3.92 
MR, C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
adjustment may be effected by turning screw Q, by which the orifice is moved up or 
down, or screw q, by which it is moved laterally. The tube T is connected by a piece 
of rubber-tubing, Pt, to the stout brass tube M, fixed permanently in the wooden 
upright, L. The end of the flexible rubber tube, H, provided with the ear-piece, E, is 
slipped over the tube M. The ear-piece may thus be applied to the ear without fear 
of disturbing the adjustment of N. The distance of the jet orifice from N may be 
varied by sliding the jet tube J to and fro in the groove in which it rests. J is 
connected by rubber-tubing to the reservoir. 
For experiments with flames the tube T may be withdrawn, and a tube with burner, 
like that shown in fig. 1, may be substituted for it. 
With this apparatus it is easy to repeat the above-described experiments, and to 
verify the following statements. 
A jet of air at moderate pressure (below 10 mms. of water), from an orifice 1 to 
I f mm. in diameter, forms a continuous column for a certain distance, beyond which 
it becomes confused and breaks up. 
Any impulse, such as a tap on the jet-support or a short and sharp sound, causes a 
minute disturbance to start from the orifice. This disturbance increases as it 
progresses, and finally causes the jet to break. By directing the jet against a flame; 
or against a hearing orifice, it is readily perceived that such disturbances travel along 
the jet-path with a velocity which is not that of sound in air. In fact, the sound 
heard in the ear-piece resembles an echo of the disturbing sound. 
The disturbances produced by sounds of different pitch travel along the jet-path 
with the same velocity. This is evident, since otherwise distinct reproduction of the 
complex vibrations of speech 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 jet stream. 
A vibrating air-jet, playing into free air, gives rise to very feeble sounds ; but these 
sounds are much intensified when the stream impinges on any arrangement which 
serves to divide it into two parts. Of such arrangements, a surface pierced with a 
small orifice is the best, the orifice being placed in the jet axis. 
A jet of air at low pressures responds to and reproduces only sounds of low pitch. 
Sounds above a certain pitch, which varies with the pressure, either do not affect it or 
are only faintly reproduced. 
At pressures between 10 and 12 mms. of water the jet reproduces all the tones of 
the speaking voice, and those usually employed in music, with the exception of very 
shrill or hissing sounds. When the pressure in the reservoir equals about 13 mms. of 
water, hissing sounds are well reproduced, while sounds of very low pitch become 
fainter. At higher pressures, up to about 25 mms. of water, shrill or hissing noises 
produce very intense disturbances, while ordinary speech tones have but little effect. 
But at these pressures sounds of high pitch frequently cause the jet to emit lower 
sounds, of which they are harmonics. 
