210 On Negative Fluid Pressure on a given Surface. 



Professor Tyndall. In his " sensitive flames " the pressure of the 

 issuing gas is so adjusted that the slightest augmentation of 

 pressure makes the flame roar, the roaring being always accom- 

 panied by elongation in bat-wing and fish-tail burners, and by 

 considerable shortening in a jet from a single orifice. 



The pressure at the orifice of the jet is of course the difference 

 of the external and internal pressures, and may be equally aug- 

 mented by the diminution of the former, or the increase of the 

 latter : and the pressure is momentarily augmented by external 

 diminution each time that the phase of rarefaction of a progres- 

 sive sound-wave reaches the orifice; and, within wide limits, the 

 more rapid the vibrations, the more decided will be the effect. I 

 apprehend that there is not the slightest alteration of the mean 

 pressure (measured by the quantity of gas issued in a given 

 time), whether the sensibility of the flame be brought into action 

 or not. 



With all due respect for the author's acknowledged experi- 

 mental ability, I must demur to the inferences arising from his 

 concluding note. I cannot conceive that the vis viva, or energy, 

 of the few molecules at the orifice can be sufficient to impart sen- 

 sible vibration to the contents of the gas-pipe. Could, for ex- 

 ample, a flute or an organ-pipe be made to speak with an em- 

 bouchure no larger than the orifice of a gas-burner ? "I guess u 

 not : and the continuous gas-pipe would not even have the advan- 

 tage of these, in reinforcing by reciprocation the impressed wave- 

 motion. And moreover, in one at least of the forms of burner (the 

 fish-tail), the supposed in-flowing wave-motion would be in pe- 

 culiarly disadvantageous circumstances ; for the waves in similar 

 phases entering by the two orifices would, after reflexion from 

 the opposite sides, of necessity interfere ; and would be in much 

 the same condition as light-waves diffracted at the sides of a 

 small object, as a pin, the resultant motion being nil, and, there- 

 fore, the effect darkness. But even granting the vibrations in 

 the gas-pipe, I do not see how they can affect the phenomena in 

 question ; because, from the progressive motion of the column 

 of gas, they would not be synchronous with those propagated in 

 the surrounding stationary atmosphere. Is the final injunction 

 respecting ample gas-ways the result of experiment, or of infer- 

 ence only ? 



I remain, 



Yours faithfully, 



Charles Brooke. 



16Fitzroy Square, 

 February 13, 1867. 



