Waves venerated by Impact. 



1.01 



line joining the centres, and that it gradually diminishes 

 until it practically vanishes at an angle of about 67° with the 

 line joining the centres, when it again increases rather 

 abruptly until it attains a second maximum value at an 

 angle of 90°. The experiment has been tried with pairs of 

 spheres of various materials, viz. (1) billiard-balls, (2) marble, 

 (3) aluminium, and (4) wood, and analogous results are noted 

 in all the cases. 



It is not difficult to see, in a general way, that the distri- 

 bution of intensity shown in fig. 1 is in accordance with the 

 hypothesis as to the origin of the sound with which we started. 

 As a result of the impact the balls undergo very rapid changes 

 of velocity in opposite directions. The case is somewhat 

 analogous to the well-known effect of the zones of silence 

 noticed in the neighbourhood of the prongs of a tuning-fork, 

 but differs from it somewhat owing to the spherical shape of 

 the balls, the non-periodic character of their motion, and their 

 close contiguity at the instant of impact. 



That the results shown in fig. 1 are quite reliable has been 

 further tested by three methods. By measuring the deflexions 

 of the spot of light for impacts at different distances from 

 the mouth of the horn, the balls being made to impinge 

 always with the same velocity and in the same direction, 

 the deflexions are found practically to vary inversely as the 

 square of the distances from the mouth of the horn. The 





Fig. 2. 



* 16 



^' 2 



i 1 n i | I , i o 



><l i j I j j I ; ; i : i i 



a ! ! I ! i III! 



4 6 8 10 12 



Inverse sauare rf the distance — * 



Tesults are shown in fig. 2, where the squares of the reci- 

 procal of the distances have been plotted against the 

 deflexions. The curve shows that over a very wide range 

 the deflexions practically vary inversely as the square of the 



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