SENSITIVE FLAMES AND SOUND-SHADOWS. 19 



cated by the water-gauge, the flame flares, becoming much shorter 

 and broader, like a little Indian club (Fig. 4, y), and producing a 

 low roaring sound, due to the escape of unburned gas. Let the 

 pressure now be diminished until this flaring barely ceases. The 

 flame is now in its most sensitive condition. Sounds 

 of low or even medium pitch have no efliect upon it ; 

 but on blowing a shrill whistle, or rattling a bunch 

 of keys anywhere within thirty or forty feet, it flares. 

 Perhaps the most beautiful illustration of its sensi- 

 tiveness is given by placing an open watch near the 

 nozzle but not touching it ; every tick causes a mo- 

 mentary sinking and spreading of the flame, so that 

 the effect may be seen across an audience-room. If 

 the audience applauds with clapping of hands, the 

 flame shrinks in acknowledgment. 



A very sensitive flame, but not so convenient as 

 that of Prof. Barrett, and not visible at so great a 

 distance, may be obtained with no pressure greater 

 than that of the street mains, by causing the gas to 

 issue from a small tube, over the orifice of which, at 

 a height of an inch or two, is placed a piece of wire 

 gauze (Fig. 5). The mixture of coal-gas and air is 

 ignited above the gauze, and a glass tube may be used 

 to protect the flame from currents of air, though this 

 is not usually necessary. Very little adjustment is 

 needed to find the distance between nozzle and gauze 

 at which the flame is most sensitive. This arrange- 

 ment was devised independently by Prof. Govi, of 

 Turin, and Mr. Barry, of Ireland. The flame is de- 

 ficient in brightness, and is only a few inches high at 

 its best, but has the advantage of not requiring any 

 appliances that may not be easily supplied in any 

 town. If Barrett's flame is available, however, it is 

 decidedly preferable to anything else. 



With such a flame as Barrett's it becomes possible 

 to explore the air and detect regions of relative noise and silence* 

 just as a delicate thermometer enables us to determine variations of 

 temperature in different layers of air or water. If the pitch be too 

 high for the ear to estimate or even detect it, the sensitive flame 

 is more delicate than the ear. Armed with a whistle yielding a 

 pitch of twelve or fifteen thousand vibrations per second, and with 

 a good flame, many beautiful analogies between sound and light 

 may be exhibited with entire satisfaction to an audience of deaf- 

 mutes, if the lecturer's fingers are fairly nimble, since there is no 

 necessity for the sounds to be heard. Most of the experiments 

 about to be described were devised by Lord Rayleigh, the suc- 



FiQ. 4. 



