i88 THE POPULAR SCIENCE MONTHLY 



Another way of showing the existence of this force is to take a 

 pendulum whose bob is a cylinder, or some other symmetrical body, 

 mounted so that it can be set in rapid rotation about a vertical axis. 

 When the bob of the pendulum is not spinning the pendulum keeps 



swinging in one plane, but when 

 the bob is set spinning the plane 

 in which the pendulum swings no 

 longer remains stationary, but ro- 

 tates slowly in the same sense as 

 the bob is spinning (Fig. 9). 



We shall now pass on to the 



consideration of how these forces 



arise. They arise because when 



a rotating body is moving through 



^ ■^----- Xv the air the pressure of the air on 



~ ^ _ _ - ~ -,- - ^ ~ X 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 the 

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

 than that on the side ADB, 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 and the other 

 having the ordinary bramble markings, p^^ ^q 



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 can be raised or lowered so as to bring either 

 the smooth or the bramble-marked ball opposite to the blast. The pres- 

 sure is measured in the following way: LM are two tubes connected 

 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. 



