380 Dynamic Theory. 



movement. The forward and back movement of particles involved in the 

 wave motion of liquids is shown to take the form of minute vertical circles. 

 The particle moves forward or away from the center of agitation on the 

 upper segment of the circle and returns on the lower segment. In sound 

 the movement of the particles of the vibrating body is " directly away 

 from and toward the center of agitation in right lines. In radiant heat 

 and light, the vibrations are at right angles to the direction of advance, 

 like the waving of the clothesline or the vibration of a fiddle string. It 

 is in all directions from the central axis or, as it is styled, in every azi- 

 muth. Every one has some idea of concord and discord. When two 

 simultaneous sounds are produced we generally notice that the effect of 

 the blending is a smooth, even sound pleasing to the ear ; or it may be 

 harsh, grating, full of loud spots and low spots and generally disagreeable. 

 The first we characterize as concordant, harmonious, &c. , and the last 

 as discordant and inharmonious. The pleasant or unpleasant effects on 

 us depend upon the habit or cultivation of our auditory brain, or the in- 

 herited effects of the habit and cultivation of our ancestors. There are, 

 in the relationship of sounds, certain actual conditions which make har- 

 mony and inharmony real and objective in nature, and it is the reflec- 

 tion of these real conditions, in our brain, that constitutes our cultiva- 

 tion and habit. The physical basis of this musical sense is found in 

 the effect of the sonorous pulsations upon each other. A vibration of 

 a sonorous body consists in its making, under the impulse of a blow, an 

 excursion to the end of its amplitude and back again. The outward 

 motion communicated to the air is called a condensed pulse, because the 

 air is condensed -within the limit of the length of the wave, or the 

 length of the spoke referred to above, and this condensation answers to 

 the shortening of the spoke by the blow on its end. The elasticity of 

 the air causes it to recover from this condensation, and when it flies 

 back it leaves behind it a stratum of rarified air for each wave, and this 

 is called a rarified pulse. It answers to the lengthening of the spoke 

 after its recovery from the blow of compression. Now, if two similar 

 sonorous bodies near to each other are set to vibrating in such a way 

 that the outward or condensed pulse is made by both at once, the re- 

 sult is an increased amplitude and a greater condensation to the con- 

 densed pulse, producing greater loudness. If the condensed pulse of 

 one is made at the same instant as the rarefied pulse of the other, the 

 two will neutralize each other , that is, the air remains still and no 

 sound results. It is like two bodies running against each other from 

 opposite directions ; they both stop. This is called interference. 

 There is often partial interference, in which sounds reduce without 

 quenching each other. If two sounding bodies are not in unison, that 

 is, have a different rate of vibration, they will, when sounded together, 



