December 22, 1910] 



NATURE 





The beam, as you will see, rotates in the same direction 

 as the cylinder, which an inspection of Fig 8 will show 

 you is just what it would do if the cylindt.- were acted 

 upon by a force in the direction in which its nose (which, 

 in this case, is the point on the cylinder first struck by 

 the blast) is moving. If I stop the blast the beam does 

 not move, even though I spin the cylinder, nor does it 

 move when the blast is in action if the rotation of the 

 cylinder is stopped ; thus both spin of the cylinder and 



Blast 



Fig. 8. 



movement of it through the air are required to develop 

 the force on the cylinder. 



Another way of showing the e.xistence of this force is 

 to take a pendulum the bob of which is a cylinder, or some 

 other symmetrical body, mounted so that it can be set in 

 rapid rotation about a vertical a.xis. When the bob of 

 the pendulum is not spinning the pendulum keeps swing- 

 ing in one plane, but when the bob is set spinning the 

 plane in which the pendulum swings no longer remains 

 stationary, but rotates slowly in the same sense as the 

 bob is spinning (Fig. 9). 



-;>>-- 



and the other having the ordinary bramble markings, are 

 mounted on an axis, and can be set in rapid rotation by 

 an electric motor. An air-blast produced by a fan comes 

 through the pipe B, and can be directed against the balls ; 

 the instrument is provided with an arrangement by which 

 the supports of the a.xis carrying the balls can be raised 

 or lowered so as to bring either the smooth or the bramble- 

 marked ball opposite to the blast. The pressure is 

 measured in the following way : — LM are tw'o tubes con- 

 nected with the pressure-gauge PQ ; L and M are placed 

 so that the golf balls can just fit in between them ; if the 

 pressure of the air on the side M of the balls is greater 

 than that of the side L, the liquid on the right-hand side 

 Q of the pressure-gauge will be depressed ; if, on the other 

 hand, the pressure at L is greater than that at M, the left- 

 hand side P of the gauge w-ill be depressed. 



I first show that when the golf balls are not rotating 

 there is no difference in the pressure on the two sides 

 when the blast is directed against the balls ; you see there 

 :s no motion of the liquid in the gauge. Next I stop the 

 blast and make the golf balls rotate ; again there is no 

 motion in the gauge. Now when the golf balls are 

 spinning in the direction indicated in Fig. 11 I turn on the 

 blast, the liquid falls on the side Q of the gauge, rises 

 on the other side. Now I reverse the direction of rotation 

 of the balls, and you see the motion of the liquid in the 

 gauge is reversed, indicating that the high pressure has 

 gone from one side to the other. You see that the pressure 

 is higher on the side M, where the spin carries this side 

 of the ball into the blast, than on L, where the spin tends 

 to carrv the ball awav from the blast. If we could 





F G. 9. 



We shall now pass on to the consideration of how these 

 forces arise. They arise because when a rotating body is 

 moving through the air the pressure of the air on one 

 side of the body is not the same as that on the other ; the 

 pressures on the two sides do not balance, and thus the 

 body is pushed away from the side where the pressure is 

 greatest. 



Thus, when a golf ball is moving through the air, 

 spinning in the direction shown in Fig. 10, the pressure 



Fig. 10. 



on the side ABC, where the velocity due to the spin 

 conspires with that of translation, is greater than that on 

 the side .\DB, where the velocity due to the spin is in 

 the opposite direction to that due to the translatory motion 

 of the ball through the air. 



I will now try to show you an experiment which proves 

 that this is the case, and also that the difference between 

 the pressure on the two sides of the golf ball depends upon 

 the roughness of the ball. 



In this instrument. Fig. 11, two golf balls, one smooth 



NO. 2147, VOL. 85] 



Fig. II. 



imagine ourselves on the golf ball, the wind would be 

 stronger on the side M than on L, and it is on the side 

 of the strong wind that the pressure is greatest. The case 

 when the ball is still and the air moving from right to 

 left is the same from the dynamical point of view as when 

 the air is still and the ball moves from left to right ; 

 hence we see that the pressure is greatest on the side 

 where the spin makes the velocity through the air greater 

 than it would be without spin. 



Thus, if the golf ball is moving as in Fig. 12, the spin 

 increases the pressure on the right of the ball and 

 diminishes the pressure on the left. 



To show the difference between the smooth ball and the 

 rough one, I bring the smooth ball opposite the blast ; \ou 

 observe the difference between the levels of the liquid in 

 the two arms of the gauge. I now 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 )ears ago. Writing to Oldenburg in 167 1 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 



