SECT. XVI. OBSTRUCTION OP SOUND. 135 



experiment, it was computed that the temperature of a mass of 

 air is raised nine-tenths of a degree when the compression is 

 equal to TJg 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 con- 

 sequently capable of conveying sound even more rapidly than 

 air, the velocity in the former 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 experi- 

 ments 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 interposition of any object, 

 such as a projecting wall ; consequently sound under water re- 

 sembles light in having a distinct shadow. It has much less in 

 air, being transmitted all round buildings or other obstacles, so 

 as to be heard in every direction, though often with a considerable 

 diminution of intensity, as when a carriage turns the corner of 

 a street. 



The velocity of sound in passing through solids is in propor- 

 tion to their hardness, and is much greater than in air or water. 

 A sound which takes some time in travelling through the air 

 passes almost instantaneously along a wire six hundred feet long ; 

 consequently it is heard twice first as communicated by the 

 Avire, and afterwards 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 falling 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 partial 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 of the Orinoco, which seemed to be three 

 times as loud by night as by day. This he illustrated by ex- 



