OF ARTS AND SCIENCES. 269 



IX. — DIFFRACTION OF SOUND. 



By William W. Jacques, 



Presented, May 10, 1876. 



The following experiments were made in order to test the possibility 

 of applying to our atmosphere the principles of Fresnel and Huyghens, 

 which, in their application to the ether, have been attended with such 

 fruitful results. 



There seems to be no a priori reason why the particles of air, form- 

 ing, as they do, a medium which, so far as the transmission of wave 

 motion is concerned, is essentially similar to the ether, should not be 

 so acted upon as to produce the interferences known in optical science 

 as diffraction fringes. The following experiments bear upon this 

 point. 



The apparatus was so arranged, in the first course of experiments, 

 as to give the best conditions for the study of external fringes, or those 

 produced outside of the geometric shadow of a sharp edge, on which 

 sound waves, diverging from a centre, were allowed to fall. 



In the second course, similar waves were made to impinge upon an 

 isolated naiTow obstacle, and so to give rise to a system of interior 

 fringes. 



All other phenomena of diffraction may be classed as particular 

 cases of one or the other of these two kinds. 



First series. A board one hundred and fifty centimetres wide was 

 placed at right angles to, and in contact with, the side of the hall. One 

 hundred and fifty centimetres from the edge, in a line at right angles to 

 the plane of the board, was placed a B^ stopped lead organ pipe. On 

 the other side of the board, a system of co-ordinates was established 

 by means of light wooden rods, running parallel and perpendicular to 

 the board ; these rods being divided into centimetres, it became very 

 easy to locate the points of interference by referring them to these 

 co-ordinates, and so to trace out the bands of interference. 



In order to distinguish these bands, I first tried applying my ear to 

 diflPerent points along one of the rods ; but they were not sufficiently 

 well marked to make them apparent to the unaided ear. I then 

 arranged a resonator, of proper size to resound to the pipe, to slide 

 along the rod ; connecting this, by a piece of firm rubber tubing, with 

 the canal of my ear. The other ear I filled first with cotton, and then 

 with putty ; so that it was entirely deaf to all sounds. I was thus en- 



