1886.] 



On the Sympathetic Vibrations of Jets. 



369 



In these experiments the primary action of the impressed vibra- 

 tions was undoubtedly exerted on the air-jet ; but a singular and per- 

 plexing fact was that no sound, or at best very faint sounds, could be 

 heard from the latter when the flame was removed, and the ear or the 

 end of a wide tube connected with the ear, was substituted for it. 

 Suspecting, finally, that the changes in the jet, effective in producing 

 sound from the flame, must be relative changes of different parts of it, 

 the author was led to try a very small hearing orifice, about as large 

 as the jet orifice. The results were most striking. By introducing 

 this little hearing orifice into the path of a vibrating air- jet, the 

 vibrations can be heard over a very wide area. Close to the jet orifice 

 they are so faint as to be scarcely audible ; but they increase in 

 intensity in a remarkable way as the hearing orifice is moved away 

 along the axis of the jet, and reach their maximum at a certain 

 distance. Experiments with smoked air showed that this point of 

 maximum sound is that at which the jet loses its rod-like character, 

 and expands rapidly ; it has been named the " breaking point," 

 because just beyond it the sounds heard from the jet acquire a broken 

 or rattling character, and at a greater distance are completely lost. 

 The distance of the breaking point from the orifice diminishes as the 

 intensity of the disturbing vibrations is increased, and also depends 

 to some extent on their pitch, and on the velocity of the jet. With 

 orifices of 1 to \\ mm. in diameter it usually varies from 1 to 6 cm. 

 The vibrations of an air- jet may also be heard at points not situated on 

 the axis ; but they are always most intense along the axis, and become 

 rapidly fainter as the distance from it increases. 



With glass jet and hearing tubes, and a light gas bag to serve as 

 reservoir, these experiments are easily repeated ; bat simple apparatus 

 for more careful experiments is described. The author's general 

 conclusions from his experiments and those of others are as 

 follows : — 



A jet of air at moderate pressure (below 10 mm. of water) from 

 an orifice 1 to 1\ mm. in diameter, forms a continuous column for a 

 certain distance, beyond which it expands and becomes confused. 



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 in area as it progresses, and finally causes the 

 jet to break. By directing the jet against a flame or 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 stmnds of different pitch travel 

 along the jet path with the same velocity. This is evident ; since 

 otherwise accurate reproduction of the complex vibrations of speech 



2 c 2 



