380 Prof. Tyndall on the Action of Sonorous 



In fig. 11 is represented another smoky flame, which, when 

 the whistle sounds, breaks up into the form shown in fig. 12. 



The foregoing experiments illustrate the lengthening and 

 shortening of flames by sonorous vibrations. They are also able 

 to produce rotation. I have here several home-made burners, 

 from which issue flat flames, each about 10 inches high, and 3 

 inches across at their widest part. The burners are purposely 

 so formed that the flames are dumpy and forked. When the 

 whistle sounds, the plane of each flame turns 90° round, and 

 continues in its new position as long as the sound continues. 



A flame of admirable steadiness and brilliancy now burns be- 

 fore you. It issues from a single circular orifice in a common 

 iron nipple. This burner, which requires great pressure to make 

 its flame flare, has been specially chosen for the purpose of en- 

 abling you to observe with distinctness the gradual change from 

 apathy to sensitiveness. The flame is now 4 inches high, and is 

 quite indifferent to sound. By increasing the pressure I make 

 its height 6 inches ; it is still indifferent. I make it 12 inches ; 

 a barely perceptible quiver responds to the whistle. I make it 

 16 or 17 inches high; and now it jumps briskly the moment the 

 anvil is tapped or the whistle sounded. I augment the pressure ; 

 the flame is now 20 inches long, and you observe a quivering at 

 intervals, which announces that it is near roaring. A slight 

 increase of pressure causes it to roar, and shorten at the same 

 time to 8 inches. I diminish the pressure a little ; the flame is 

 again 20 inches long, but it is on the point of roaring and short- 

 ening. Like the singing flames which were started by the 

 voice, it stands on the brink of a precipice. The proper sound 

 pushes it over. It shortens when the whistle sounds, exactly 

 as it did when the pressure was in excess. The action reminds 

 one of the story of the Swiss muleteers, who are said to tie up 

 their bells at certain places lest the tinkle should bring an 

 avalanche down. The snow must be very delicately poised be- 

 fore this could occur. I believe it never did occur ; but our flame 

 illustrates the principle. We bring it to the verge of falling, and 

 the sonorous pulses precipitate what was already imminent. 

 This is the simple philosophy of all these sensitive flames. 



When the flame flares, the gas in the orifice of the burner has 

 been thrown into vibration ; conversely, when the gas in the orifice 

 is thrown into vibration, the flame, if sufficiently near the flaring 

 point, will flare. Thus the sonorous vibrations, by acting on the 

 gas in the passage of the burner, become equivalent to an aug- 

 mentation of pressure in the holder. In fact we have here re- 

 vealed to us the physical cause of flaring through excess of pres- 

 sure, which, common as it is, has never, I believe, been hitherto 

 explained. In the orifice of the burner the gas encounters 



