MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OP JETS. 
,397 
The motions of both disc and ring increase with the intensity of the sound, so long 
as they are within the breaking point. 
What relation subsists between the diminution in pressure along the axis of a 
vibrating jet and the intensity of the disturbing sound is as yet unknown. But it is 
by no means improbable that, when discovered, it may supply a means of directly 
comparing the intensities of sounds of different pitches. 
The conclusion to be drawn is, I think, that any impulse of the nature of sound, or 
of any other nature, causes a disturbed transverse layer to start from the orifice of an 
air-jet; that this layer increases in area as it advances with the jet, the ratio of the 
pressure at its centre to the pressure at its circumference becoming continually 
smaller ; and that the impact of a succession of these disturbances against a hearing 
orifice or a dame gives rise to a sound whose constituent vibrations are copies of the 
disturbing vibrations. 
The foregoing experiments supply an explanation of the fact that very little sound 
results when a vibrating jet plays into free air or into a large hearing-tube. Under 
these circumstances the diminutions in velocity along the jet axis, due to its vibrations, 
are balanced by the excesses along its outer portions, an almost complete interference 
ensuing when the jet breaks. But this interference is prevented when the jet strikes 
upon an edge or a perforated plate, in fact upon any arrangement which divides the 
stream into two parts, and sound is the result. A flame probably acts in this way ; 
for inspection shows that while it is emitting sound there is no combustion at the 
centre of the plane of impact, but very active combustion at the circumference. 
This region of active combustion under the best conditions takes the form of a 
ring, and the explanation of the loud sounds proceeding from it is to be found in 
rapid changes in the rate of combustion. The occurrence of such changes may be 
easily proved. Let a fine slip of very thin platinum foil be attached by brazing to 
two stout copper rods, and let the foil be bent and inserted in the flame in such a 
way that it coincides with the blue flame-ring, and is raised to incandescence. If 
now the copper rods be connected in circuit with a battery and telephone, both of low 
resistance, the changes occurring in the temperature of the flame-ring when the jet is 
vibrated will be communicated to the little platinum slip, causing corresponding 
changes in its resistance ; hence will necessarily result corresponding changes in the 
electric circuit, so that the sound or other disturbance acting on the jet will be feebly 
reproduced in the telephone. 
Another simple experiment may suitably find mention here. If a plate of 
platinum foil be raised to incandescence either by a battery current or by a noiseless 
blow-pipe flame, and if an air-jet at proper distance be projected against the incandes¬ 
cent surface (of course on the opposite side from the blow-pipe), any vibration or dis¬ 
turbance imparted to the jet orifice will be heard as sound from the plate, and this sound 
will be louder the higher the temperature to which the plate is raised. This must, 
of course, be due, either to expansions and contractions of the plate, caused by the 
