MR. 0. A. BELL ON THE SYMPATHETIC VIBRATIONS OP JETS. 
389 
tube with a very small opening in its end (about as large as the jet orifice), which 
could be applied to different points in the jet-path. With this arrangement the 
results were strikingly different. When the end of the hearing-tube was moved 
about in front of, and not too close to, the orifice from which a vibrating jet issued, 
the vibrations were distinctly audible over a wide area, and at points where the large 
hearing-tube gave absolutely no sound. When the hearing orifice was held at any 
point on the axis of the jet, the sound heard was much louder than at surrounding 
points in the same section, occasionally attaining a remarkable degree of intensity ; 
and this intensity invariably reached its maximum at a certain distance from the jet 
orifice. When the hearing-tube was moved from this position away from the jet 
orifice, the sound first became confused, and then changed to an unmusical roar ; but 
when it was moved towards the orifice the sound gradually died away, and nothing 
could be heard when the two orifices were close together. The diminution in loudness 
was also very rapid when the hearing orifice was moved from a point on the jet axis 
in any direction towards the circumference. 
Pig. 2. 
It now became of interest to discover the connexion between the point of maximum 
sound-intensity and the structure of the jet. To this end I made use of a well-known 
device for rendering visible the motions of air, viz., mixing it with smoke. Professor 
Tyndall (loc. cit.) showed that when certain sounds were produced in the neighbour¬ 
hood of a smoked air-jet at suitable pressure, the result was a shortening of the 
continuous column, the jet appearing to become confused, and to lose its rod-like 
character sooner than it would if undisturbed. 
Experiments with smoked air soon threw some light on the phenomena above 
described. A jet of air charged with tobacco-smoke was allowed to escape at a 
pressure of about -g-th of an inch of water from an orifice, about 1-| mm. in diameter, 
in the end of a glass tube. The atmosphere being perfectly still, and strict silence 
observed, the issuing stream was continuous and rod-like for a length of about 35 
centims. This rod-like portion was slightly conical, the apex of the cone being at the 
jet orifice. Beyond this distance the jet expanded and rapidly became dissipated. 
Any continuous vibration of moderate pitch, impressed upon the orifice in any way, 
caused the continuous column to become shorter, and the jet to “ break ” or become 
confused at a distance from the orifice varying from 6 centims. to 1, according to the 
intensity of the vibrations. 
