1874.] Transmission of Sound by the Atmosphere. 61 



visible on the 1st, while the sounds were cut off at one sixth of the dis- 

 tance. At 2 P.M. neither guns nor trumpets were able to pierce the 

 transparent air to a depth of 3, hardly to a depth of 2 miles. This extraor- 

 dinary opacity was proved conclusively to arise from the irregular admix- 

 ture with the air of the aqueous vapour raised by a powerful sun. This 

 vapour, though perfectly invisible, produced an acoustic cloud impervious 

 to the sound, and from which the sound-waves were thrown back as the 

 waves of light are from an ordinary cloud. The waves thus refused trans- 

 mission produced by their reflection echoes of extraordinary strength and 

 duration. This I may remark is the first time that audible echoes have 

 been proved to be reflected from an optically transparent atmosphere. By 

 the lowering of the sun the production of the vapour was checked, and 

 the transmissive power of the atmosphere restored to such an extent that, 

 at a distance of 2 miles from the Foreland, at 7 P.M., the intensity of the 

 sound was at least thirty-six times its intensity at 2 P.M. 



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

 Canadian whistle of great power being added to the list of instruments. 

 A boiler had its steam raised to a pressure of 70 Ibs. 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 convert this stream into a 

 series of separate strong puffs. This was done by causing a disk with 

 12 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 pres- 

 sure generated a sonorous wave of great intensity ; the successive waves 

 linking 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 7| 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 previous 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 depen- 

 dence 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 trans- 

 missive power had diminished, the maximum range being 7| miles. On 



