22
ACOUSTICS.
ether countries of Europe . The shops have been usually suppliedfrom the Levant with dried roots which do not appear to be su-perior to those of our own growth. The root of this plant has avery agreeable flavour, which is greatly improved by drying. It :is reckoned carminative and stomachic, having a warm, pungent, !bitterish taste, so is frequently used as an ingredient in bitters. |The A. gramineus, a native of China , has been cultivated at the jroval gardens at Kew. !
Acorus adulterinus, or vulgaris, in the Materia Medica, jthe root of the iris lutea palustris, or common yellow water flag ;tlower. r
ACOSTAN, a mountainous island in the north seas between jjAsia and America, observed by Captain Cook . J
ACOUSMATICI, [from nxmc, to hear,] an appellation given!to such among the disciples of Pythagoras , as had not completed?
their 5 years probation. They stood opposed to the mathenialici,who were tliow initiated into the secrets of science, and the acous-malic philosophy, to the mathematic.
ACOUSTIC Duct, in anatomv, the meatus auditorium, orexternal passage of the ear. See Anatomy.
Acoustic Instrument, or Auricular tube. Sec Acoustics.
Acoustic Medicines, remedies against imperfections anddisorders of the ear.
Acoustic Nerve, the auditory Nerve. See Anatomy.v Acoustic Vessels, in the ancient theatres, were a kind ofVessels made of brass, shaped in the bell fashion, which being offall tones within the pitch of the voice, or even of instruments, ren-' dered the sounds more audible, so that the actors could be heardthrough all parts of the theatres, which were even 400 feet iudiameter.
THE Science of ACOUSTICS, [from rexaw, to hear,] calledalso PHONICS, [from "kiwi, vox, rcl souus, a voice or sound,]comprehends every thing relative to the nature, operation, andetlects of Sound. Some authors divide it into Diacoustics, whichtreats of the properties of sounds that come directly to the ear,and Catacouslics which illustrates the nature of rejtccted sounds.That Air is the principal vehicle of sound is universally admitted,but some philosophers assert that water is also a medium of sound,since a bell sounds distinctly in water, and fishes are sensible ofsound even at the bottom of deep rivers. Perhaps every substanceis in some measure a conductor of sound; but sound is much en-feebled by passing from one medium to another. If a man stop-ping one of his ears with his finger, stops the other also by pressingit against the end of a long stick, and a watch be applied to the op-posite end of the stick, or of a piece of timber, be it ever so long,the beating of the watch will be distinctly heard; whereas, in theusual way, it can scarcely he heard at the distance of 15 or 18 feet.The same effect will take place if he stops both his ears witluhishands, and rests his teeth, his temple, or the cartilagenous part ofone of his ears against the end of the stick. Instead of a watch,a gentle scratch may be made at one end of a pole or rod, and theperson who keeps the ear in close contact with the other end of thepole will hear it very plainly. Thus persons who are dull of hear-ing, may, by applying their teeth to some part of an harpsichord,or other sounding body, hear the sound much better than other-wise. If a person tie a poker or any other piece of metal on tothe middle of a strip of flannel about a yard long, then press withhis thumbs or fingers the ends of the flannel into his ears, while heswings the poker against any obstacle, as an iron or steel fender,-hewill hear a sound very like that of a large church bell. Sound isproduced by the vibrations of elastic bodies, as appears by the tre-mors communicated to the surrounding bodies, and the sensation iscaused by' certain particles or pulses of the air striking on the tym-panum of the ear. Several theories have been framed to accountfor the manner and cause of the propagation of sound, though thesubject bas not yet been satisfactorily cleared, yet it seems mostprobable that sound is conveyed by means of an elastic fluid whichw either the air itself, or is contained medium of sound, whetherthat medium be air, water, or any other substance; and perhapsthis elastic fluid is no other than the electric fluid. Philosophers,however, are still much divided in their opinions respecting themanner in which sound is conveyed to the ear; i. e. Whether itis diffused in the air, or in a circular or undulatorv manner, likethe tvavfcs occasioned in a smooth water, by