570 THE HUMAN BODY. 



given to a pendulum; if these be repeated at such intervals 

 of time as to always help the swing and never to retard it, 

 the pendulum will soon be set in powerful movement. If 

 the taps are irregular, or when regular come at such inter- 

 vals as sometimes to promote and sometimes retard the move- 

 ment, no great swing will be produced; but if they always 

 push the pendulum in the way it is going at that instant, 

 they need not come every swing in order to set up a powerful 

 vibration; once in two, three, or four swings will do. A 

 stretched string, such as that of a piano, is so far like a 

 pendulum that it tends to vibrate at one rate and no other; 

 if aerial waves hit it at exactly the right times they soon set 

 it in sufficiently powerful vibrations to cause it to emit an 

 audible note. By using such strings we might hope to de- 

 tect the separate pendular vibrations in any non-pendular 

 aerial periodic movement if such really existed; certain 

 strings would pick out the pendular component agreeing 

 in rate with their own vibrational period and be soon set 

 in powerful movement; while those not vibrating in the 

 same period as any of the pendular components, would 

 remain practically at rest, like the pendulum getting taps 

 which sometimes helped and sometimes impeded its swing. 

 If the dampers of a piano be raised and a note be sung 

 loudly to it, it will be found that several strings are set 

 in vibration, such vibrations being called sympathetic. The 

 human voice emits compound tones which can be mathe- 

 matically analyzed into simple vibrations, and if the piano 

 strings set in movement by it be examined, they will be 

 found to be exactly those which answer to these pendular 

 vibrations and to no others. We thus get experimental 

 grounds for believing that compound tones are really made 

 up of a number of simple vibrations, and get an additional 

 justification for the supposition that in the ear each note is 

 analyzed into its pendular components; and that the differ- 

 ence of sensation which we call timbre is due to the effect of 

 the secondary partial tones thus perceived. If so, the ear 

 must have in it an apparatus adapted for sympathetic reso-^ 

 nance. 



It may be asked why, if the ear analyzes vibrations in 

 this way, do we not commonly perceive it ? How is it that 

 what we ordinarily hear is the timbre of a given tone and not 



