MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
393 
In general, a pressure of about 12 mins, 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 reproduce speech, singing, and irregular sounds 
included under the term “ noises.” 
It must be understood that the pressures here given are only suitable for jets about 
1 mm. in diameter. When the diameter is only a small fraction of a millimeter the 
above limits may be greatly exceeded, since the velocity of efflux no longer depends 
solely on the pressure. 
A jet escaping from a perfectly circular orifice into free air does not vibrate spon¬ 
taneously so as to produce 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 spon¬ 
taneously 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 con¬ 
stitutes one of the chief drawbacks to the use of a flame as a means of converting 
the jet vibrations into sound. The disturbances excited in the surrounding air by 
the impact of the jet upon it are so intense as to react easily upon the stream as it 
escapes from the orifice. When, therefore, the jet is thrown into any state of 
vibration it tends to continue in the same state, even after the exciting sound has 
ceased. 
A jet of air usually responds most energetically to some particular tone or set of 
related tones. Such a particular tone may be called its fundamental. The practical 
inconvenience arising from this may be diminished by increasing the air pressure, 
until the jet fundamental is higher than any of the tones to be reproduced. 
The influence of reflected vibrations in disturbing the jet may be well shown as 
follows :—Let one end of a short piece of rubber-tubing be brought close to the orifice 
of a silent jet playing against a hearing-tube, but not in such a way as to obstruct it. 
The jet will remain silent. But if the other end of the tube be held near the hearing 
orifice the jet will begin to “sing,” and will continue singing so long as the tube is 
held in this position. The tone produced, while depending mainly on the jet, is not 
independent of the resonance of the tube, since a change in the length or diameter of 
the latter frequently alters the pitch. It is noteworthy that the arrangements used 
by Sondhauss and Masson in their experiments were precisely such as would cause 
disturbances produced by the jet, either by impact or otherwise, to be thrown back 
on the orifice. This may account for some of the irregular and apparently anomalous 
results obtained by them. 
* Lord Rayleigh (Phil. Mag., ser. 5, vol. 17 (1884), p. 189) suggested that the musical sound, sometimes 
obtained when an air-jet is directed against a flame, results from the tendency of any regular cycle in the 
mode of disintegration of the jet to propagate itself, when a connexion exists between the orifice and 
the body on which it strikes, as in Savart’s experiments with water-jets. 
MDCCCLXXXVT. 3 E 
