546 THE HUMAN BODY. 



exactly on the other side of the abscissa, are known as 

 pendular vibrations. 



The Composition of Vibrations. The vibrations of a 

 second's pendulum set the air-particles in contact with it 

 in similar movement, but the aerial waves succeed one- 

 another too slowly to produce in us the sensation of a 

 musical note. If for the pendulum we substitute a tuning- 

 fork (the prongs of which move in a like way), and the 

 fork vibrates 132 times per 1", then 132 aerial waves will fall 

 on the tympanic membrane in that time, and we will hear 

 the note c of the unaccented octave. If the larger con- 

 tinuous curve in Fig. 150 represent the aerial vibrations in 

 this case, the distance to 1 on the abscissa will represent 

 rj-J-g- of a second. Let, simultaneously, the air be set in 

 movement by a fork of the next higher octave, c', making 

 2 64 vibrations per 1"; under the influence of this second fork 

 alone, the aerial particles would move as represented by the 

 line 0, J 1 , b*, and so on, the waves being half as long 

 and cutting the abscissa twice as ofter. But when both 

 forks act together the aerial movement will be the algebraic 

 sum of the movements due to each fork; when both drive the 

 air one way they will reinforce one another, and vice versa; 

 the result will be the movement represented by the dotted 

 line, which is still periodic, repeating itself at equal intervals 

 of time, but no longer pendular, since it is not alike on the 

 ascending and descending limbs of the curves. We thus 

 get at the fact that non-pendular vibrations may be pro- 

 duced by the fusion of pendular, or, in technical phrase, by 

 their composition. 



Suppose several musical instruments, as those of an or- 

 chestra, to be sounded together. Each produces its own 

 effect on the air-particles, whose movements, being the 

 algebraical sum of those due to all, must at any given in- 

 stant be very complex; yet the ear can pick out at will and 

 follow the tones of any one instrument. From the com- 

 plex aerial movement it can select that fraction of it which 

 one vibrating body produces. The air in the external 

 auditory meatus at any given moment can only be in 

 one state of rarefaction or condensation and at one rate 



