402 
MR. C. A. BELL ON THE SYMPATHETIC. VIBRATIONS OF JETS. 
entirely removed by increasing the pressure until the jet responds to a note of 4000 
double vibrations per second, or to the highest tone easily whistled. In speech at 
least tones of such extreme pitch are always very feeble, and even if the jet break 
above the membrane when such a note is loudly sounded, it will probably not do so 
under the influence of spoken words. 
Between the musical tones of the ordinary speaking voice, which correspond 
roughly with the tones of the pianoforte, and sibillant or hissing sounds, there 
appears to be a considerable interval. And if a jet be at such pressure as to repro¬ 
duce all the tones of the piano, a much higher pressure will be required to make it 
respond to hissing noises. But when this point is just reached small jets are still 
sensitive to all lower sounds. Accordingly with small jets, the vibrations of which 
may be rendered audible in a manner presently to be described, this is the pressure 
best adapted for reproducing speech, music, and other sounds of all kinds. Moreover, 
at such pressure the tendency to reinforce particular tones is entirely lost. 
At much higher pressures the sensitiveness of small jets to hissing noises becomes 
pronounced, but the capacity to reproduce lower tones gradually disappears, the 
deeper tones being the first to lose effect. 
The connexion between the spontaneous vibration of a jet and its impact on some 
solid, is well seen as follows. While the jet is undisturbed by external sounds let the 
hearing-tube, or a branch from it, be brought near to the sound-board, the open, 
trumpet-shaped end being held quite close to it. The receiving-tube being introduced 
into the path of the jet near the orifice, no effect will be produced. But if the mem¬ 
brane be gradually drawn away from the orifice a point will be reached at which the 
jet will begin to “ sing,” at first faintly, afterwards more loudly. The singing will 
continue so long as the same state of affairs is maintained, but will cease when the 
ear-piece is withdrawn from the sound-board. Even when no special means are used 
to make the jet self-disturbing, the impact vibrations communicated from the mem¬ 
brane to the sound-board through the intervening air may be sufficient to keep the 
jet in vibration. If the locality be very quiet the point at which this occurs may even 
be reached, and the jet remain silent; but any accidental noise or vibration may dis¬ 
turb this state of quietude, and cause the jet to sing. 
It is evidently true for liquid as for gaseous jets that all sound disturbances are 
propagated along the jet-path with the same velocity.. 
Only a highly practised ear can distinguish between the disturbing sounds and the 
echo-like reproduction by the jet when both are within range of hearing. In order, 
therefore, to form an adequate idea of the intensity of sound that may arise from the 
jet impact, and the marvellous accuracy with which impressed vibrations are amplified 
and reproduced, it is absolutely necessary that the source of these impressed vibrations 
should be at a distance from the experimenter. Vibrations may be very simply con¬ 
veyed to a jet along the wire or cord of a “ lover’s telegraph.” A thin cord is attached 
at one end directly to the support of the jet, or to the tube itself, and at the other to 
