144 UNDULATIONS OF CORN. [SECT. xvi. 



A sudden blast depresses each ear equally and successively 

 in the direction of the wind, out, in consequence of the elas- 

 ticity 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 backwards and forwards 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 suc- 

 cessively, 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 together at regular inter- 

 vals. Between these there will occur equal spaces, where the 

 ears will be few, in consequence 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 conse- 

 quence of these changes the field will be marked by an alter- 

 nation 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 independent of the extent of the 

 oscillations of each individual ear, though both take place 

 in the same direction. The length of a wave is equal to the 

 space between two ears precisely in the same state of motion, 

 or which are moving similarly, and the time of the vibration 

 of each ear is equal to that which elapses between the arrival 

 of two successive waves at the same point. The only differ- 

 ence 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 com- 



