1910] on the Dynamics of a Golf Ball. 797 



flight of the ball the nose is moving to the right ; hence by our rule 

 the ball will move off to the right, and its path will resemble the 

 dotted line in Fig. 5, in fact, the ball will behave like a sliced ball. 

 Such a ball, as a matter of fact, has spin of this kind about a vertical 

 axis. 



If the ball spins about a vertical axis in the opposite direction as 

 in Fig. 6, then, looking along the line of flight, the nose is moving to 

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



\ 

 \ 



Fig. 5. 



indicated by the dotted line; this is the spin possessed by a "pulled" 

 ball. 



If the ball were spinning about an axis along tlie 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 



Fig. 6. 



would swerve while in the air, but would not break on hitting the 

 ground. If the ball were spinning 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 cf 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 travelling 



