. XVI. EXTENT OP HEARING. 125 



vary from a few inches to several feet. The various 

 musical instruments, the human voice and that of ani- 

 mals, the singing of birds, the hum of insects, the roar 

 of the cataract, the whistling of the wind, and the other 

 nameless peculiarities of sound, show at once an infinite 

 variety in the modes of ae'rial vibration, and the aston- 

 ishing acuteness and delicacy of the ear, thus capable of 

 appreciating the minutest differences in- the laws of 

 molecular oscillation. 



All mere noises are occasioned by irregular impulses 

 communicated to the ear, and if they be short, sudden, 

 and repeated beyond a certain degree of quickness, the 

 ear loses the intervals of silence and the sound appears 

 continuous. Still such sounds will be mere noise: in 

 order to produce a musical sound, the impulses, and 

 consequently the undulations of the air must be all ex- 

 nctly similar in duration and intensity, and must recur 

 after exactly equal intervals of time. If a blow be given 

 to the nearest of a series of broad, flat, and equidistant 

 palisades set edgewise in a line direct from the ear, 

 each palisade will repeat or echo the sound ; and these 

 echoes returning to the ear at successive equal intervals 

 of time will produce a musical note. The quality of a 

 musical note depends upon the abruptness, and its in- 

 tensity upon the violence and extent of the original im- 

 pulse. In the theory of harmony the only property of 

 sound taken into consideration is the pitch, which varies 

 with the rapidity of the vibrations. The grave or low 

 tones are produced by very slow vibrations, which in- 

 crease in frequency as the note becomes more acute. 

 Very deep tones are not heard by all alike, and Dr. Wol- 

 laston, who made a variety of experiments on the sense 

 of hearing, found that many people though not at all 

 deaf are quite insensible to the cry of the bat or the 

 cricket, while to others it is painfully shrill. From his 

 experiments he concluded that human hearing is limited 

 to about nine octaves, extending from the lowest note of 

 the organ to the highest known cry of insects ; and he 

 observes with his usual originality that, " as there is 

 nothing in the nature of the atmosphere to prevent the 

 existence of vibrations incomparably more frequent than 

 any of which we are conscious, we may imagine that 

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