THE SCIENTIFIC WORK OF TYNDALL. 665 



graver sounds. I doubt whether I am wise in trying to exhibit 

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

 tion 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 

 undisturbed, it pursues a straight course and remains cylindrical. 

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

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

 upon the screen. The real motion of the jet can not, of course, 

 be ascertained by mere inspection. It consists in a continuously 

 increasing 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 un- 

 disturbed 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 car- 

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

 the ordinary lights fail, an attempt was 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 moder- 

 ate distance. Two explanations 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 be- 

 hind, and the wave is liable to be tilted upward and so to be car- 

 ried 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 im- 

 material. The sound is heard by an observer down wind, and not 

 so well by an observer up wind. The explanation by refraction 



