222 Lord Rayleigh [March 16, 



to operate successfully upon it ; because, as Tyndall soon found, a 

 smoke-jet is much more difficult to deal with than flames, and is sen- 

 sitive to much graver sounds. I doubt whether I am wise in trying 

 to exhibit smoke-jets to an audience, but I have a special means of 

 projection by which I ought at least to succeed in making them 

 visible. It consists in a device by which the main part of the light 

 from the lamp is stopped at the image of the arc, so that the only 

 light which can reach the screen is light which by diffusion has been 

 diverted out of its course. Thus we shall get an exhibition of a jet 

 of smoke upon the screen, showing bright on a dark ground. The 

 jet issues near the mouth of a resonator of pitch 256. When un- 

 disturbed it pursues a straight course, and remains cylindrical. 

 But if a fork of suitable pitch be sounded in the neighbourhood, the 

 jet spreads out into a sort cf fan, or even bifurcates, as you see 

 upon the screen. The real motion of the jet cannot of course be 

 ascertained by mere inspection. It consists in a continuously in- 

 creasing sinuosity, leading after a while to complete disruption. If 

 two forks slightly out of unison are sounded together, the jet 

 expands and re-collects itself, synchronously with the audible beats. 

 I should say that my jet is a very coarse imitation of Tyndall's. 

 The nozzle that I am using is much too large. With a proper nozzle, 

 and in a perfectly undisturbed atmosphere — undisturbed not only 

 by sounds, but free from all draughts — the sensitiveness is wonderful. 

 The slightest noise is seen to act instantly and to bring the jet down 

 to a fraction of its former height. 



Another important part of Tyndall's work on Sound was carried 

 out as adviser of the Trinity House. When in thick weather the 

 ordinary lights fail, an attempt is made to replace them with sound 

 signals. These are found to vary much in their action, sometimes 

 being heard to a very great distance, and at other times failing to 

 make themselves audible even at a moderate distance. Two explana- 

 tions have been suggested, depending upon acoustic refraction and 

 acoustic reflection. 



Under the influence of variations of temperature refraction occurs 

 in the atmosphere. For example, sound travels more quickly in 

 warm than in cold air. If, as often happens, it is colder above, the 

 upper part of the sound wave tends to lag behind, and the wave is 

 liable to be tilted upwards and so to be carried over the head of the 

 would-be observer on the surface of the ground. This explanation 

 of acoustic refraction by variation of temperature was given by 

 Prof. Osborne Reynolds. As Sir G. Stokes showed, refraction is also 

 caused by wind. The difference between refraction by wind and by 

 temperature variations is that in one case everything turns upon the 

 direction in which the sound is going, while in the second case this 

 consideration is immaterial. The sound is heard by an observer 

 down wind, and not so well by an observer up wind. The explana- 

 tion by refraction of the frequent failure of sound signals was that 

 adopted by Prof. Henry in America, a distinguished worker upon 



