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MR. C. A. BELL ON THE SYMPATHETIC VIBRATIONS OP JETS. 
In experimenting with flames it is well to observe the following precautions. The 
flame should be short: the gas should escape at very low pressure from a circular 
orifice about the size of the jet orifice. If the burner orifice is small, and the gas 
pressure high, the flame itself may become a source of vibrations ; and these vibrations, 
intensified by the impact of air upon it, may be thrown back on the jet orifice. 
The loudest sounds are obtained by directing the jet below the apex of the blue 
flame-zone. In this region the gas has not yet been mixed with the surrounding air, 
and the intensity of its combustion will therefore depend on the supply derived from 
the jet. It is remarkable that vibrations are very feebly reproduced when pure 
oxygen is substituted for air in the jet. In this case the blue flame ring formed by 
impact becomes very small, the combustion of the oxygen becomes localized, and the 
necessary condition of the separation of the jet into two portions is probably not 
fulfilled. 
The flame ought to be of sufficient size to apparently absorb the jet. A complete 
blue flame ring will then appear, the plane of which intersects the angle between the 
jet and the flame. The centre of this ring will then appear dark, and very little light 
will be given out. 
The best distance of flame or hearing-tube from the jet orifice must be determined 
by trial. The adjustment is, however, a matter of a few seconds’ manipulation, and 
may easily be effected with eye and ear as guides. When it is desired to reproduce 
accurately a series of sounds, the distance should be such that the jet will not 
“ break ” in front of the hearing orifice or flame, under the influence of the most 
intense vibration to which it is to be subjected. 
When a flame and an air-jet meet at right angles, sounds may also result from 
vibrations impressed on the flame orifice. The properties of flames are in general 
parallel to those of gaseous jets. Without going into details, the chief differences 
may be summarised as follows : 
A flame behaves as a more rigid body than an unignited jet, and, except in the 
so-called “sensitive” state, i.e., at high pressure, it is not easily affected by aerial 
sound vibrations. 
The pressure necessary to make a flame respond to anj T given range of tones is 
higher than that required for an air-jet from the same orifice. In these two relations, 
therefore, a flame is intermediate between a gaseous and a liquid jet. 
Except at high pressures, when it is approaching the sensitive state, a vibrating 
flame from a circular orifice does not emit sound. But its vibration may always be 
rendered audible by blowing air against it. The best result is obtained when a stream 
of air at gentle pressure is directed from a wide tube just below the apex of the blue 
zone (fig. 4). 
The phenomenon of interference is strikingly shown when vibrations of slightly 
different pitches are impressed on the flame and air-jet orifices respectively. 
Having now briefly described the more prominent characteristics of vibrating 
