Mr. F. W. Barrett on Sensitive Flames. 221 



a most wonderful thing to consider, how almost infinitely small 

 is the amount of vibratory motion sufficient to alter so com- 

 pletely the aspect of a large flame of gas : and this sensitiveness 

 of such a gas-flame to certain sounds would lead one to hope 

 that it might be put to some use for experimental or prac- 

 tical purposes. The chirp of a cricket would, I have no doubt, 

 have an energetic action upon the flame. Speaking to the 

 flame in an ordinary voice at a distance of 30 or 40 feet away 

 caused a very marked divergence. It was noticed that the 

 letter s had a very strong effect on the flame ; and it was very 

 curious to watch the flame as it apparently mocked any person 

 who happened to be speaking*. 



Whilst making some of the foregoing experiments last summer, 

 I was led to observe that the pressure of the gas had an important 

 influence upon the divergence of the flame, and remarked that 

 an increased pressure acted like a shrill sound in spreading out 

 the flame, which gave at the same time a roaring noise. Pro- 

 fessor Leconte has, however, decidedly the prior claim to this 

 observation, which Professor Tyndall has raised to an expla- 

 nation of the phenomenon. Professor Tyndall remarks, " The 

 gas issues from its burner with a hiss, and an external sound 



* In the last Number of the Philosophical Magazine, Professor Tyndall 

 has already remarked on the striking effect upon the flame produced by 

 the letter s, or by the hiss given when compressed air issues from an orifice. 

 Associated with this observation there are some other remarkable pecu- 

 liarities connected with the sound of the letter s. When listening from 

 some elevated point to the singing of a large assemblage of people, I have 

 frequently noticed that when a word containing any sibilant was sung, the 

 sound of the peoples' voices united itself into a sharp, loud and somewhat 

 prolonged hiss, like the escape of high-pressure steam from a small nozzle, 

 and quite unlike anything produced by any other letter. In a paper pub- 

 lished in the Philosophical Magazine for June 1849, the Astronomer Royal 

 has stated his belief that the sound of s or z is produced by " an interrup- 

 tion of the continuity of the particles of air" — that it is in fact a broken 

 wave, in its progress resembling the rush of a bore on a river ; and has 

 compared it to a broken-headed sea which, meeting an embankment, is 

 not regularly reflected as the larger waves would be, but runs along the 

 side to a far distance. In support of this analogy, Mr. Airy mentions the 

 fact that a sibilant sound is not returned by an ordinary echo, and that in 

 whispering-galleries the buzzing sound of a whisper is carried along close 

 to and never quits the surface of the dome, being only heard at the oppo- 

 site side by applying the ear close to that surface, while an ordinary sound 

 is not transmitted along the surface, and is not so strikingly heard at the 

 opposite side. From theoretic considerations, Mr. Airy remarks that any 

 clear musical sound would have a tendency to degenerate into a hiss by 

 mere distance of transmission, and that this conversion would soonest take 

 place with loud sounds on a high key. That eminent philosopher is, how- 

 ever, unaware whether there exists any instance of such a conversion : 

 might not a sensitive flame be applied to the determination of this interest- 

 ing point ? 



