June 4, 1886.] 



SCIENCE. 



497 



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 

 reproduce speech, music, and the irregular sounds 

 classified as 'noises.' 



It must be understood that the pressures here 

 given are only suitable 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 mucji 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 exciting sound has 

 ceased. 



A jet of air usually responds most energetically 

 to some particular tone or set of related tones 

 (Sondhauss). Such a particular tone may be called 

 the jet fundamental. The practical inconvenience 

 arising from this may be diminished by raising the 

 air-pressure until the jet fimda mental is higher 

 than any of the tones to be reproduced. 



When a flame and an air-jet meet at right 

 angles, vibrations impressed upon the flame- 

 orifice also yield sound. The conditions of press- 

 ure, etc., are somewhat different ; but the 

 changes produced at the orifice grow in the same 

 way as those in an air-jet. The best results are 

 obtained when a gentle current of air is directed 

 from a wide tube just below the apex of the blue 

 zone. 



It is difficult, at first sight, to account for the 

 fact that a vibrating jet gives rise to sound only 

 when it strikes upon some object which divides it 

 into two parts. The following experiments, how- 

 ever, in some sense explain this. The relative 

 normal velocity at different points in the stream 

 may be measured by introducing into its path the 



open end of a capillary tube which is connected 

 with a water manometer. This velocity dimin- 

 ishes continuously along the axis from the orifice 

 to the breaking-point, and also diminishes con- 

 tinuously from any point of the axis outwards 

 towards the circumference. Now, a sudden dis- 

 turbance communicated to the air at the orifice 

 will be found to produce a fall in velocity along 

 the axis of the jet, but a rise in velocity along its 

 extreme outer portions. It thus appears that the 

 changes along the axis and along the circum- 

 ference, produced by a disturbance, are of op- 

 posite character. When the jet plays into free 

 air, these opposing changes neutralize each other 

 in the main ; but this interference is prevented 

 when the jet strikes upon any object which serves 

 to divide it. 



When a vibrating air- jet plays against a small 

 flame, the best sounds are heard when the stream 

 strikes the flame just below the apex of the blue 

 zone. At the plane of contact an intensely blue 

 flame ring appears, and this ring vibrates visibly 

 when the jet is disturbed. The production of 

 sound from it doubtless depends on changes in 

 the rate of combustion of the gas. This may be 

 proved by inserting into the ring a fine slip of 

 platinum, connected in circuit with a battery and 

 a telephone (fig. 5). When the jet is thrown into 

 vibration, the consequent variations in the tem- 

 perature of the platinum affect its conductivity, 

 and hence a feeble reproduction of the jet-vibra- 

 tion may be heard in the telephone. 



To Savart we are mainly indebted for our 

 knowledge of the sympathetic vibrations of liquid 

 jets. This physicist showed that a liquid jet 

 always tends to separate into drops at a distance 

 from the orifice in a regular manner ; and that 

 this tendency is so well marked, that when the 

 jet strikes upon any object, such as a stretched 

 membrane, so arranged that the disturbances 

 caused by impact may be conducted back to the 

 orifice, a definite musical sound is produced. The 

 pitch of the sound, or the number of drops sep- 

 arated in a given time, varies directly as the 

 square root of the height of liquid in the reser- 

 voir, and inversely as the diameter of the orifice. 

 Savart further showed that external vibrations 

 impressed upon the orifice may act like the im- 

 pact disturbances, and cause the jet to divide into 

 drops. Impact on a stretched membrane may 

 then cause the reproduction as sound of the im- 

 pressed vibrations. The tones capable of produ- 

 cing this effect were considered to lie within the 

 limits of an octavo below and a fifth above the 

 jet normal. 



The author has found, however, that jets of 

 every mobile liquid are capable of responding to 



