SOUND 297 



be heard a considerable distance away by a person whose 

 ears are in the water. These facts show that sound can 

 be carried from its source to the ear by solids, liquids, or 

 gases. 



204. Speed of Sound. In our rooms during conversa- 

 tion we might suppose that sound travels instantly from 

 one person to another. But when we see the condensed 

 steam from the whistle of a locomotive evaporate or be- 

 come invisible before we hear the sound, we know that 

 sound does not travel instantly. If we see a flash of light- 

 ning, it is sometimes several seconds before we hear the 

 thunder. 



The French Academy of Science appointed a com- 

 mission in 1832 to determine the speed of sound. They 

 placed cannon on two hilltops that were 11.5 miles apart. 

 The cannon were fired at night so that the flash could 

 be seen, and the time was determined from the time the 

 flash was seen till the sound was heard. This commis- 

 sion found the speed of sound to be 331.2 meters per 

 second, when the air was at freezing temperature or o C. 

 The accepted rate for the speed of sound is 331.3 meters, 

 or 1087 feet, per second at o C. The speed of sound in 

 water is 1400 meters per second, or about four times the 

 speed in air. The speed in iron is 5100 meters per second, 

 or about sixteen times the speed in air. 



The speed of sound in air increases with an increase in 

 temperature. The amount of this increase is about 60 

 centimeters, or two feet, per second per degree centigrade. 

 So the speed of sound at 20 C. is 40 feet per second 

 more than at o C., or the speed at 20 C. is about 343.3 

 meters, or 1126 feet, per second. 



205. Reflection of Sound. An echo is a reflected 

 sound. All solid objects reflect sound just as nearly all 



