2n2 



NATURE 



[December 22, igio 



be the nose. If it is moving in an inclined direction CP, 

 as in Fig. 2, then A will be the nose. 



Now let the ball have a spin on it about a horizontal 

 axis, and suppose the ball is travelling horizontally as in 

 Fig. 3, and that the direction of the spin is as in the 



KiG. 2. 



figure, then the nose A of the ball is moving upwards, and 

 since by our rule the ball tries to follow its nose, the 

 ball will rise and the path of the ball will be curved as 

 in the dotted line. If the spin on the ball, still about a 

 horizontal axis, were in the opposite direction, as in 



,4 



F... 3. 



Fig. 4, then the nose A of the ball would be moving 

 downwards, and as the ball tries to follow its nose it 

 will duck downwards, and its path will be like the dotted 

 line in Fig. 4. 



Let us now suppose that the ball is spinning about a 



vertical axis, then if the spin is as in Fig. 5, as we look 

 along the direction of the flight of the ball the nose is 

 movmg 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. 



Fig. 



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 



x^gglpv 



Fig. 6. 



off to the left, describing the path indicated 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 



NO. 2147, VOL. 85] 



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 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 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 down- 

 ward 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 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 tli 

 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 centre of the ball to rest, but to 



2:^ 



Fig. 7. 



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

 the velocity \'. 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 about the room ; it 

 is much more convenient to keep the ball still and make 

 the air move. 



The first e.xperiment I shall try is one made by Magnus 

 in 1852 ; its object is to show that a rotating body moving 

 relatively to the air is acted on by a force in the direction 

 in which the nose of the body is moving relatively to its 

 centre ; the direction of this force is thus at right angles 

 both to the direction in which the centre of the body is 

 moving and" also to the axis about which the body is 

 spinning. For this purpose a cylinder A (Fig. 7) is 

 mounted on bearings so that it can be spun rapidly about 

 a vertical axis ; the cylinder is attached to one end of the 

 beam B, which is weighted at the other end, so that when 

 the beam is suspended by a wire it takes up a horizontal 

 position. The beam yields readily to any horizontal force, 

 so that if the cylinder is acted on by such a force this will 

 be indicated by the motion of the beam. In front of the 

 cylinder there is a pipe. D, through which a rotating fan 

 driven by an electric motor sends a blast of air which 

 can be directed against the cylinder. I adjust the beam 

 and the beam carrying the cylinder so that the blast of 

 air strikes the cylinder symmetrically ; in this case, when 

 the cylinder is riot rotating the impact against it of the 

 stream of air does not give rise to any motion of the 

 beam. I now spin the cylinder, and you see that when 

 the blast strikes against it the beam moves off sideways. 

 It goes off one way when the spin is in one direction, and 

 in the opposite way when the direction of spin is reversed. 



