134 UNDULATIONS OF THE AIR. SECT. XVI. 



independent of the extent of the oscillations of each in- 

 dividual ear, though both take place in the same direc- 

 tion. The length of a wave is equal to the space be- 

 tween two ears precisely in the same state of motion, 

 or which are moving similarly, and the time of the vi- 

 bration of each ear is equal to that which elapses be- 

 tween the arrival of two successive waves at the same 

 point. The only difference between the undulations of 

 a corn-field and those of the air which produce sound 

 is, that each ear of corn is set in motion by an external 

 cause and is uninfluenced by the motion of the rest ; 

 whereas in air, which is a compressible and elastic fluid, 

 when one particle begins to oscillate, it communicates 

 its vibrations to the surrounding particles, which trans- 

 mit them to those adjacent, and so on continually. 

 Hence from the successive vibrations of the particles of 

 air the same regular condensations and rarefactions take 

 place as in the field of corn, producing waves through- 

 out the whole mass of air, though each molecule, like 

 each individual ear of corn, never moves far from its 

 state of rest. The small waves of a liquid and the un- 

 dulations of the air like waves in the corn, are evidently 

 not real masses moving in the direction in which they 

 are advancing, but merely outlines, motions, or forms 

 passing along, and comprehending all the particles of an 

 undulating fluid which are at once in a vibratory state. 

 It is thus that an impulse given to any one point of the 

 atmosphere is successively propagated in all directions, 

 in a wave diverging as from the center of a sphere to 

 greater and greater distances, but with decreasing in- 

 tensity, in consequence of the increasing number of par- 

 ticles of inert matter which the force has to move ; like 

 the waves formed in still water by a falling stone, which 

 are propagated circularly all around the center of' dis- 

 turbance (N. 156). 



The intensity of sound depends upon the violence 

 and extent of the initial vibrations of air ; but whatever 

 they may be, each undulation when once formed can 

 only be transmitted straight forward, and never returns 

 back again unless when reflected by an opposing ob- 

 stacle. The vibrations of the aSrial molecules are al- 

 ways extremely small, whereas the waves of sound 



