Feb. 5, 1874] 



NATURE 



267 



THE ACOUSTIC TRANSPARENCY AND 



OPACITY OF THE ATMOSPHERE* 



II. 



■yyE have now to consider the complementary side of the 

 phenomena. A stratum of air, 3 miles thick, on a perfectly 

 calm day, has been proved competeat to stifle both the cannon- 

 ade and the horn-sounds employed at the South Foreland ; 

 while the observations just recorded, one and all, point to the 



mixture of air and aqueous vapour as the cause of this extra- 

 ordinary phenomenon. Such a mixture could fill the atmosphere 

 with an impervious acoustic cloud on a day of perfect optical 

 transparency. But, granting this, it is incredible that so 11 eat 

 a body of sound could utterly disappear in so short a distance 

 without rendering any account of itself. Supposing, then, 

 instead of placing ourselves behind the acoustic cloud, we were 

 to place ourselves In front of it, might we not, in accordance 

 with the law of conservation, expect to receive by reflection the 

 sound which had failed to reach us by transmission ? The case 



[A tunnel 2 in. square, 4 ft. S in. long, open at both ends, and having a glass front, runs through the box a b. The space 

 above and below is divided into cells opening into the tunnel by oblong orifices exactly corresponding vertically. Each alternate 

 cell of the upper series — the ist, 3rd, 5th, &c. — communicates by a tube (■/</) with the upper reservoir [g], its counterpart in the 

 lower series having a free outlet into the air. In like manner the 2nd, 4th, 6th, &c., of the lower series of cells are connected with 

 the lower reservoir (/) ; and each has its direct passage into the air through the cell immediately above it. The gas distriautors are 

 filled from both ends at the same time ; the upper with carbonic acid gas, the lo^-er with coal-gas, by brancbes from their respec- 

 tive supply pipes (/i /(). A well-padded box (/) opening upon the end of the tunnel forms a little cavern, whence the sound-waves are 

 sent forth by an electric bell. A fe v feet from the other end of the tunnel, in a direct line, is a sensitive flams (/(•), provided with a 

 funnel as sound collector, and guarded from chance currents by a shade. 



The bell was set ringing. The flame, with quick response to each blow of the hammer, emitted a sort of musical roar, so 

 regular were its alternate shortenings and lengthenings as the successive sound-pulses reached it. The gases were then admitted. 

 Twenty-five flat jets of coal gas ascended from the tubes below, and twenty-five cascades of carbonic acid poured down from the 

 tubes above. That which was a homogeneous medium had now fifty limiting surfaces, from each of which a portion of the sound 

 was thrown back. In a few moments ihese successive reflections became so elTective that not a single sound-wave having sufficient 

 power to affect a flame so sensitive as to be knocked down, crushed, as it were, by a chirrup, or jingle, at twenty feet distance, 

 could pierce the clear, optically-transparent, but acoustically-opaque atmosphere in the tunnel. So long as the gases continued to flow, 

 the flame remained perfectly tranquil. When the supply was cut olT, the gases rapidly diffused into the air. The atmospliere of the 

 tunnel became again homogeneoui, and therefore acoustically transparent, and the flame bowed down to each sound-pulse as before. 

 Alternate layers of common air and air saturated with various vapours produce the same effect.] 



would then be strictly analogous to the .reflection of light from 

 an ordinary cloud to an observer placed between it and the sun. 

 My fint care, in the early part of the day in question, was to 

 assure myself that our inability to hear the sound did not arise 

 from any derangement of the instruments. At one P.M. I was 



" Royal Inslitution, Friday evening Discourse by Prof. Tynd,ill, D.C. L. 

 LL.D., F.R.S. Jan. 16. (Continued from page 25.3.) 



rowed to the shore, and landed at the base of the South 

 Foreland cliff. The body of air which had already shown such 

 extraoriinary power to intercept sound, and which manifested 

 this power still more impressively later in the day, was now in 

 front of us. On it the sonorous waves impinged, and from it 

 tiiey were sent back to us with astonishing intensity. The 

 instruments, hidden from view, were^ on the summit of a cliff 



