SENSE OF HEARING. 429 



it will divide itself into a number of parts, which will reciprocate the original 

 sound, so as greatly to increase its intensity; and the same sound-board will 

 act equally well for tuning-forks of several different degrees of pitch. When 

 a smaller body is used for resonance, however, it is essential that there should 

 be a relation between its fundamental note and that of the sonorous body; 

 otherwise no distinct resonance is produced. Thus, if a tuning-fork in vibra- 

 tion be held over a column of air in a tube, of such a length that the same note 

 would be given by its vibration, its sound will be reciprocated. And if it be 

 held over a pipe, the column of air in which is a multiple of this, the column 

 will divide itself into that number of shorter parts, each of which will recipro- 

 cate the original sound, and the total action will be one of resonance. But if 

 the length of the pipe bear no such correspondence with the note sounded by 

 the tuning-fork, no resonance is given by the column of air it contains. 3. 

 Vibrations of conduction are the only ones by which sounds can strictly be 

 said to be propagated. These are distinguishable into various kinds, into 

 which it is not requisite here to inquire. It should be remarked, however, 

 that all media, fluid, liquid, or solid, are capable of transmitting sound in this 

 manner, a vacuum being the only space through which it cannot pass. 

 The transmission is usually much more rapid through solid bodies, than 

 through liquid ; and through liquid, than through gaseous. The greatest 

 diminution in the intensity of sound is usually perceived, when a change takes 

 place in the medium through which it is propagated, especially from the aeri- 

 form to the liquid. 



561. The detailed application of these principles has been most elaborately 

 worked out by Miiller ; and the following statement of what may be regarded 

 as the present condition of our knowledge of the subject, is little more than 

 an abstract of his results. Considering it desirable, in the first place, to estab- 

 lish the conditions under which those animals hear that are constantly im- 

 mersed in water, he made a series of experiments, from which he draws the 

 following conclusions : i. Sonorous vibrations, excited in water, are imparted 

 with considerable intensity to solid bodies. n. Sonorous vibrations of solid 

 bodies are communicated with greater intensity to other solid bodies brought 

 in contact with them, than to water ; but with much greater intensity to 

 water, than to atmospheric air. in. Sonorous vibrations are communicated 

 from air to water with great difficulty, with very much greater difficulty 

 than they are propagated from one part of the air to another ; but their transi- 

 tion from air to water is much facilitated, by the intervention of a membrane 

 extended between them. iv. Sonorous vibrations are not only imparted from 

 water to solid bodies with definite surfaces, which are in contact with the 

 water, but are also returned with increased intensity by these bodies to the 

 water ; so that the sound is heard loudly in the vicinity of those bodies, in 

 situations where, if it had its origin in the conducting power of the water 

 alone, it would be faint. v. Sonorous undulations, propagated through water, 

 are partially reflected by the surfaces of solid bodies. vi. Thin membranes 

 conduct sound in water without any loss of its intensity, whether they be 

 tense or lax. From HI., iv., and vi., we learn the mode in which the sound 



be made to give a great variety of sounds. Thus, if a stretched string be divided by a bridge 

 into two equal parts, each will sound the octavo of the fundamental note, or the 8th note 

 above it. If it be divided into three parts, each will give the 12th above the fundamental 

 note; if into four, the 15th or double, octave will be heard; if into five, the 17th; if into six, 

 the 1 9th ; if into seven, the 20th (flat seventh above the second octave); if into eight, the 

 22d or triple octave. A string forcibly set in vibration has a tendency to sound these har- 

 monics with the fundamental note, by spontaneous division into several distinct segments of 

 vibration ; as may be easily made evident by striking one of the lower keys of the piano, 

 and listening to the sounds heard whilst the fundamental note is dying away. 



