800 



Professor Sir J. J. Thomson 



[March 18, 



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 on the side ABC, where 



Fig. 10. 



the velocity due to the spin conspires with that of translation, is 

 greater than that on the side A D B, 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. 



Fig. 11. 



In this instrument. Fig. 11, two golf -balls, one smooth 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 axis carrying the balls- 



