MB, C. A. BELL OK THE SYMPATHETIC VIBRATIOHS OP JETS. 
421 
as a perforated plate, which divides each ring into two parts, this interference roust 
he more or less incomplete. 
When a jet of liquid escapes into a medium which offers little resistance to its 
motion, no change, or at any rate only a slight change, can be produced by vibration 
in the mean velocity along its axis. For here the accelerations are balanced by the 
retardations ; and as these changes succeed each other very rapidly, they are incapable 
of affecting the sluggishly moving manometer, although quite capable of acting on 
the highly sensitive mechanism of the ear. When, however, the jet plays within 
a medium which sensibly resists it, the case is different. Even in its normal state 
the stream gradually parts with its energy to the surrounding fluid, and therefore 
expands while its velocity diminishes. But when it is thrown into vibration, owing 
to the greater surface exposed to the medium by the swellings and contractions, the 
loss of energy and consequent expansion become more rapid ; and hence results a 
diminution of mean volocity. For the same reason, we have in the outer layers, or 
in the medium immediately adjoining, an increase of mean velocity when the jet is 
thrown into vibration. 
It has been assumed so far that the disturbances communicated to the jet are of 
the nature of impulses. But the same reasoning can be extended to cases in which 
the motions of the orifice are undulations, even of such long period that one whole 
vibration cannot be included in the continuous part of the jet. For here each 
infinitely thin section of the jet can be regarded as an independent ring or system 
of rings, which grows or contracts at the expense of adjacent sections. 
I have also assumed that the motions of the orifice take place in the direction of 
the jet. But it is evident that lateral impulses may also alter the vorticity of the 
stream, and the disturbances will then not be symmetrical. Want of symmetry is 
not apparent when a jet is disturbed by sounds produced at some distance, whatever 
their direction ; but it is very evident in the photographs of liquid jets thrown into 
vibration by tuning forks applied to the support. The planes of the rings in the jets, 
and the transverse diameters of the drops into which it is resolved, are seen not to 
be always perpendicular to the axis. 
It hardly needs to be pointed out that impulses communicated to the fluid either 
behind or external to the orifice, may cause disturbances precisely similar to those 
resulting from motions of the orifice itself. Experimentally we know that vortex 
rings must be produced in this way. 
The properties of a jet may undoubtedly be profoundly modified by the viscosity or 
surface tension of the fluid. When the fluid is highly viscous an impulse com¬ 
municated to the orifice will tend to produce acceleration or retardation of the whole 
layer of fluid included within it; and even if the fluid is only moderately viscous, 
the inequalities of motion constituting vorticity, in virtue of which the initial 
disturbances tend to grow, must rapidly disappear. On the other hand, the surface 
tension of a liquid may be conceived to accelerate, to a certain extent, the growth 
