

Fig. 6. 



1 86 THE POPULAR SCIENCE MONTHLY 



the left, hence the ball moves off to the left, describing the path indi- 

 cated by the dotted line ; this is the spin possessed by a " pulled " ball. 

 If the ball were spinning about an axis along the line of flight, 

 the axis of spin would pass through the nose of the ball, and the spin 

 would not affect the motion of the nose; the ball following its nose 

 would thus move on without deviation. 



Thus, if a cricket ball were 

 ^ spinning about an axis parallel to 

 ^^"^ the line joining the wickets, it 



'^ would not swerve in the air, it 



would, however, break in one way 

 or the other after striking the 

 ground; if, on the other hand, the 

 ball were spinning about a vertical axis, it would swerve while in the 

 air, but would not break on hitting the ground. If the ball were spin- 

 ning about an axis intermediate between these directions it would both 

 swerve and break. 



Excellent examples of the effect of spin on the flight of a ball in 

 the air are afforded in the game of base ball; an expert pitcher by 

 putting on the appropriate spins can make the ball curve either to the 

 right or to the left, upwards or downwards; for the sideway curves 

 the spin must be about a vertical axis, for the upward or downward 

 ones about a horizontal axis. 



A lawn-tennis player avails himself of the effect of spin when he 

 puts " top spin " on his drives, i. e., hits the ball on the top so as to 

 make it spin about a horizontal axis, the nose of the ball traveling 

 downwards, as in Fig, 4 ; this makes the ball fall more quickly than it 

 otherwise would, and thus tends to prevent it going out of the court. 

 Before proceeding to the explanation of this effect of spin I will 

 show some experiments which illustrate the point we are considering. 

 As the forces acting on the ball depend on the relative motion of the 

 ball and the air, they will not be altered by superposing the same 

 velocity on the air and the ball; thus, suppose the ball is rushing 

 forward through the air with the velocity V, the forces will be the same 

 if we superpose on both air and ball a velocity equal and opposite to that 

 of the ball ; the effect of this is to reduce the center of the ball to rest, 

 hut to make the air rush past the ball as a wind moving with the 

 velocity V. Thus, the forces are the same when the ball is moving 

 and the air at rest, or when the ball is at rest and the air moving. 

 In lecture experiments it is not convenient to have the ball flying 

 alx)ut the room, it is much more convenient to keep the ball still and 

 make the air move. 



The first experiment I shall try is one made by Magnus in 1858; 

 its object is to show that a rotating body moving relatively to the air 



