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MR, C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
Again, if the manometer orifice be placed at any point in the jet axis, and if the jet 
be thrown into vibration, a diminution of mean velocity will be indicated, which 
increases alike with the intensity of the impressed vibrations and the distance from 
the jet orifice ; and becomes most pronounced when the disturbances are sufficient to 
make the jet break in front of the manometer orifice. These changes are precisely 
similar to those noticed in vibrating air-jets. 
It is interesting to contrast with these the conditions of motion in a water jet 
projected into air. Using a manometer with a very small orifice in the manner already 
described, it is easy to show that the velocity of the jet particles at the orifice, and 
for some distance outside of it, is greatest along the axis, and diminishes from this 
outwards. At a distance from the orifice, however, a similar change is by no means 
so evident, partly because most large jets, which can alone be studied in this way, are 
usually so unsteady that it is not easy to keep the manometer orifice constantly in 
one position with respect to the stream ; but chiefly, no doubt, because, owing to the 
viscosity of the liquid, the motion of the particles tends to become uniform low down 
in the stream. Nevertheless I have little doubt that such a fall in velocity occurs 
throughout the continuous portion of the jet. 
In order to detect changes in velocity along the axis of the jet, and to get rid of 
the acceleration due to gravity, the jet must be projected horizontally : and here the 
curvature of the path described introduces a new element of difficulty in the 
observations. But as the result of many experiments it may, I think, be stated that 
there is a distinct retardation of the axial portions of the jet at a distance from the 
orifice. 
Finally, it is quite impossible to say definitely that vibration of the jet causes a 
diminution in the mean velocity along the axis, within its continuous portion, or that 
it does not. Any such change must be slight, and would of course be mainly attri¬ 
butable to friction of the jet against the surrounding air. That this friction is not 
wholly to be neglected may be shown by allowing a water jet to fall through a 
horizontal flame from a small orifice, near its base (fig. 11). When the flame is at a 
sufficient distance from the jet orifice, vibrations impressed upon the jet will cause it 
to emit sounds even when it is still within the continuous portion. These sounds are 
no doubt due to the jet carrying along with it a layer of air, which is projected 
against the flame. 
I have now given a brief description of the chief properties of jets as observed by 
myself and others. Nothing like a complete account of the experiments made has 
been attempted; simply those have been selected which throw any new light on the 
subject. 
It does not appear, so far, that the phenomena admit of any obvious explanation, 
properly so called. At the very outset we are met by the difficulty that the motions 
taking place within a fluid jet, even when it is not subjected to vibration, have been 
only partially investigated. In wlmt follows, therefore, I shall simply indicate a few 
