ACOUSTICS. 



another, the sensation of the motion of sound dors 

 not assist us to understand iis precise nature, as 

 compared with visible motion*. Tims the ear at 

 oner petccivcs the dill'ereiice between a grave Olid nil 

 acute sound ; but it is only from attentive obscr\ation 

 by the eye, that we din-over the different rapidity 

 of succession in the vibrations which produce them. 

 The vibrations of a great many bodies, :is strings, 

 IK-US, and ineml>ranes when emitting sounds, nmy, 

 however, be distinctly seen, and even Ml; but 

 they may often be rendered more sensible to the eye 

 by a little artifice, such as sprinkling the vibrating 

 body witli suul, or some light granular substance. 

 Sound may be produced without \ ibrations or alterna- 

 tions ; thus, if we pass the nail quickly over the teeth 

 of a comix the rapia succession of singleshocks or noises 

 produces all the effect of vibrations. It must be evident 

 that the rapid motions here described, whether ori- 

 ginating in vibrations, or a succession of concussions, 

 must l)e communicated from the body in which they 

 are excited, to the sheet of air, or wliatever else be 

 in contact with it, and from this again to another 

 sheet beyond the first : thus diffusing the motion in 

 every direction. The agitation of the sounding body 

 must thus be communicated to the surrounding me- 

 dium to a great distance, and impressed upon any 

 body situated within this distance ; if this body be 

 the ear, the tremor excited in it by these agitations 

 will be perceived by the mind. The necessity of 

 some medium for the transmission of sound is proved 

 by ^experiment. If a bell be rung in an exhausted 

 receiver, the sound will be hardly perceptible, while 

 the tones will lx>come clear and distinct, on re-admit- 

 ting the air. Having thus given a general outline 

 of uie source and propagation of sound, we sliall pro- 

 c< nl to consider, with as much minuteness as the 

 limits of this work will permit, some of the more 

 important facts connected with them. The most ob- 

 vious characteristics, by which we distinguish differ- 

 ent sounds, consist of differences in their degrees of 

 what we call loudness, and acuteness, or pitch. We 

 can produce, at pleasure, sounds having different de- 

 grees of loudness, from the same sonorous body, by 

 making the concussions upon it more or less violent ; 

 disturbing in a greater or less degree the arrange- 

 ment of its parts. So two bodies of like substance 

 and figure, but unlike mass, when subjected to the 

 same shock, emit sounds unlike in loudness ; and, 

 again, bodies, of like mass and figure, but unlike sub- 

 stance, form sounds more or less loud, when subjected 

 to the same shock. In this latter case, the loudness 

 1ms a relation to the quantity of elasticity possessec 

 by the bodies ; and in all cases, when the disturbance 

 of the parts is carried beyond the elastic power o; 

 the body, so as to produce a permanent change o; 

 figure, no increase of loudness is induced. From a 

 consideration of the preceding facts, we may con- 

 clude, that loudness depends upon the quantity of 

 motion, or sonorous vibration, in which it originates 

 The other principal characteristic of sound, its acute- 

 ness or pitch, depends upon the frequency with which 

 the concussions or vibrations of the sonorous body 

 succeed each other. That sounds may be audible 

 to a common ear, it is necessary that the concussions 

 upon the medium, which communicates them, shouk 

 follow each other hi such succession, that not more 

 than 8192, nor less than 32, distinct concussions shal 

 be made upon the medium during the lapse of one 

 second. Some ears, however, can perceive sounds 

 emanating from vibrations a little beyond the ex 

 tremes to which the perception of other ears are 

 confined We should be careful not to confound the 

 frequency of vibrations with the velocity of vibratory 

 motion. A string may vibrate with a greater or less 

 velocity, as it passes its axis to a greater or less dis- 



ance ; yet the times of its vibrations may be nil 

 qual. The difference of velocity, M eciing the 

 nantity of motion only, would produce no change, 

 xcept in the londne- - iif the sound. To those sounds 

 vhicli proceed from infrequent vibrations, we give 

 lie name of Mat or grave , those from frequent vibra- 

 ions we call sharp or ncitir. When vibrations suc- 

 eed each other in equal times, their sound t xci 

 ileasant sensation, and they are called musical. When 

 wo bodies arc made to MUind together, if their vibra- 

 ions are performed in equal times, the sounds are 

 said to be hi unison. When the vibrations are per- 

 brmed in unequal times, so that some of those ot the 

 >ne are not accompanied by those of the other, the 

 ar perceives a degree of dis-onanre in the sounds. 

 If, however, the vibrations meet after short and re- 

 gular intervals, the dissonance is not easily detected, 

 md the sounds are said to accord. During the con- 

 tinuance of most primary sounds, however excited, 

 we perceive other and more acute sounds co-existing 

 with them. These are called their linmnntics. They 

 are supposed to originate in a scries of secondary vi- 

 brations, more short and frequent than the principal 

 vibration. Thus a sounding string, for e~xample, may 

 36 supposed not to pass its axis in a simple curve, 

 but to resolve itself into a tortuous line, formed by a 

 number of smaller curves, each of which vibrates 

 across its own axis, thus producing its harmonics. It 

 is perhaps some combination of the harmonics with 

 the primary sound, that characterizes the sound of 

 different instruments, though of the same loudness and 

 pitch, so that we can distinguish one from another. 

 The air, being the common medium which surrounds 

 the ear, is tliat by which sounds are usually trans- 

 mitted. This transmission is performed with a velo- 

 city of about 1130 feet in a second. All other bodli -. 

 however, are capable of transmitting sound. It may 

 be done perfectly, even by the solid parts of the head. 

 If, for example, we hold the stem of a watch between 

 the teeth, and cover the ears with the hands, tlu: 

 beats are heard more distinctly than when the instru- 

 ment is held at an equal distance in the air. The 

 rubbing together of two stones under water may be 

 heard, oy an ear in the same medium, at the distance 

 of half a mile. When the air, or any other body of 

 indefinite extent, is disturbed, in a point situated 

 within it, by a sonorous vibration, it forms a wave, 

 which passes from the disturbed point, as a centre, in 

 every direction. It follows that as the wave extends 

 itself, the mass to be put in motion increases until 

 the original motion is rendered insensible from the 

 magnitude of the mass to which it has communicated 

 itself. The velocity with which waves, thus formed, 

 move through any homogeneous eliistic medium, is 

 always equal to tliat which a heavy body would ac- 

 quire by falling through half the height of the mo- 

 dulus of elasticity.* In applying this law to the 

 transmission of sound by the air, it was for a long 

 time found not to give the same results as were o)>- 

 tained by experiment. This discrepancy, ho\\ 

 has been most ingeniously reconciled by a small 

 correction for the latent heat made sensible by the 

 compression; the effect of this being to inc. 

 the height of the modulus of elasticity. We ought 

 therefore, to find that liquids, and more especially 

 some of the solids, should transmit sound much 

 more rapidly than air ; and this agrees most per- 

 fectly with various experiments. Cast-iron, for ex- 

 ample, has been found to transmit sound with a 

 velocity 10 times greater than air. Sound does not 

 readily pass from one medium to another ; a sound 

 made in the air is not easily distinguished under wa- 

 ter, although the distance be very small. It is from 



The- height of the modulus of elasticity of air is 27,800 feet 



