SICT. XVI. TRANSMISSION OF SOUND. 129 



nine-tenths of a degree when the compression is equal 

 to j-} T of its volume. 



Probably all liquids are elastic, though considerable 

 force is required to compress them. Water suffers a 

 condensation of nearly 0-0000496 for every atmosphere 

 of pressure, and is consequently capable of conveying 

 sound even more rapidly than air, the velocity in the for- 

 mer being 4708 feet in a second. A person under water 

 hears sounds made in air feebly, but those produced in 

 water very distinctly. According to the experiments of 

 M. Colladon, the sound of a bell was conveyed under 

 water through the Lake of Geneva to the distance of 

 about nine miles. He also perceived that the progress 

 of sound through water is greatly impeded by the inter- 

 position of any object, such as a projecting wall ; conse- 

 quently sound under water resembles light hi having a 

 distinct shadow. It has much less in air, being trans- 

 mitted all round buildings or other obstacles, so as te be 

 heard in every direction, though often with a consid- 

 erable diminution of intensity, as when a carriage turns 

 the corner of a street. 



The velocity of sound in passing through solids is in 

 proportion to their hardness, and is much greater than 

 in air or water. A sound which takes some time in trav- 

 eling through the air passes almost instantaneously along 

 a wire six hundred feet long; consequently it is heard 

 twice first as communicated by the wire and after- 

 ward through the medium of the air. The facility 

 with which the vibrations of sound are transmitted along 

 the grain of a log of wood is well known. Indeed they 

 pass through iron, glass, and some kinds of wood, at the 

 rate of 18,530 feet in a second. The velocity of sound 

 is obstructed by a variety of circumstances, such as fall- 

 ing snow, fog, rain, or any other cause which disturbs 

 the homogeneity of the medium through which it has 

 to pass. M. de Humboldt says that it is on account of 

 the greater homogeneity of the atmosphere during the 

 night that sounds are then better heard than during the 

 day, when its density is perpetually changing from par- 

 tial variations of temperature. His attention was called 

 to this subject on the plain surrounding the Mission of 

 the Apures by the rushing noise of the great cataracts 

 9 



