Lord Rayleigh's Acoustical Observations, 283 



diminish X as far as possible, it is advisable to use sounds of 

 very high pitch, which have the additional advantage of 

 readily exciting sensitive flames. I have found it best to work 

 indoors, in which case a disk of 15 inches diameter is suitable; 

 with a much larger disk and in an ordinary room there would 

 hardly be sufficient free space on all sides. I have tried a 

 considerable variety of sources of sound, including electric 

 sparks, a small electric bell, and a Galton's whistle ; but the 

 best results were obtained with a bird-call and with a squeaky 

 toy-reed. 



On November 20 the source was a bird-call blown with a 

 pressure of 4" water, and was placed about 20" from the 

 15" disk. The observation was made at a distance of 24" on 

 the further side of the disk, and was successful both with the 

 ear and with the sensitive flame. In the former case I em- 

 ployed a plate of wood bored with a hole about |" in diameter, 

 and held against the side of the head in such a position that 

 the hole was opposite the ear-passage. The head was moved 

 about until the position of maximum sound was determined, 

 and was then withdrawn, leaving the plate in situ. In order 

 to verify conveniently that the position of maximum sound 

 was really at the centre of the shadow, a hole was bored through 

 the centre of the disk, which could be closed with a cork during 

 the adjustment of the ear-plate. When the adjustment was 

 complete, the cork was removed; and then the eye placed 

 behind the ear-plate would see the source of sound through 

 the two holes. With a little practice the central point could 

 be picked out almost as well by ear as by eye. 



With a sensitive flame the observation was even easier. 

 The most suitable is that from a pin-hole burner brought 

 near the flaring-point by a gas-pressure of about 10" of 

 water. To get the best result, the pressure must be care- 

 fully adjusted ; and in order to avoid disturbance, it is ad- 

 visable to move the source rather than the flame. When 

 the place of maximum effect has been determined, the cork 

 is removed from the central hole of the disk, and the gas 

 is lowered. By looking just over the burner it is then easy 

 to see whether or not the source of sound occupies the central 

 position. 



On November 24 the toy reed was substituted for the bird- 

 call, the disk and distances being the same as before. In the 

 case of this source the experiment succeeded better with the 

 flame than with the ear. 



On a subsequent occasion a larger disk, of 30" diameter, was 

 tried ; but the results were not so good, probably in conse- 

 quence of the increased effect of reflection from the floor and 

 walls of the room. 



