396 
MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
Further, let the manometer orifice be moved from an} 7 point on the axis of the jet 
towards the circumference; a fall in velocity may again be noted which is steep near 
the jet axis, but becomes slower as the circumference is approached. 
A similar change may be noticed just at or even inside the jet orifice; but here the 
fall is more steep than at external points. 
Now let the manometer orifice be fixed at different points along the jet axis, and 
let the jet be subjected to any continuous musical vibrations by which it is affected. 
The vibration of the jet will then be attended by a diminution of mean velocity along 
the axis, which is almost imperceptible close to the jet orifice, but becomes sensible 
at a distance depending on the intensity of the vibration. If the vibration be 
gradually increased so as to cause the breaking point to approach the manometer 
orifice, the rate of fall becomes gradually greater; and if the jet “ break” a little in 
front of the orifice the manometer may indicate no excess of pressure. 
Finally, let the manometer orifice be placed at a distance from the axis of the jet, 
and very near its circumference, and let the jet be continuously vibrated as before. 
It will then be found that changes in the mean velocity, similarly dependent on the 
intensity of the vibration and the distance from the orifice, occur along the outer part 
of the jet, but opposite in character to those along its axis, for vibration of the jet 
now causes increase of pressure in the manometer. 
It may sometimes happen that greater increase of pressure will be noticed at some 
points on the circumference of any section of the jet than at others. But by rotating 
the jet tube it can be seen that this depends on some want of symmetry about the 
orifice or of the tube behind it, and not on the direction of the sound. A jet which 
shows this peculiarity may also present in any section two (possibly more) points of 
maximum sound intensity when examined by a hearing-tube, indicating probably a 
tendency to divide or branch. 
The same changes may be shown in a simpler way as follows :—Cut out of thin 
writing paper a small disc about a quarter of an inch in diameter, on the end of a 
slender slip of paper, which may serve as a handle, and let this be fixed by the stem 
at some distance in front of a jet, in such a way that the stream impinges on the 
centre of the disc. The latter will then naturally appear to be repelled, but will be 
kept in position by the stem, which acts as a kind of spring. Any sound will now 
cause the disc to move towards the orifice, showing a diminution of pressure against 
its face. 
Now remove the disc and substitute for it a small flat ring of paper, on the end of 
a slender stem, the internal diameter of the ring being about 6 mms., and its external 
diameter about 12 mms.; and let this be so held at some distance from the orifice that 
the central portions of the jet pass through the ring, while the extreme outer portions 
strike its face. The ring will also appear to be slightly repelled while the jet is 
steady ; but any vibration impressed upon the jet will cause it to move further away 
from the orifice, showing an increase of pressure against its face. 
