150 VELOCITY OF SOUND. [SECT. xvi. 



problem, and assuming the velocity of sound as given by ex- 

 periment, it was computed that the temperature of a mass 

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

 sion is equal to 1 j B 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 former being 4708 feet in a 

 second. A person under water hears sounds made in air 

 feebly, but those produced in water very distinctly. Accord- 

 ing 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 interposition of any object, such as a projecting wall ; 

 consequently sound under water resembles 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 pro- 

 portion 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 communi- 

 cated by the wire 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 ho- 

 mogeneity of the atmosphere during the night that sounds 



