110 Mr. Sudhansukumar Banerji on Aerial 



we have assumed for the origin of the sound. This hypo- 

 thesis may therefore be regarded as confirmed by expe- 

 riment to the practical exclusion of any others. It was, 

 however, considered that further experimental study of 

 the character of the sound-wave emitted in different direc- 

 tions by the colliding spheres would be of very great 

 interest. 



Attempts have been made to obtain photographic records 

 of the motion of the mica disk under the action of the sound- 

 wave by various methods. The first, due to Siegbahn *, was 

 that of optically recording the motion of a pointer attached 

 to the disk by the use of two microscopes focussed on it, one 

 •on either side. Another method which was used was simply 

 to fix a small mirror to the mica disk at the place of greatest 

 angular deflexions and to photograph on a falling plate the 

 motions of the image of an illuminated slit formed by 

 reflexions from the surface of the mirror. While both of 

 these methods gave results confirming the broad indications 

 of theory, the photographs obtained could not be regarded 

 as satisfactory records of the character of the sound-wave 

 owing to the free vibrations of the mica disk excited by the 

 sound of the impact which continued for an appreciable 

 period. Even the first two or three swings, which were 

 much larger in amplitude than the others, showed the free 

 vibrations somewhat prominently. This was evidently due 

 to the highly impulsive character of the sound-wave. Some 

 improvement was obtained by using a mica disk stiffened by 

 attachment to a wire stretched in front of it under tension. 

 (This was taken out from an old gramophone.) The motion 

 of the wire was recorded photographically. Even with this 

 arrangement, however, the free vibrations of the receiving 

 apparatus were prominent. Unless a sufficiently sensitive 

 and at the same time strongly damped recorder is found, 

 there does not appear much hope of obtaining a satisfactory 

 direct record of the character of the sound-pulse. The 

 writer hopes shortly to try the use of an acet3dene-gas 

 manometric flame and will also make further experiments 

 with receivers of various types for obtaining an accurate 

 record of the character of the sound-wave. 



Experiments have also been made with the ballistic 

 apparatus described in the first part of the paper to compare 

 the effects of the impacts of balls of the same size but of 



* Phil. Mag., May 1914. 



