TRANSMISSION OF SOUND BY THE ATMOSPHERE. 
177 
on the preceding one, plainly proving that something else than 
the wind must play an important part in shortening the sound- 
range. 
On July 1 we experimented upon a rotating horn, and heard 
its direct or axial blast, which proved to he the strongest, at a 
distance of 10 J miles. The sounds were also heard at the 
Yarne light-ship, which is 12 J miles from the Foreland. The 
atmosphere had become decidedly clearer acoustically, but not 
so optically ; for on this day thick haze obscured the white- 
cliffs of the Foreland. In fact, on days of far greater optical 
purity, the sound had failed to reach one-third of the distance 
attained to-day. In the light of such a fact, any attempt to 
make optical transparency a measure of acoustic transparency, 
must be seen to be delusive. On July 1 a 12- inch American 
whistle, of which we had heard a highly favourable account, 
was tried in place of the 12-inch English whistle ; but, like its 
predecessor, the performance of the new instrument fell behind 
that of the horns. An interval of twelve hours sufficed to con- 
vert the acoustically clear atmosphere of July 1 into an opaque 
one ; for on July 2 even the horn-sounds, with paddles stopped 
and all noiseless on board, could not penetrate further than 
4 miles. 
Thus each succeeding day provided us with a virtually new 
atmosphere, clearly showing that conclusions founded upon one 
day’s observations might utterly break down in the presence of 
the phenomena of another day. This was most impressively 
demonstrated on the day now to be referred to. The acoustic 
imperviousness of July 3 was found to be still greater than 
that of the 2nd, while the optical purity of the day was 
sensibly perfect. The cliffs of the Foreland could be seen to- 
day at ten times the distance at which they ceased to be visible 
on the 1st, while the sounds were cut off at one-sixth of the dis- 
tance. At two p.m. neither guns nor trumpets were able to 
pierce the transparent air to a depth of 3, hardly to a depth 
of 2 miles. This extraordinary opacity we consider to arise 
from the irregular admixture with the air of the aqueous 
vapour raised by a powerful sun. This vapour, though per- 
fectly invisible, produced an acoustic cloud impervious to the 
sound, and from which the sound-waves were thrown back as 
the waves of light are from an ordinary cloud. The waves thus 
refused transmission produced by their reflection echoes of 
extraordinary strength and duration. This we may remark is 
the first time that audible echoes have been proved to be 
reflected from an optically transparent atmosphere. By the 
lowering of the sun the production of the vapour was checked, 
and the transmissive power of the atmosphere restored to such 
an extent that, at a distance of 2 miles from the Foreland, at 
VOL. XIII. — NO. LI. N 
