AS A VEHICLE OF SOUND. 
201 
therefore is knowledge of direction given by a sound, but it may also be given by the 
aerial echoes of the sound. 
On various occasions, when the atmosphere was perfectly free from clouds, I have had 
myself rowed from the ‘ Galatea ’ to the Foreland Cliff to listen to the echoes. On the 
16th of October, at half a mile from the shore, we stopped and found the syren-echoes 
at this distance very distinct. The sky being absolutely cloudless, the gig was manned, 
and we rowed in. As we approached the cliff, thus deepening the reflecting layer of 
air, the echoes augmented in intensity and duration. We did not quit the boat, but 
halted as near as possible to the water-mark. The echoes returned by the transparent 
and perfectly invisible atmosphere were of astonishing strength and sweetness. 
On this day the whistles and syren were compared. The average duration of the 
syren-echoes was 11 seconds ; that of the whistle-echoes 6 seconds. In all cases the 
sound with the longest echo had the greatest range. 
With sounds of the same pitch the duration of the echoes might be taken as a 
measure of the penetrative power of the sound. 
The earliest, strongest, and most trumpet-like tones are thrown back from the stratum 
of air less than a quarter of a mile in thickness, which is first pierced by the sound. 
After we had placed this air-stratum between us and the shore its echoes were with- 
drawn, the residual echoes being in consequence much feebler. 
One additional illustration of this character will suffice. On the 17th of October, 
at about 5 p.m., the air being perfectly free from clouds, we rowed towards the base of 
the Foreland Cliff. The ‘Galatea’ being broadside on returned a loud echo to both 
syren and horns. It was a simple but wonderfully distinct copy of the original sound. 
At a certain moment it struck in, raised the intensity of the sound, then ceased, suddenly 
lowering the intensity, and leaving the long-drawn aerial echoes to pursue their course 
and die gradually into silence. 
We landed, and passed over the seaweed to the base of the cliff. As I reached the 
base the position of the ‘ Galatea ’ was such that an echo of astonishing intensity was 
sent back from her side ; it came as if from an independent source of sound established 
on board the steamer. As before, this echo ceased suddenly, leaving the aerial echoes 
to fade gradually away. 
At the base of the cliff a series of concurrent observations gave the duration of the 
aerial syren-echoes from 13 to 14 seconds. 
Lying on the shingle under a projecting roof of chalk, the somewhat enfeebled dif- 
fracted sound reached me, and I was able to hear with great distinctness, about a second 
after the starting of the syren-blast, the echoes striking in and reinforcing the direct 
sound. The first rush of echoed sound was very powerful, and it came, as usual, from 
a stratum of air 600 or 700 feet in thickness. On again testing the duration of the 
echoes, it was found to be from 14 to 15 seconds. The perfect clearness of the afternoon 
caused me to choose it for the examination of the echoes. It is worth remarking that this 
was our day of longest echoes, and it was also our day of greatest acoustic transparency, 
MDCCCLXXIV. 2 D 
