664 I'HE POPULAR SCIENCE MONTHLY. 



I come now to the second division of my subject, that relating 

 to sound. Tyndall, as you know, wrote a book on sound, founded 

 on lectures delivered in this place. Many interesting and origi- 

 nal discoveries are there embodied. One, that I have been espe- 

 cially interested in myself, is on the subject of sensitive flames. 

 Prof. Le Conte in America made the first observations at an 

 amateur concert, but it was Tyndall who introduced the remark- 

 able high-pressure flame now before you. It issues from a pin- 

 hole burner, and the sensitiveness is entirely a question of the 

 pressure at which the gas is supplied. Tyndall describes the 

 phenomenon by saying that the flame under the influence of a 

 high pressure is like something on the edge of a precipice. If 

 left alone, it will maintain itself ; but under the slightest touch 

 it will be pushed over. The gas at high pressure will, if undis- 

 turbed, burn steadily and erect, but if a hiss is made in its neigh- 

 borhood it becomes at once unsteady, and ducks down. A very 

 high sound is necessary. Even a whistle, as you see, does not act. 

 Smooth, pure sounds are practically without effect unless of very 

 high pitch. 



I will illustrate the importance of the flame as a means of in- 

 vestigation by an experiment in the diffraction of sound. I have 

 here a source of sound, but of pitch so high as to be inaudible. 

 The waves impinge perpendicularly upon a circular disk of plate 

 glass. Behind the disk there is a sound shadow, and you might 

 expect that the shadow would be most complete at the center. 

 But this is not so. When the burner occupies this position the 

 flame flares ; but when by a slight motion of the disk the position 

 of the flame is made eccentric, the existence of the shadow is 

 manifested by the recovery of the flame. At the center the in- 

 tensity of sound is the same as if no obstacle were interposed. 



The optical analogue of the above experiment was made at the 

 suggestion of Poisson, who had deduced the result theoretically, 

 but considered it so unlikely that he regarded it as an objection 

 to the undulatory theory of light. Now, I need hardly say, it is 

 regarded as a beautiful confirmation. 



It is of importance to prove that the flame is not of the es- 

 sence of the matter, that there is no need to have a flame, or to 

 ignite it at the burner. Thus, it is quite possible to have a jet of 

 gas so arranged that ignition does not occur until the jet has 

 lost its sensitiveness. The sensitive part is that quite close to 

 the nozzle, and the flame is only an indicator. But it is not neces- 

 sary to have any kind of flame at all. Tyndall made observations 

 on smoke jets, showing that a jet of air can be made sensitive to 

 sound. The difiiculty is to see it, and to operate successfully 

 upon it ; because, as Tyndall soon found, a smoke jet is much 

 more difiicult to deal with than flames, and is sensitive to much 



