MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OF JETS. 
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gaseous jets, I propose to give a few experiments which may be of value when the 
nature of the changes occurring within them comes under discussion. This subject is 
one of some difficulty. No complete theory of jets has been worked out, and I have 
searched through the literature of the subject without finding anything that would 
materially assist in explaining their peculiar properties.* We have here to deal with 
extremely rapid motions of a fluid which is invisible, and the evidence under such 
circumstances must be mainly indirect. Many obscure points will, however, become 
clear when the very similar phenomena of liquid jets are considered. 
Let an orifice in the end of a capillary tube, which is connected with a small water 
manometer, be moved about in the path of a, jet of air. The plane of the orifice being 
always kept at right angles to the jet, it will be found that the velocity of the air 
along the axis of the stream diminishes continuously from the jet orifice to the 
breaking point. This fall in velocity is at first slow, but becomes very rapid when 
the manometer orifice reaches the point where the jet begins to expand. 
* [August 26, 1886.— Since writing the above, my attention has been directed to two papers by Lord 
Rayleigh (Proc. Lond. Math. Soc., vol. 10 (1878), p. 4, and voh 11 (1879), p. 1), in which the 
explanation of the normal tones, at least, of air-jets, is doubtless to he found. 
In the first of these papers it is shown mathematically that in the case of a cylindrical jet of fluid 
moving in still fluid (supposed frictionless) disturbances impressed upon the jet give rise to waves 
which move forward with the velocity of the jet, and increase in amplitude very rapidly with the time ; 
and that the instability of the jet increases without limit with the pitch of the disturbances. 
The second paper deals, amongst other topics, with cases of jets of frictionless fluid under certain 
hypothetical conditions which approximate to those of the actual jets which come under experiment. It 
is shown that the instability of the jet, so far from increasing without limit with the pitch of the 
disturbances, soon reaches a maximum; and that for disturbances of still shorter wave-length than those 
which are most effective this instability is finally exchanged for actual stability. 
It is to be hoped that the distinguished author of these papers may himself discuss their full bearing 
on the facts here recorded.] 
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