1874.] Transmission of Sound by the Atmosphere. 63 



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 found 

 inferior to it. This was our day of greatest acoustic transparency, the 

 sound reaching a maximum of 15 miles for the syren, and of more than 

 16 for the gun. The echoes on this day were continued longer than on 

 any other occasion. They continued for 15 seconds, their duration 

 indicating the atmospheric depth from which they came. 



On October 18, though the experiments were not directed to deter- 

 mine the transmissive power of the air, we were not without proof that 

 it continued to be high. From 10 to 10.30 A.M., while waiting for the 

 blasts of the syren at a distance of 3 miles from the Foreland, the con- 

 tinued reports of what we supposed to be the musketry of skirmishing 

 parties on land were distinctly heard by us all. We afterwards learned 

 that the sounds arose from the rifle-practice on Kingsdown beach, 

 5| miles away. On July 3, which, optically considered, was a far more 

 perfect day, the 18-pounder, howitzer, and mortar failed to make them- 

 selves heard at half this distance. The 18th was mainly occupied in 

 determining the influence of pitch and pressure on the syren-sound. 

 Taking the fluctuations of the atmosphere into account, I am of opinion 

 that the syren, performing from 2000 to 2400 revolutions a second, or, 

 in other words, generating from 400 to 480 waves per second, best meets 

 the atmospheric conditions. We varied the pressure from 40 to 80 Ibs. 

 on the square inch ; and though the intensity did not appear to rise in 

 proportion to the pressure, the higher pressure yielded the hardest and 

 most penetrating sound. 



The 20th was a rainy day with strong wind. Up to a distance of 

 5 1 miles the syren continued to be heard through the sea- and paddle- 

 noises. In rough weather, indeed, when local noises interfere, the syren- 

 sound far transcends all other sounds. On various occasions to-day it 

 proved its mastery over both gun and horns. On the 21st the wind 

 was strong and the sea high. The horn-sounds, with paddles going, 

 were lost at 4 miles, while the syren continued serviceable up to 6| miles. 

 The gun to-day was completely overmastered. Its puffs were seen at the 

 Foreland ; but its sound was unheard when the syren was distinctly heard. 

 Heavy rain failed to damp the power of the syren. The whistles were 

 also tried to-day, but were found far inferior to the syren. On the 22nd 

 it blew a gale, and the 'Galatea' quitted us. We made observations 

 on land on the influence of the wind and of local noises. The shelter 

 of the Coastguard Station at Cornhill enabled us to hear gun-sounds 



