Page:Popular Science Monthly Volume 83.djvu/206

202 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 rotating 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 demonstration. The air is driven past the ball by a centrifugal blower and the pressures on two opposite sides of the ball are indicated by manometers as shown in Fig. 7.

When the ball is not rotating, the 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 which 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 manometers indicate unequal pressures on the two sides of the ball (shown