AS A VEHICLE OE SOUND. 
197 
Thus far, therefore, the explanation given above entirely agrees with the results of 
observation ; but we pushed the test further by steaming further out. At 5f miles we 
halted and' heard the sounds : at 6 miles we heard them distinctly, but so feebly that 
we thought we had reached the limit of the sound-range ; but while we waited the 
sounds rose in power. We steamed to the Yarne buoy, which is 7f miles from the 
signal-station, and heard the sounds there better than at 6 miles distance. We con- 
tinued our course outwards to 10 miles, halted there, but heard nothing. 
Steaming, however, on to the Varne light-ship, which is situated at the other end of the 
Yarne shoal, we hailed the master, and were informed by him that up to 5 p.m. nothing 
had been heard, but that at that hour the sounds began to be audible. He described 
one of them as “ very gross, resembling the bellowing of a bull,” which very accurately 
characterizes the sound of the large American steam-whistle. At the Yarne light-ship, 
therefore, the sounds had been heard towards the close of the day, though it is 12f 
miles from the signal-station. 1 think it probable that, at a point 2 miles from the 
Foreland, the sound at 5 p.m. possessed fifty times the intensity which it possessed at 2 p.m. 
On our return to Dover Bay at 10 p.m., we heard the sounds, not only distinct but loud, 
where nothing could be heard in the morning. 
In consequence of the position of the promontory very curious winds and currents 
establish themselves round the South Foreland. Mr. Holmes was, as usual, at the 
Foreland on July 3 ; and he informed me that from the motion of the smoke of some 
passing steamers, and from the sails of sailing-vessels, he could recognize a curious circu- 
lation of the air. A slight wind would sometimes hug the shore to the N.E., then bend 
round and move towards the South Sand Head light-ship. And, in point of fact, the 
wind at the light-vessel had been S.W., with a force of 3, nearly the whole of the day ; 
whereas with us it had passed from S.W. by W. to a dead calm, and afterwards to S.E. 
On shore also it had shifted from S.W. to S.E. The atmospheric conditions between 
the light-vessel and the Foreland were therefore different from those between us and 
the Foreland; and the consequence was that at the time when we were becalmed and 
heard nothing, the light-keepers at South Sand Head heard the sounds plainly. 
§ 8. Aerial Echoes. 
But both the argument and the phenomena have a complementary side, which we 
have now to consider. A stratum of air less than 3 miles thick on a calm day 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 was due to 
the irregular admixture of air and aqueous vapour, which filled the atmosphere with an 
impervious acoustic cloud on a day of perfect optical transparency. But, granting this, 
it is incredible that so great 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 
