TRANSMISSION OF SOUND BY THE ATMOSPHERE. 
179 
have been heard at fifteen times the distance attained to wind- 
ward. The momentary character of the gun-sound renders it 
liable to he quenched by a single puff of wind ; but sounds of 
low pitch generally, whether momentary or not, suffer more 
from an opposing wind than high ones. We had on the 13th 
another example of the powerlessness of heavy rain to deaden 
sound. 
On the 14th the maximum range was 10 miles, but the 
atmosphere did not maintain this power of transmission. It 
was a day of extreme optical clearness ; but its acoustic clear- 
ness diminished as the day advanced. In fact the sun was in 
action. We proved to-day that by lowering the pitch of the 
Canadian whistle, its sound, which had previously been in- 
audible, became suddenly audible. The day at first was 
favourable to the transmission of the longer sound-waves. After 
a lapse of three hours the case was reversed, the high-pitched 
syren being then heard when both gun and horns were inaudible. 
But even this state of things did not continue, so rapid and 
surprising are the caprices of the atmosphere. At a distance of 
5 miles, at 3.30 p.m., the change in transmissive power reduced 
the intensity of the sound to at least one half of what it 
possessed at 11.30 a.m., the wind throughout maintaining the 
same strength and direction. Through all this complexity the 
knowledge obtained on July 3 sheds the light of a principle 
which reduces to order the apparent confusion. 
October 15 was spent at Dungeness in examining the per- 
formance of Daboll’s horn. It is a fine instrument, and its 
application was ably worked out by its inventor ; still it would 
require very favourable atmospheric conditions to enable it to 
warn a steamer before she had come dangerously close to the 
shore. The direction in which the aerial echoes return was 
finely illustrated to-day, that direction being always the one in 
which the axis of the horn is pointed. 
The 16th was a day of exceeding optical transparency, but of 
great acoustic opacity. The maximum range was only 5 miles. 
On this day the howitzer and all the whistles were clearly over- 
mastered by the syren. It was, moreover, heard at 3^ miles 
with the paddles going, while the gun was unheard at 2J miles. 
With no visible object that could possibly yield an echo in 
sight, the pure aerial echoes, coming from the more distant 
southern air, were distinct and long-continued at a distance of 
2 miles from the shore. Near the base of the Foreland cliff we 
determined their duration and found it to be 11 seconds, while 
that of the best whistle echoes was 6 seconds. On this day three 
whistles, sounded simultaneously, were pitted against the syren, 
and found clearly inferior to it. 
On the 17th four horns were compared with the syren and 
