HARMONY IN MUSIC. 73 



To return from waves of water to waves of sound. 

 Imagine an elastic fluid like air to replace the water, and 

 the waves of this replaced water to be compressed b}^ an 

 inflexible plate laid on their surface, the fluid being pre- 

 vented from escaping laterally from the pressure. Then 

 on the waves being thus flattened out, the ridges where 

 the fluid had been heaped up will produce much greater 

 density than the hollows, from which the fluid had been 

 removed to form the ridges. Hence the ridges are re- 

 placed by condensed strata of air, and the hollows by 

 rarefied strata. Now further imagine that these com- 

 pressed 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 spreading 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 



