THE DYNAMICS OF A QOLF BALL 



189 



the pressure at L is greater than that at M the left-hand side P of the 

 gauge will 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 



Fig. 11. 



against the balls; you see there is 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 carry the ball 

 away from the blast. If we could 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 mov- 

 ing 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 great- 

 est 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 dimin- 

 ishes 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; you observe the difference 

 between the levels of the liquid in the two arms of the gauge. I now 



Pia. 12. 



