130 NATURE OF WAVES. SECT. XVI. 



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 

 ^he 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 

 9scillation are equal for all, they will be crowded together at 

 regular intervals. 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 consequence of these 

 changes the field will be marked by an alternation of bright and 

 dark bands. Thus the successive waves which fly over the com 

 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 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 communi- 

 cates its vibrations to the surrounding particles, which transmit 

 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 throughout 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 undulations 

 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 



