Transparency and Opacity of the Atmosphere. Z77 



from the Foreland, at 7 p.m., the intensity of the sound was at least 

 thirty-six times its intensity at 2 p.m. 



That layers of dried air alternating with layers of air saturated 

 with the vapour of a volatile liquid can powerfully intercept sound, 

 has been experimentally established. 



On October 8 the observations were resumed, a steam syren and 

 a Canadian whistle of great power being added to the list of instru- 

 ments. A boiler had its steam raised to a pressure of 70 lbs. to the 

 square inch ; on opening a valve this steam would issue forcibly in 

 a continuous stream ; and the sole function of the syren was to con- 

 vert this stream into a series of separate strong puffs. This was 

 done by causing a disk with twelve radial slits to rotate behind a 

 fixed disk with the same number of slits. When the slits coincided 

 a puff escaped ; when they did not coincide the outflow of steam 

 was interrupted. Each puff of steam at this high pressure gene- 

 rated a sonorous wave of great intensity, the successive waves link- 

 ing themselves together to a musical sound so intense as to be best 

 described as a continuous explosion. 



During the earlier part of October 8 the optical transparency of 

 the air was very great ; its acoustic transparency, on the other 

 hand, was very defective. Clouds blackened and broke into a rain- 

 and hail-shower of tropical violence. The sounds, instead of being 

 deadened, were improved by this furious squall ; and after it had 

 lightened, thus lessening the local noises, the sounds were heard at 

 a distance of 7g miles distinctly louder than they had been heard 

 through the preceding rainless atmosphere at a distance of 5 miles. 

 At 5 miles distance, therefore, the intensity of the sound had been 

 at least doubled by the rain — a result entirely opposed to all pre- 

 vious assertions, but an obvious consequence of the removal by con- 

 densation and precipitation of that vapour the mixture of which 

 with the air had been proved so prejudicial to the sound. On this 

 day a dependence was established between the pitch of a note and 

 its penetrative power — the syren generating 480 waves being 

 slightly inferior to the horns, while generating 400 waves a second 

 it was distinctly superior. The maximum range on October 8 was 

 9 miles. On October 9 the transmissive power had diminished, the 

 maximum range being 7i miles. On both these days the syren 

 proved to be superior to the horns, and on some occasions superior 

 to the gun. 



On the 10th and 11th, a gale having caused our steamer to seek 

 safety in the Downs, we made land-observations. The duration of 

 the aerial echoes was for the syren and the gun 9 seconds, for the 

 horns 6 seconds. The duration varies from day to day. We sought 

 to estimate the influence of the violent wind, and found that the 

 sound of the gun failed to reach us in two cases at a distance of 

 550 yards to windward, the sound of the syren at the same time 

 rising to a piercing intensity. To leeward the gun was heard at 

 five times, and certainly might have been heard at fifteen times the 

 distance attained to windward. The momentary character of the 

 gun-sound renders it liable to be quenched by a single puff of wind ; 



