SENSE OF HEARING. 897 



of producing a definite tone at all. This is the case, for example, when a tun- 

 ing-fork in vibration is placed upon a sound-board ; for, even though the whole 

 board have no definite fundamental note, 1 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 pro- 

 duced. Thus, if a tuning-fork in vibration 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 reciprocate 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 distin- 

 guishable 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 aeriform to the liquid. 



900. 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 establish 

 the conditions under which those animals hear, that are constantly immersed in 

 water, he made a series of experiments, from which he draws the following con- 

 clusions : I. Sonorous vibrations, excited in water, are imparted with consider- 

 able intensity to solid bodies. n. Sonorous vibrations of solid bodies are com- 

 municated with greater intensity to other solid bodies brought in contact with 

 them, than to water ; but with much greater intensity to water than to atmo- 

 spheric air. in. Sonorous vibrations are communicated from air to water with 

 great difficulty, this difficulty very much exceeding that with which they are 

 propagated from one part of the air to another ; but their transition from air 

 to water is much facilitated by the intervention of a membrane extended be- 

 tween 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. 



1 The fundamental note of a body is the lowest tone which it will yield, when the whole 

 of it is in vibration together. By dividing the body into two or more distinct parts, it 

 may 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 octave 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 19th ; 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 harmonics 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. 



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