98 Mr. Sudhansukumar Banerji on Aerial 



whole of the sound of impact must be principally due to the 

 impulse given to the fluid medium by the surfaces of the 

 spheres, which undergo a sudden change of velocity as a 

 result of the impact. The only alternative explanation that 

 might be suggested is some kind of action, namely, a sudden 

 compression or rarefaction in the neighbourhood of the region 

 of contact ; but this, it seems, can hardly be correct, as the 

 spherical shape of the balls and the smallness of the relative 

 velocity of impact would not readily admit of any specially 

 intense compression or rarefaction being set up in the medium 

 round the region of contact. Probably some kind of local 

 reciprocating motion would be set up in this region, but 

 this would not be of much importance. 



The first hypothesis suggested in the preceding paragraph 

 regarding the origin of the sound can be fully tested by an 

 experimental and theoretical investigation of the distribution 

 of intensities in different directions round the colliding spheres, 

 and by studying the manner in which the sound depends 

 (1) on the duration of the impact, (2) on the coefficient of 

 restitution, (3) on the diameter of the balls, and (4) on the 

 relative velocity of impact and possibly other factors also. 



2. Measurement of the Intensity. 



The distribution of intensities in different directions round 



the colliding spheres is found to possess many remarkable 



peculiarities which would be very difficult to reconcile with 



any other hypothesis regarding the origin of the sound. 



Even by the unaided ear one can perceive that the intensity 



of the sound is greatest when heard in the direction of 



movement of the colliding spheres, and is comparatively 



quite feeble in the plane at right angles to this line. Inside 



a laboratory the reflexions from the walls of the room give 



some trouble. The contrast between the intensities in the 



two directions is therefore best appreciated by the unaided 



ear when the observations are made in the open air, so as to 



avoid such reflexions as far as possible. A rough estimate 



of the ratio of the intensities can be made by varying the 



distance of the colliding spheres from the observer. So far 



as could be judged, the sound in the direction of impact 



appeared at least three or four times more intense in one 



direction than in the other. Some uncertainty was caused 



by the difference in the character of the sound from various 



directions, this difference being so marked that by its aid 



alone the angle made by the line of collision with the direction 



of the observer could be judged with fair accuracy. Other 



remarkable peculiarities were revealed when it was arranged 



