370 



Mr. C. A. Bell. 



[May 13, 



at a distance from the orifice would be impossible. This velocity is 

 much less than that of sound in air, and is probably the mean velocity 

 of the stream. 



A vibrating air-jet playing into free air gives rise to very feeble 

 sounds, but these sounds are much intensified when the jet impinges 

 on any obstacle which serves to divide it into two parts. Of such 

 arrangements the best is a perforated surface, the orifice being placed 

 in the axis of the jet. 



A jet of air at low pressure responds to and reproduces only 

 sounds of low pitch. Sounds above a certain pitch, which depends 

 on the pressure, either do not affect it or are only faintly reproduced. 



At pressures between 10 and 12 mm. of water an air-jet reproduces 

 all the tones of the speaking voice, and those usually employed in 

 music, with the exception of very shrill or hissing noises. When 

 the pressure in the reservoir equals about 13 mm. of water, hissing 

 sounds are well reproduced, while sounds of low pitch become fainter. 

 At higher pressures, up to about 25 mm. of water, shrill or hissing 

 noises produce very violent disturbance, while ordinary speech tones 

 have little effect. But at these pressures sounds of high pitch 

 frequently cause the jet to emit lower sounds of which they are 

 harmonics. 



In general a pressure of about 12 mm. of water will be found 

 most suitable for reproducing speech or music. Under this condition 

 the jet is very sensitive to disturbances of all kinds, and will repro- 

 duce speech, music, and the irregular sounds classified as " noises." 



It must be understood that the pressures here given are only suit- 

 able for jets of not too small diameter. When the diameter of the 

 orifice is only a small fraction of a millimetre the above limits may be 

 much exceeded ; since the velocity of efflux no longer depends solely 

 on the pressure. 



A jet of air escaping from a perfectly circular orifice does not 

 vibrate spontaneously so as to emit a musical sound. But musical 

 vibrations may be excited in it by the passage of the air on its way 

 to the orifice through a resonant cavity, or through any irregular 

 constriction. 



An air- jet impinging on any obstacle, such as a flame, frequently 

 vibrates spontaneously, if the obstacle is at sufficient distance and of 

 such a nature as to diffuse the disturbances produced by impact or 

 throw them back on the orifice. This constitutes one of the chief 

 objections to the use of a flame as a means of rendering audible the 

 vibrations of a jet. The disturbances excited in the surrounding air 

 by the impact of the stream upon it are so intense as easily to react 

 on the orifice. When, therefore, the jet is thrown into any state of 

 vibration it tends to continue in the same state, even after the excit- 

 ing sound has ceased. 



