386 LECTURE XXXII. 



ting of the lips, in blowing the trumpet or French horn, can scarcely be 

 called a vibration, and the pitch of the sound is here determined 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 chord, capable of vibrating, either accurately, or very nearly, with 

 the same frequency, it causes a sympathetic tone, resembling that from 

 which it originated ; and the chord may produce such a sound either by 

 vibrating as a whole, or by dividing itself into any number of equal parts. 

 Thus, if the daniper 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 chords 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 chords 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 

 lliTOugh 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 

 motion?, 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 accu- 

 rately of the termination of a sound. Besides, a sympathetic vibration may 

 be excited not only by a sound producing vibrations of equal frequency, 

 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 listen- 

 ing 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 is, however, no doubt that the muscles^ 

 with which the different parts of the ear are furnished,are concerned in accom- 

 modating the tension of some of them to the better-transmission of sound; 



