565 



HEARING. 



in consequence of the duration of the impres- 

 sion upon the auditory nerve. The frequency 

 of repetition necessary for the production of a 

 continued sound from single impulses is, 

 according to Sir J. Ilerschel, probably not less 

 than sixteen times in a second, though the 

 Jimit would appear to differ in different ears. 



We distinguish in musical sounds, 1, the 

 pitch ; 2, the intensity or loudness ; 3, the 

 quality. The pitch of the sound depends on 

 the rapidity with which the vibrations succeed 

 each other, and any two sounds produced by 

 the same number of vibrations or impulses in 

 the same time are said to be in unison. The 

 loudness or intensity depends upon the violence 

 and extent of the primitive impulse. The quality 

 is supposed by Sir J. Herschel to depend on the 

 greater or less abruptness of the impulses, or 

 generally, on the law which regulates the excui- 

 sions of the molecules of air originally set in 

 motion. 



Sound may be communicated by air, aeri- 

 form fluids, liquids or solids, with variable 

 degrees of velocity. In air at the temperature 

 of 62 Q Fahr. sound travels at the rate of 1125 

 feet in a second, or 1090 feet in a second in 

 dry air at the freezing temperature. 



The velocity with which sound travels is, 

 however, quite independent of its intensity or 

 its tone; sounds of all pitches and of every 

 quality travel with equal speed, as is proved 

 by the fact that distance does not destroy the 

 harmony of a rapid piece of music played by 

 a band. If notes of a different pitch travelled 

 with different velocities, they would not reach 

 the ear in the order in which they were played. 

 Moreover, Biot put it to the test of direct ex- 

 periment ; he caused several tunes to be played 

 on a flute at the end of a pipe 3120 feet long, 

 and found that they could be distinctly heard 

 without the slightest derangement. 



Neither is the velocity of sound affected by 

 an increase of density in the air. It is, how- 

 ever, greater in warm than in cold air in conse- 

 quence of the greater elasticity of the former. 

 In the different gases much variety has been 

 observed in the velocity of sound ; through car- 

 bonic gas the rate of the velocity is said to be 

 one-third slower than ordinary, but through hy- 

 drogen gas, which is twelve times more elastic 

 than common air, the speed exceeds the usual 

 rate three and a half times. A more striking 

 difference is as regards the intensity of sound 

 or the impression it is capable of producing on 

 our organs of hearing. This varies conside- 

 rably with the increase or diminution in the 

 density of the transmitting gas. By means of 

 a piece of clock-work, which caused a ham- 

 mer to strike at regular intervals, the conduct- 

 ing power of the gas could be estimated, the 

 clock-work being placed in a glass receiver filled 

 with the gas. It was thus that Priestley ascer- 

 tained that in hydrogen the sound was scarcely 

 louder than in vacuo; in carbonic acid and in 

 oxygen it was somewhat louder than in air. 



Water can transmit sound, as the anatomist 

 would infer must be the case from the fact 

 that fishes are provided with distinct and highly 

 developed organs of hearing. Ilauksbee, An- 



deron, the Abb6 Nollet, and Franklin have 

 abundantly proved this by their experiments. 

 M. Colladon, by means of a tin cylinder three 

 yards long and eight inches in diameter, closed 

 at its lower end but open to the air above, 

 plunged vertically in the water, was enabled to 

 hear the sound of a bell at a distance of about 

 nine miles, and from numerous observations 

 he concluded that the velocity of sound in 

 water at about 46 Fahr. was equal to 4703 

 feet in the second. 



Solids convey sound as well as or even 

 better than air or liquids. Elasticity and ho- 

 mogeneousness are the qualities which best 

 adapt solids for the conveyance of sound: 

 hard substances, then, which are the most 

 elastic, conduct sound best. An interesting 

 experiment of Hassenfratz and Gay Lussac, in 

 the quarries of Paris, affords a striking contrast 

 of the relative conducting powers of air and 

 solids. A blow of a hammer against the rock 

 produced two sounds which separated in their 

 progress; that propagated through the stone 

 arrived almost instantly, while the sound con- 

 veyed by the air lagged behind. A more re- 

 markable experiment was that of Herhold and 

 Rahn, related by Chladni: a metallic wire 

 600 feet long was stretched horizontally, and 

 at one end a plate of sonorous metal was 

 attached ; when the plate was slightly struck, 

 a person at the opposite end, holding the wire 

 in his teeth, heard at every blow two distinct 

 sounds, the first transmitted almost simulta- 

 neously by the metal, the other arriving later 

 through the air. Biot, with the assistance of 

 Messrs. Boulard and Malus, concluded the 

 velocity of sound in cast iron at the tempera- 

 ture 51 Fah. to be 11,090 feet in a second. 



Reflexion of sound. Sonorous undulations 

 in passing from one medium to another always 

 experience a partial reflexion, and when they 

 encounter a fixed obstacle, they are wholly 

 reflected ; and in both cases the angle of in- 

 cidence is, as in the reflexion of light, equal to 

 the angle of reflexion. 



Echos are sounds reflected from some ob- 

 stacle which is placed in their way, as the wall 

 of a house, or those of an apartment, or the 

 surface of a rock, or the vaulted roof of a 

 church, &c.; and a sound thus reflected may, 

 by meeting another similar obstacle, be again 

 reflected, and thus the echo may be repeated 

 many times in succession, becoming, however, 

 fainter at each repetition till it dies away alto- 

 gether. The phenomena of echos illustrate 

 beautifully the analogy between sound and 

 light. Thus, the reflexion of sound from con- 

 cave and convex surfaces takes place exactly as 

 in the case of light : if a reflecting surface be 

 concave towards an auditor, the sounds re- 

 flected from its several points will converge 

 towards him, exactly as reflected rays of light 

 do; and he will receive a sound more intense 

 than if the surface were plane, and the more 

 so the nearer it approaches to a sphere con- 

 centric with himself; the contrary is the case 

 if the echoing surface be convex. If the 

 echo of a sound excited at one station be 

 required to be heard most intensely at another, 



