SOUND 213 



carries sound to the ears. This is correct ; but air is not 

 the only substance that will carry sound. Solids, liquids, 

 and other gases will also carry sound. If the ear is held 

 near the steel rails of a railroad, a train approaching at 

 considerable distance may be heard. It is also quite a 

 familiar fact that if the ear is held under water, the noise 

 made by hitting two stones together some distance away 

 may be heard distinctly. The speed of sound in air at 

 C. is 1087 feet a second. In water the speed is about 

 4600 feet a second and in iron 16,700 feet. The speed of 

 sound in air increases with the temperature. 



When two boards are brought together suddenly, air 

 is driven from between them with considerable force and 

 the surrounding air is pushed back in all directions. This 

 motion is communicated to the next layer of air and to 

 succeeding layers as wave motion. As the wave moves 

 farther and farther from its source, a larger volume of 

 air is affected and the intensity of the wave decreases. 

 If an ear is within the range of these waves, the sensa- 

 tion of sound is produced. Sounds may be heard in all 

 directions, which is evidence that the wave advances in 

 the form of the surface of a sphere. 



When a stone is dropped in still water, waves are pro- 

 duced. These waves move in circles outward from the 

 source. That the water in these waves does not move 

 out with the wave may be proved by scattering some 

 light material on the surface of the water. Sound waves 

 are similar to water waves. Water waves advance as 

 the circumference of a circle, while sound waves in the 

 air advance in the form of the surface of a sphere. In 

 such a wave the air does not actually move forward. 

 Air is very elastic. When an impulse is given to the air by 

 some sound-producing source, the adjacent layer of air is 



