OF SOUND. 229 



called an octave. If one gives two while the other 

 gives three, the harmony of a fifth is produced. 

 In a fourth the vibrations are as three to four, &c. 

 All sounds arrive at the ear in the same time 

 from sounding bodies equally distant, and sound 

 travels through the air at the rate of 1130 feet in a 

 second. This knowledge of the velocity of sound 

 is of use in determining the distances of ships, or 

 other objects: for instance, if a ship fires a gun, 

 the motion of the light from the flash being almost 

 instantaneous, the sound is not heard till some 

 time after the flash is seen; then if 1130 feet be 

 multiplied by the number of seconds between the 

 sight of the flash and the sound, it will give the 

 distance nearly. 



Sounds may be heard in water, but faintly, 

 though it travels through water much faster than 

 through air. 



Different musical sounds do not destroy each 

 other, but may be heard together, the reason of 

 which may be illustrated by observing, that the 

 circular waves made in the water, by two stones 

 thrown in at the same time, cross each other 

 without disturbance. 



When the aerial vibrations meet with an obstacle 

 of a hard nature, they are reflected; and conse- 

 quently, an ear placed in the course of these 

 reflected waves, will perceive a sound similar to 

 the original sound, but which will seem to proceed 

 from a body situated in like position and distance 

 behind the plane of reflection, as the real sounding 

 body is before it. This reflected sound is called 

 an echo. 



The waves of sound being thus reflexible, nearly 

 in the same manner as the rays of light, may be 

 deflected or magnified by much the same contri- 



Q 3 