throwing in a stone; orwhether it runs in straight lines, like the rays of light diffused froma centre, in the manner that live electrical fluid runs along a rod ofiron. Sir Isaac Newton adopted the former theory, and illustratedthe propagation of sound by an undulatory or rather vermicularmotion m the particles of the air. Every sound may be consideredas driven off from the sounding body in straight lines, and im- jpre-sed upon the air in one direction only: but whatever ini pres- !sion is made upon a fluid in one direction’, is diffused upon itssur- jfice into all directions; so that the sound first driven directly for- !ward soon fills up a wide sphere, and is heard on every side. Thus, >as it is impressed, it instantaneously travels forward with a verysat*, motion, resembling the velocity with which we know elec-I
tricity flies from one end of a line to another. Now, as to tirepulses, or close shakes, as the musicians express it, which a sound-ing body is known to make, each pulse is itself a distinct andperfect sound, and the interval between every two pulses is pro-foundly silent. Continuity of sound from the same body is only adeception of the hearing; for as each distinct sound succeeds atvery small intervals, the auditory nerves have no time to transmitits images with equal swiftness to the mind, and the interval is thuslost to sense. Sir I. Newton’s theory appears to be the least ex-ceptionable ; the principal'objection to it, is, that it requires anela-lic medium, whereas sound moves through water which is notelastic, but no one will doubt that water contains an elastic fluid,though it is not elastic itself, for the electric fluid is known to existin water. Ilence we consider the theory of the undulatory motionas the best, and shall endeavour to illustrate it. The parts of a so-norous body being put into motion by percussion, excite con-centric vibrations in the air all around the said body; so that let aperson be any how, or any where, situated witlun the verge ofthose motions, and he will equally hear the sound, at equal dis-tances from the body whence it comes. See Plate L. fig. 1. whereI) D represents a drum, and D 1,2, 3, 4, 5, Sec. the circularpulses of the air, made bv, and conveying the sound of, the beatsto our ears. For the pai .icles of air contiguous to the sonorousbody, being compelled by. the first impulse to move forwards,propel those next to them, and these, others again, and so on to.aconsiderable distance, according to the intensity of the percussiveforce. But when the particles of the sononorous body make thesecond part of the vibration, by returning back again, the par-ticles of air also, by their repulsive power, repel each other to-wards their proper places, and thus again expand themselves. Nowsince motion, once generated in elastic, bodies, continues sometime before it can be destroyed by the resistance and counter-ac-tion of contiguous bodies, it follows, that the particles of the so-norous body, and consequently those of the adjacent air, have torsome time a reciprocal vibratory motion, by going forwards andbackwards through very small spaces in indefinitely small por-tions of time; which motion gradually decreases till it be totallydestroyed. To illustrate this, let A C (Fig. 2.) be an elastic stringor chord, fixed at the points A and C; and let it be drawn out otits natural position A C into another, A B C, upon which, beinglet go, it will, by its elasticity, not only fly back to its first posi-tion A C', but into another A EC, near as far on the other sideA C , as A B C was on the first; after this it will return again al-most to B, and then return almost to E; and these courses and re-courses of the string growing still less and less, it will at last settlein its first and natural position A D C. When the chord begins itsmotion at first from B, it strikes the particle of air contiguous toit; and that will, by its approach towards the next particle, affectit, bv means of its repulsive power, and so on, through such a_number of particles as can receive the motion while the stringmoves from B to D. Let, therefore, A, B, C, D, li, F, G, &c.(Fig. 4.) represent such a series of parades of air, at an equaldistance, and the first particle A contiguous to the middle point Itof such a string be agitated by it in its motion. The string begin-ning to move, all the particles A, B, C, will begin to move lor-
, let E be,the
wards also; and, since this motion is propagated in tune