CH. V] MECHANICAL RECREATIONS 105 



suppose to represent the section of the vertical wall in a racquet- 

 court. In other words, the ball tends to move at right angles to 

 the line in which its centre is moving and in the direction 

 in which the surface of the front of the ball is being carried by 

 the rotation. Sir J. J. Thomson has pointed out that if we 

 consider the direction in which the nose (or foremost point) of 

 the ball is travelling, we may sum up the results by saying that 

 the ball always follows its nose. Lord Rayleigh has shown that 

 the line of action of resulting force on the ball is perpendicular 

 to the plane containing the direction (m) of motion of the centre 

 of the ball and the axis (s) of spin, and its magnitude varies 

 directly as the velocity of translation, the velocity of spin, and 

 the sine of the angle between the lines m and s. 



In the case of a lawn tennis-ball, the shape of the ball is 

 altered by a strong cut, and this introduces additional compli- 

 cations. 



Spin on a Cricket-Ball. The curl of a cricket-ball in its 

 flight through the air, caused by a spin given by the bowler 

 in delivering the ball, is explained by the same reasoning. 



Thus suppose the ball is delivered in a direction lying in a 

 vertical plane containing the middle stumps of the two wickets. 

 A spin round a horizontal axis parallel to the crease in a 

 direction which the bowler's umpire would describe as positive, 

 namely, counter clock-wise, will, in consequence of the friction 

 of the air, cause it to drop, and therefore decrease the length 

 of the pitch. A spin in the opposite direction will cause it to 

 rise, and therefore lengthen the pitch. A spin round a vertical 

 axis in the positive direction, as viewed from above, will make 

 it curl sideways in the air to the left, that is, from leg to off. 

 A spin in the opposite direction will make it curl to the right. 

 A spin given to the ball round the direction of motion of the 

 centre of the ball will not sensibly affect the motion through 

 the air, though it would cause the ball, on hitting the ground, 

 to break. Of course these various kinds of spin can be combined. 



Flight of Golf-Balls. The same argument explains the 

 effect of the spin given to a golf-ball by impact with the club. 

 Here the motion takes place for a longer interval of time, and 



