ON TH SOURCES AND EFFECTS OF SOUND. 301 



ficiently in\ssatigated. A glass, or a bell, divides in general into four 

 porlioTS? vibrating separately, and sometimes into six or eight ; they may 

 readily be distinguished by means of the agitations excited by them in a 

 fluid contained in the glass. It is almost unnecessary to observe, that the 

 fluid thus applied, by adding to the mass of matter to be moved, makes the 

 vibration slower, and the sound more grave. 



In some cases the vibrations of fluids and solids are jointly concerned in 

 the production of sound : thus, in most of the pipes of an organ denomi- 

 nated reed pipes, the length of a tongue of metal is so adjusted as to be 

 capable of vibrating in the same time with the air contained in the pipe. 

 Sometimes, however, the air only serves to excite the motion of the solid, 

 as in some other organ pipes, which are usually much shorter than would 

 be required for producing the proper note alone, and probably in the 

 glottis, or organ of the voice of animals. On the other hand, the alternate 

 opening and shutting of the lips, in blowing the trumpet or French horn, 

 can scarcely be called a vibration, and the pitch of the sound is here de- 

 termined by the properties of the air in the pipe only. The vibrations of a 

 solid may be excited by an undulation propagated through a fluid ; thus, 

 when a loud sound strikes against a cord, capable of vibrating, either ac- 

 curately, or very nearly, with the same frequency, it causes a sympathetic 

 tone, resembling that from which it originated ; and the cord may pro- 

 duce such a sound either by vibrating as a whole, or by dividing itself into 

 any number of equal parts. Thus, if the damper be raised from any of the 

 strings of a harpsichord, it may be made to vibrate, by striking or singing 

 any note, of which the sound corresponds either to that of the whole string, 

 or to that of any of its aliquot- parts. Sometimes also two cords that are 

 very nearly alike, appear, when sounding together, to produce precisely 

 the same note, which differs a little from each of those which the cords 

 would produce separately ; and a similar circumstance has been observed 

 with respect to two organ pipes placed near each other. In these cases the 

 vibrating substances must affect each other through the medium of the air ; 

 nearly in the same manner as two clocks, which rest on the same support, 

 have been found to modify each other's motions, so as to exhibit a perfect 

 coincidence in all of them. 



It is uncertain whether any fibres in the ear are thus sympathetically agi- 

 tated in the process of hearing, but if there are any such vibrating fibres, 

 their motions must necessarily be of short duration, otherwise there would 

 be a perpetual ringing in our ears, and we should never be able to judge 

 accurately of the termination of a sound. Besides, a sympathetic vibration 

 may be excited not only by a sound producing vibrations of equal fre- 

 quency, but also by a sound, of which every alternate, or every third or 

 fourth vibration, coincides with its motions : it would, therefore, be 

 difficult to distinguish such sounds from each other, if hearing depended 

 simply on the excitation of sympathetic vibrations. It is true that we 

 generally distinguish, in listening to a loud and deep sound, precisely such 

 notes as would be thus produced ; but it is only when the sounding body 

 is capable of affording them from the nature of its vibrations ; for we may 

 listen for them in vain in the sound of a bell or of a humming top. There 



