194 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
steamed abreast of it at a distance of \ a mile : the sound of the horn was here exceed- 
ingly powerful. At a distance of 3 miles from the station, and on the line between it 
and the South Sand Head light-ship, the sounds were stronger than at the other side ; 
but we were here both nearer to the axis and further removed from the interfering 
influence of the shore. Close to the shore on both sides of the station the turning 
of the horn produced greater variations of the sound than in the open sea in front of 
the station: in one case, indeed, where the maximum sound was marked 10, the mini- 
mum was set down at 2. 
An interval of 12 hours sufficed to change in a surprising degree the acoustic trans- 
parency of the air. On the 1st of July the sound had a range of nearly 18 miles; 
on the 2nd the range did not exceed 4 miles. 
§ 6. Contradictory Results. 
Thus far the investigation proceeded with hardly a gleam of a principle to connect 
the inconstant results. The distance reached by the sound on the 19th of May was 3^ 
miles; on the 20th it was 5J miles; on the 2nd of June 6 miles; on the 3rd more than 
9 miles ; on the 10th it was also 9 miles ; on the 25th it fell to 6^ miles ; on the 26th 
it rose again to more than 9J miles ; on the 1st of July, as we have just seen, it reached 
12f, whereas on the 2nd the range shrunk to 4 miles. None of the meteorological 
agents observed could be singled out as the cause of these fluctuations. The wind exerts 
an acknowledged power over sound, but it could not account for these phenomena. On 
the 25th of June, for example, when the range was only 6^ miles, the wind was favour- 
able ; on the 26th, when the range exceeded 9J miles, it was opposed to the sound. 
Nor could the varying optical clearness of the atmosphere be invoked as an explanation; 
for on July 1, when the range was 12f miles, a thick haze hid the white cliffs of the 
Foreland, while on many other days, when the acoustic range was not half so great, the 
atmosphere was optically clear. Up to July 3 all remained enigmatical; but on this 
date observations were made which seemed to me to displace surmise and perplexity by 
the clearer light of physical demonstration. 
§ 7. Aerial Reflection and its Causes; solution of contradictions. 
On July 3 we first steamed to a point 2-9 miles S.W. by W. of the signal-station. 
No sounds, not even the guns, were heard at this distance. At 2 miles they were 
equally inaudible. But this being the position in which the sounds, though strong in 
the axis, invariably subsided, we steamed to the exact bearing from which our observations 
had been made upon July 1. At 2.15 p.m., and at a distance of 3f- miles from the station, 
with calm clear air and a smooth sea, the horns and whistle (American) were sounded, but 
they were inaudible. Surprised at this result, I signalled for the guns. They were all 
fired, but, though the smoke seemed at hand, no sound whatever reached us. On July 1, 
in this bearing, the observed range of both horns and guns was 10^ miles, while on the 
bearing of the Varne light-vessel it was nearly 13 miles. We steamed in to 3 miles, 
