64 ON THE PHYSIOLOGICAL CAUSES OF 



will produce much greater density than the hollows, from which 

 the fluid had been removed to form the ridges. Hence the 

 ridges are replaced by condensed strata of air, and the hollows 

 by rarefied strata. Now further imagine that these compressed 

 waves are propagated by the same law as before, and that also 

 the vertical circular orbits of the several particles of water 

 are compressed into horizontal straight lines. Then the waves 

 of sound will retain the peculiarity of having the particles of 

 air only oscillating backwards and forwards in a straight line, 

 while the wave itself remains merely a progressive form of 

 motion, continually composed of fresh particles of air. The 

 immediate result then would be waves of sound spr/eading out 

 horizontally from their origin. 



But the expansion of waves of sound is not limited, like 

 those of water, to a horizontal surface. They can spread out in 

 any direction whatsoever. Suppose the circles generated by 

 a stone thrown into the water to extend in all directions of space, 

 and you will have the spherical waves of air by which sound is 

 propagated. 



Hence we can continue to illustrate the peculiarities of the 

 motion of sound by the well-known visible motions of waves 

 of water. 



The length of a wave of water, measured from crest to 

 crest, is extremely different. A falling drop, or a breath of air, 

 gently curls the surface of the water. The waves in the wake 

 of a steamboat toss the swimmer or skiff severely. But the 

 waves of a stormy ocean can find room in their hollows for the 

 keel of a ship of the line, and their ridges can scarcely be 

 overlooked from the mast-head. The waves of sound present 

 similar differences. The little curls of water with short lengths 

 of wave correspond to high tones, the giant ocean billows to 

 deep tones. Thus the contrabass C has a wave thirty-five feet 

 long, its higher octave a wave of half the length, while the 

 highest tones of a piano have waves of only three inches in 

 length. 



1 The exact lengths of waves corresponding to certain notes, or symbols of 

 tone, depend upon the standard pitch assigned to one particular note, and 



