INTENSITY OF SOUND. 1 7 1 



each other, and impart a motion to the elastic medium air, so a sound 

 comes to us 



The intensity of sounds gets less the farther it goes from us, or the 

 loudness of sound is less the greater its distance. The law is, that in 

 an unvarying medium the loudness varies inversely as the square of the 

 distance. But Poisson has shown that when air-strata, differing in density, 

 are existing between the ear and the source of the sound, the intensity or 

 loudness with which it is heard depends only on the density of the air at the 

 place the sound originated. This fact has been substantiated by balloonists 

 who heard a railway whistle quite distinctly when they were nearly 20,000 

 feet above the ground. It therefore follows that sound can be heard in a 

 balloon equally well as on the earth at certain given distances. But as the 

 density of the air diminishes the sound becomes fainter, as has been proved 

 by the bell rung in the receiver of an air-pump. The velocity of sound, to 

 a certain extent, depends upon its intensity, as Earnshaw sought to prove ; for 

 he instanced a fact that in the Arctic regions, where sound can be heard for 

 an immense distance, in consequence of the still and homogeneous air, the 

 report of a cannon two miles and a half away was heard before the loud 

 command to " fire," which must have preceded the discharge. Another 

 instance showing the difference in hearing through mixed and homogeneous 

 media may be referred to. In the war with America, when the English and 

 their foes were on opposite sides of a stream, an American was seen to beat 

 his drum, but no sound came across. " A coating of soft snow and a thick 

 atmosphere absorbed the noise." Glazed, or hard snow, would have a con- 

 trary effect. Reynault also experimentally verified his theory, that sound 

 when passing through a space of nearly 8,000 feet lost velocity as its 

 intensity diminished, and in that distance between its arrival at 4,000 feet 

 and at 7,500 feet, the sound velocity diminished by 2*2 feet per second. He 

 also tried to demonstrate that sound velocity depended upon its pitch, and 

 that lower notes travelled with the greater speed. 



The reflection and refraction of sound follows the same fundamental 

 laws as the reflection and refraction of light. The reflection of sound is 

 termed an Echo, which is familiar to all tourists in Switzerland and Ireland 

 particularly. There are several very remarkable echoes in the world : at 

 Woodstock, and at the Sicilian cathedral of Gergenti, where the confessions 

 poured forth near the door to priestly ears were heard by a man concealed 

 behind the high altar at the opposite end. It is curious that such a spot 

 should have been accidentally chosen for the Confessional. The whispering 

 gallery in St. Paul's is another instance of the echo. 



Echoes are produced by the reflection of sound waves from a plane or 

 even surface. A wall, or even a cloud, will produce echoes. Thunder is 

 echoed from the clouds. (The celebrated echo of " Paddy Blake," at 

 Killarney, which, when you say "How do you do," is reported to reply, "Very 

 well, thank you," can scarcely be quoted as a scientific illustration.) And 

 the hills of Killarney contain an echo, and the bugle sounds are beautifully 



