196 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
generated should so rise and mingle with the air as to form an absolutely homogeneous 
medium I considered in the highest degree improbable. It would be sure, I thought, 
to rise in streams, breaking through the superincumbent air now at one point now at 
another, thus rendering the air flocculent with wreaths and striae, charged in different 
degrees with the' buoyant vapour. At the limiting surfaces of these spaces, though 
invisible, we should have the conditions necessary to the production of partial echoes 
and the consequent waste of sound. 
Curiously enough, the conditions necessary for the testing of this explanation imme- 
diately set in. At 3.15 p.m. a solitary cloud threw itself athwart the sun, and shaded the 
entire space between us and the South Foreland. The production of vapour was 
suddenly checked by the interposition of this screen ; hence the probability of suddenly 
improved transmission. To test this inference the steamer was immediately turned 
and urged back to our last position of inaudibility. The sounds, as I expected, were 
distinctly though faintly heard. This was at 3 miles distance. At 3f miles the guns 
were fired, both point-blank and elevated. The faintest pop was all that we heard ; but 
we did hear a pop, whereas we had previously heard nothing, either here or three quarters 
of a mile nearer. We steamed out to 4^- miles, where the sounds were for a moment 
faintly heard; but they fell away as we waited; and though the greatest quietness 
reigned on board, and though the sea was without a ripple, we could hear nothing. 
We could plainly see the steam-puffs which announced the beginning and the end of a 
series of trumpet-blasts, but the blasts themselves were quite inaudible. 
It was now 4 p.m., and my intention at first was to halt at this distance, which was 
beyond the sound-range, but not far beyond it, and see whether the lowering of the sun 
would not restore the power of the atmosphere to transmit the sound. But after waiting 
a little, the anchoring of a boat was suggested, so as to liberate the steamer for other 
work ; and though loth to lose the anticipated revival of the sounds myself, I agreed 
to this arrangement. Two men were placed in the boat and requested to give all atten- 
tion so as to hear the sound if possible. With perfect stillness around them they heard 
nothing. They were then instructed to hoist a signal if they should hear the sounds, 
and to keep it hoisted as long as the sounds continued. 
At 4.45 we quitted them and steamed towards the South Sand Head light-ship. Pre- 
cisely 15 minutes after we had separated from them the flag was hoisted : the sound 
had at length succeeded in piercing the body of air between the boat and the shore. 
We continued our journey to the light-ship, went on board, and heard the report of 
the lightsmen. Returning towards the Foreland, in answer to a signal expressing a 
wish to communicate with us, we manned a boat and pulled to the shore. The 
exhaustion of the ammunition was reported, but the horns and whistle continued to 
sound. We steamed out to our anchored boat, and then learned that when the flag was 
hoisted the horn-sounds were heard, that they were succeeded after a little time by the 
whistle-sounds, and that both increased in intensity as the evening advanced. On our 
arrival, of course we heard the sounds ourselves. 
