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42 CHELTENHAM COLLEGE NATURAL HISTORY SOCIETY. 
escaping from the jet and heing sent back into it. Thus the sound- 
producing waves of air were the cause of the alternating motions of 
the burning jet. He should now endeavour to show that when once 
a sound, however small, was started in the tube, the alternating mo- 
tions of the jet themselves reinforced that sound, increased it, and 
maintained it. They might suppose that to start the sound, a very 
small disturbance of the column of air in the tube accidentally 
arose. This disturbance acted upon the jet, and the disturbance 
caused in the jet reacted upon the disturbed column of air, in- 
creased it and maintained it. 
A complete and satisfactory explanation of the way in which 
the flame acts to maintain the vibrations had been given by Lord 
Rayleigh. His explanation was as follows. The effect of the jet 
burning in the tube was by its heat to expand the air in the tube. 
If now the conditions under which the jet was alternately growing 
large and small were such that the jet was giving most heat at the 
time when the column of air in the sounding tube was expanding 
and least heat when contracting, the heat of the jet was helping the 
expansion during the expansion more than it was hindering con- 
traction during the contraction. The effect of the alterations in 
the jet on the whole would then be to reinforce the vibrations. 
The question then arose, Does the flame impart most heat to the 
air during the expansion? Are the conditions such that they might 
conclude that the flame zs giving most heat during the expansion, 
or just atthe commencement of the expansion? Now they might 
at first be disposed to say that the greatest issue of gas, and there- 
fore greatest development of heat, would take place when the pres- 
sure in the sounding tube was least or at the moment of greatest 
expansion, and just after it, 7.e., during contraction. If this were so, 
the flame would be alternating in a manner unfavourable for the 
maintenance of vibrations. But it has been noticed by Seeback 
and others that the length of tube delivered the jet of hydroden has 
a great effect in determining the excitability of the flame. The tube 
must be of certain length before the flame will maintain the vibra- 
tions. The length of the tube, there can be doubt, acts to retard 
the moment of greatest development of heat after the moment of 
greatest rarefaction of air in the tube, so that it shall take place during 
the expansion or at a moment favourable for maintaining the 
vibrations. 
The lecturer then showed a piece of apparatus lately devised by 
Lord Rayleigh for illustrating the mechanics of this subject. A piece 
