63 VELOCITY OF SOUND. 



exhausting the receiver, you cut off the communication be- 

 tween the air and bell ; and the latter, therefore, cannot 

 impart its motion to the air. It has been ascertained that 

 liquids as well as air are capable of conveying the vibratory 

 motion of a sonorous body to the organ of l>earing ; for 

 sound can be heard under water. Dr. Franklin imagined, 

 that with his ear under water, he heard the collision of 

 stones in that medium, at the distance of a mile. 



The vibration of a sonorous body gives a tremulous mo- 

 tion to the air around it, very similar to the motion commu- 

 nicated to smooth water when a stone is thrown into it. 

 This first produces a small circular wave around the spot in 

 which the stone falls ; the wave spreads, and gradually com- 

 municates its motion to the adjacent waters, producing simi- 

 lar waves to a considerable extent. The same kind of waves 

 are produced in the air by the motion of a sonorous body, 

 but with this difference, that as air is an elastic fluid, the 

 motion does not consist of regularly extending waves, but of 

 vibrations, and are composed of a motion forwards and back- 

 wards, similar to those of a sonorous body. They differ also 

 in the one taking place in a plane, the other in all directions : 

 the aerial undulations being spherical. The first sphere of 

 undulations .^jvhich are produced immediately round the so- 

 norous body, by pressing against the contiguous air, con- 

 denses it. The condensed air, though impelled forward by 

 the pressure, reacts on the first set of undulations, driving 

 them back again. The second set of undulations which 

 have been put in motion, in their turn communicate their 

 motion, and are themselves driven back by reaction. Thus 

 there is a succession of waves in the air, corresponding with 

 the succession of waves in the water. 



The air is a fluid so much less dense than water, that 

 motion is more easily comiimnicated to it. The firing of a 

 cannon produces vibrations of the air which extend to se- 

 veral miles around. Distant sound, however, takes some 

 time to reach us, and we see the light of the flash long be- 

 fore we hear the report. The velocity of sound is commonly 

 computed at the rate of eleven hundred and forty-two feet 

 in a second. Its velocity varies according to the tempe- 

 rature, density, and humidity of the atmosphere. It is in- 

 fluenced also by the force and direction of the wind. The 

 velocity of sound has been applied to the measurement of 



