118 TERRESTRIAL ADAPTATIONS. 



over the field, in the direction in which the 

 wind blows, but this appearance of an object 

 moving is delusive. The only real motion is that 

 of the ears of grain, of which each goes and 

 returns, as the stalk stoops and recovers itself. 

 This motion affects successively a line of ears in 

 the direction of the wind, and affects simultane- 

 ously all those ears of which the elevation or de- 

 pression forms one visible wave. The elevations 

 and depressions are propagated in a constant 

 direction, while the parts with which the space 

 is filled only vibrate to and fro. Of exactly such 

 a nature is the propagation of sound through the 

 air. The particles of air go and return through 

 very minute spaces, and this vibratory motion 

 runs through the atmosphere from the sounding 

 body to the ear. Waves, not of elevation and 

 depression, but of condensation and rarefaction, 

 are transmitted ; and the sound thus becomes an 

 object of sense to the organ. 



Another familiar instance of the propagation of 

 vibrations we have in the circles on the surface 

 of smooth water, which diverge from the point 

 where it is touched by a small object, as a drop 

 of rain. In the beginning of a shower for in- 

 stance, when the drops come distinct, though fre- 

 quent, we may see each drop giving rise to a ring, 

 formed of two or three close concentric circles, 

 which grow and spread, leaving the interior of 

 the circles smooth, and gradually reaching parts 



