THE ATMOSPHERE AND FOG-SIGNALING. 553 



I learn from a subsequent letter that daring the battle the air was 

 still. J. T. 



Echoes pkom Invisible Acoustic Clouds. But both the argu- 

 ment and the phenomena have a complementary side, which we have 

 now to consider. A stratum of air less than three miles thick on a 

 calm clay has been proved competent to stifle both the cannonade and 

 the horn-sounds employed at the South Foreland ; while, according to 

 the foregoing explanation, this result w T as due to the reflection of the 

 sound from invisible acoustic clouds which filled the atmosphere on a 

 day of perfect optical transparency. But, granting this, it is incredi- 

 ble that so great a body of sound could utterly disappear in so short 

 a distance without rendering some 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 would then be 

 strictly analogous to the reflection of light from an ordinary cloud to 

 an observer between it and the sun. 



My first 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 de- 

 rangement of the instruments on shore. Accompanied by the private 

 secretary of the Deputy Master of the Trinity House, at 1 p. m. I was 

 rowed to the shore, and landed at the base of the South Foreland Cliff. 

 The body of air which had already shown such extraordinary power 

 to intercept the sound, and which manifested this power still more im- 

 pressively later in the day, was now in front of us. On it the sonorous 

 waves impinged, and from it they were sent back with astonishing in- 

 tensity. The instruments, hidden from view, were on the summit of a 

 cliff 235 feet above us, the sea was smooth and clear of ships, the at- 

 mosphere was without a cloud, and there was no object in sight which 

 could possibly produce the observed effect. From the perfectly trans- 

 parent air the echoes came, at first with a strength apparently little 

 less than that of the direct sound, and then dying gradually and con- 

 tinuously away. A remark made by my talented companion in his 

 note-book at the time shows how the phenomenon affected him : " Be- 

 yond saying that the echoes seemed to come from the expanse of 

 ocean, it did not appear possible to indicate any more definite point 

 of reflection." Indeed, no such point was to be seen ; the echoes 

 reached us, as if by magic, from the invisible acoustic clouds with 

 which the optically transparent atmosphere was tilled. The existence 

 of such clouds in all weathers, whether optically cloudy or serene, is 

 one of the most important points established by this inquiry. 



Here, in my opinion, we have the key to many of the mysteries and 

 discrepancies of evidence which beset this question. The foregoing 

 observations show that there is no need to doubt either the veracity 



