192 THE POPULAR SCIENCE MONTHLY 



one per cent, of the atmospheric pressure to produce a force on the ball 



greater than its weight. The ball leaves the club in a good drive with 



a velocity sufficient to produce far greater pressures than this. The 



consequence is that when the ball starts from the tee spinning in the 



, direction shown in Fig. 14, this is often 



called underspin, the upward force due 



to the spin is greater than its weight, 



V' ^^.^-^ thus the resultant force is upwards, 



,'',^'-^^ and the ball is repelled from the earth 



^__^^ ^y^^^- " instead of being attracted to it. The 



f j^ consequence is that the path of the 



V_</ ball curves upward, as in the curve A, 



^°' ■ instead of downwards, as in B, which 



would be its path if it had no spin. The spinning golf ball is in fact 

 a very efficient heavier than air flying machine, the lifting force may 

 be many times the weight of the ball. 



The path of the golf ball takes very many interesting forms as 

 the amount of spin changes. We can trace all these changes in the 

 arrangement which I have here, and which I might call an electric 

 golf links. With this apparatus I can subject small particles to forces 

 of exactly the same type as those which act on a spinning golf ball. 



Fig. 15. 



These particles start from what may be called the tee A (Fig. 15) . This 

 is a red hot piece of platinum with a spot of barium oxide upon it, the 

 platinum is connected with an electric battery which causes negatively 

 electrified particles to fly off the barium and travel down the glass 

 tube in which the platinum strip is contained; nearly all the air has 

 been exhausted from this tube. These particles are luminous, so that 

 the path they take is very easily observed. We have now got our 

 golf balls off from the tee, we must now introduce a vertical force 

 to act upon them to correspond to the force of gravity on the golf 

 ball. This is easily done by the horizontal plates BC, which are 

 electrified by connecting them with an electric battery; the upper one 

 is electrified negatively, hence when one of these particles moves 

 between the plates it is exposed to a constant downwards force, quite 

 analogous to the weight of the ball. You see now when the particles 

 pass between the plates their path has the shape shown in Fig. 16; 



