202 THE POPULAR SCIENCE MONTHLY 



direction, the low pressure region would have been on the other side of 

 the ball and it would have curved in the opposite direction. 



In order to show this difference in pressure on the sides of a rotat- 

 ing ball as it is thrown through the air, or in practise as the air is 

 driven past the ball, the author has devised the following demonstra- 

 tion. The air is driven past the ball by a centrifugal blower and the 

 pressures on two opposite sides of tlie ball are indicated by manometers 

 as shown in Fig. 7. 



When the ball is not rotating, tlie velocity of the air on the two sides 

 of the ball is the same (shown by equal density of stream lines in top 

 view section of Fig. 8) and the manometers indicate equal pressures on 

 the two sides of the ball (end view of Fig. 8). This is equivalent to the 

 ball going through the air without rotating and without curving to 

 either side as shown by the heavy arrow. However, the pressure on 

 either side of the ball is less than the pressure in the still air outside 

 the tube whicli directs the air past the ball; that is, the high velocity 

 regions near the ball are low-pressure regions. 



When the ball is rotating as shown in Fig. 9 the friction against the 

 surface of the ball accelerates the flow of air past it on one side and 

 retards the air stream on the other side; that is, the stream lines are 

 more dense on one side (shown in top view of Fig. 9) and the man- 

 ometers indicate unequal pressures on tlie two sides of the ball (shown 



Fio. 



