SCT. XVI. UNDULATIONS OF OOBN. 123 



ble, which shows that the atmosphere is really the me- 

 dium of sound. In the small undulations of deep water 

 in a calm, the vibrations of the liquid particles are made 

 m the vertical plane, that is up and down, or at right 

 angles to the direction of the transmission of the waves. 

 But the vibrations of the particles of ah* which produce 

 sound differ from these, being performed in the same 

 direction in which the waves of sound travel. The 

 propagation of sound has been illustrated by a field of 

 corn agitated by the wind. However irregular the 

 motion of the corn may seem on a superficial view, it 

 will be found, if the velocity of the wind be constant, 

 that the waves are all* precisely similar and equal, and 

 that all are separated by equal intervals and move in 

 equal times. 



A sudden blast depresses each ear equally and suc- 

 cessively in the direction of the wind, but in conse- 

 quence of the elasticity of the stalks and the force of 

 the impulse, each ear not only rises again as soon as 

 the pressure is removed, but bends back nearly as 

 much in the contrary direction, and then continues to 

 oscillate backward and forward in equal times, like a 

 pendulum to a less and less extent, till the resistance of 

 the air puts a stop to the motion. These vibrations are 

 the same for every individual ear of corn. Yet as their 

 oscillations do not all commence at the same time, but 

 successively, the ears will have a variety of positions at 

 any one instant. Some of the advancing ears will meet 

 others in their returning vibrations, and as the times of 

 oscillation are equal for all, they will be crowded to- 

 gether at regular intervals. Between these there will 

 occur equal spaces, where the ears will be few, in con- 

 sequence of being bent in opposite directions ; and at 

 other equal intervals they will be in their natural upright 

 positions. So that over the whole field there will be a 

 regular series of condensations and rarefactions among 

 the ears of corn, separated by equal intervals where 

 they will be in their natural state of density. In con- 

 sequence of these changes the field will be marked by 

 an alternation of bright and dark bands. Thus the 

 successive waves which fly over the corn with the 

 speed of the wind, are totally distinct from, and entirely 



