204 SENSE OF HEARING. 



of apartments, and this they sometimes do with much accuracy. It is 

 well known, that if a sound be confined within a small space, it appears 

 louder than when the sonorous undulations can extend farther; hence 

 the greater noise caused directly by a pistol fired in a room than in the 

 open air. The sound indirectly produced will necessarily be modified 

 by the different reflections or echoes, that may be excited. By attend- 

 ing to these circumstances to the loudness of the voice and the inten- 

 sity of the reverberations occasioned by the walls, and calling into aid 

 experience under similar circumstances, in other words, by effecting 

 a strictly intellectual process, the blind attain the knowledge in 

 question. 



The velocity of a body is indicated by the rapid succession of the 

 vibrations that impress the ear, as well as by the change in their inten- 

 sity, if the body be moving along a surface or through the air. A car- 

 riage, approaching with great velocity, is detected by the ear, from the 

 rapidity with which the wheels strike against intervening obstacles; 

 and by the gradual augmentation in the intensity of the sound produced. 

 When opposite to us the intensity is greatest ; and a declension gradu- 

 ally takes place until the sound is ultimately lost in distance. 



Lastly ; by audition we form some judgment of the nature of bodies 

 by the difference in the sounds emitted. It has been already remarked, 

 that the timbre or quality of sound can be accurately appreciated. By 

 this quality we distinguish between the sound of wood or metal; of 

 hollow or solid bodies, &c. ; but in all these cases we are compelled to 

 call into aid our experience without which we should be completely at 

 a loss and to execute a rapid, but often very complicated, intellectual 

 operation. 



Audition may be exercised passively as well as actively ; hence the 

 difference between simply hearing, and listening. We cannot appre- 

 ciate, in man, the precise effects produced on the different portions of 

 the ear by volition ; whether, for example, the advantage be limited 

 to the better direction given to the ear, as regards the sonorous body, 

 and to avoiding all distraction by confining the attention to the im- 

 pressions made on the sense; or whether, by it, the pavilion may not 

 be made somewhat more tense by the contraction of its intrinsic and 

 extrinsic muscles ; whether the membrana tympani, and the membrane 

 of the foramen ovale may be modified by the contraction of the muscles 

 of the ossicles; or, in fine, the auditory nerve be rendered better adapt- 

 ed for the reception of the impression, and the brain for its apprecia- 

 tion. All these points are unsusceptible of direct observation and 

 experiment; and are, therefore, enveloped in uncertainty. In some 

 animals as the horse the outer ear becomes an acoustic instrument 

 under the guidance of volition; and is capable of being turned in every 

 direction in which a sonorous body may be placed. 



Like other senses, that of hearing is largely improved by education 

 or cultivation. The savage, accustomed, in the stillness of the forest, 

 to listen to the approach of enemies or his prey, has the sense so deli- 

 cate as to hear sounds, that are inaudible to one brought up in the din 

 of the busy world. The blind, for reasons more than once assigned, 



