to Oldenburg in 1671 about the dispersion of light, he says, in the 

 course of his letter, " I remembered that I had often seen a tennis- 

 ball struck with an oblique racket describe such a curved line. For 

 a circular as well as 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, and there excite a 

 reluctancy and reaction of the air proportionately greater." This 

 letter has more than a scientific interest — it shows that Newton set an 

 excellent precedent to succeeding mathematicians and physicists by 

 taking an interest in games. The same explanation was given by 

 Magnus, and the mathematical theory of the effect is given by Lord 

 Eayleigh in his paper on " The Irregular Flight of a Tennis Ball," 

 published in the Messenger of Mathematics, Vol. VI., p. 14, 1877. 

 Lord Eayleigh shows that the force on the ball resulting from this 

 pressure difference is at right angles to the direction of motion of the 

 ball, and also to the axis of spin, and that the magnitude of the force 

 is proportioned to the velocity of the ball multiplied by the velocity 

 of spin, multiplied by the sine of the angle between the direction of 

 motion of the ball and the axis of spin. The analytical investigation 



of the effects which a force of this 

 IHZZZZ!ZZZZ_ tyP e would produce on the move- 

 >, ment of a golf ball has been dis- 



cussed very freely by Professor Tait, 

 Fig. 13. who also made a very interesting 



series of experiments on the veloc- 

 ities and spin of golf balls when driven from the tee and the resistance 

 they experience when moving through the air. 



As I am afraid I can not assume that all my hearers are expert 

 mathematicians, I must endeavor to give a general explanation without 

 using symbols, of how this difference of pressure is established. 



Let us consider a golf ball, Fig. 13, rotating in a current of 

 air flowing past it. The air on the lower side of the ball will have 

 its motion checked by the rotation of the ball, and will thus in the 

 neighborhood of the ball move more slowly than it would do if there 

 were no golf ball present, or than it would do if the golf ball were 

 there but was not spinning. Thus if we consider a stream of air 

 flowing along the channel PQ, its velocity when near the ball at Q 



