i8 SENSITIVE FLAMES AND SOUND-SHADOWS. 



But there is now no longer any necessity to resort to such 

 dangerous sources of sound as dynamite. Whistles may be made 

 which yield tones exceeding twenty thousand vibrations per sec- 

 ond. The wave-length corresponding to such a pitch is less than 

 an inch. The advantage presented is that the sound is continu- 

 ous, and it may be made as constant as we please by supplying 

 the whistle from a cylinder full of compressed air, regulating the 

 pressure by means of an appropriate gauge. The disadvantage is 

 that the intensity is but slight, and the pitch is too high to be 

 perceived as sound by most persons unless the ear is closely ap- 

 plied. An artificial indicator must hence be used, whose motion 

 under the disturbances due to sound can be seen at a distance. 



In 1857 Prof. John Le Conte discovered that an ordinary naked 

 gas-flame, from a fish-tail or bat-wing burner, becomes an indicator 

 of sound by vibrating in unison with an external source, provided 

 the pressure be such that the flame is just ready to flare. This 

 can be easily shown by blowing a shrill whistle or bowing a 

 tuning-fork of high pitch in the immediate neighborhood of the 

 flame, which at once becomes forked (Fig. 3) into several long, 



vibrating tongues. The 

 effect soon ceases if the 

 pressure be gradually di- 

 minished. This result is 

 due to the disturbance 

 produced by sound - 

 waves on the outflowing 

 jet of gas at the nozzle. 

 The high temperature of 

 flame is therefore not 

 necessary for the pro- 

 duction of such co-vibra- 

 tion, but serves to make 

 it more easily manifest. 

 Nine years elapsed after Dr. Le Conte's discovery before the 

 subject was taken up again and independently by Mr. W. F. 

 Barrett, in London, who used small cylindrical jets, which were 

 found to flare under similar conditions, and could be ren- 

 dered far more sensitive. A " pin-hole lava-tip " may be fitted 

 into the end of a metal tube and connected by means of India- 

 rubber tubing to a cylinder of compressed illuminating gas. In 

 connection with this, also, there should be a water manometer 

 gauge for regulating the pressure of the outflowing gas. If the 

 pin-hole is very smoothly cylindrical, the flame mounts up to the 

 height of nearly eighteen inches (Fig. 4, x), with an apparent 

 thickness scarcely more than that of the little finger, and burn- 

 ing quietly. When the pressure approaches ten inches, as indi- 



Fis. 3.— Sensitive Batwing Flames. 



