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Show 1 10 Additwnal ]\Totes. The hissing sounds are produced by air forcibly pushed through certain passages of the mouth wit.hout being previously _rendered s?norous by the larynx; and obtain their sibilancy from their slower vibrations, occasioned by the nmcous membrane, which lines those apertures or passages, being less tense than that of the larynx. I suppose the stream of air is in both cases frequently interrupted by the closing of the sides or mouth of the passages or aperture; but that this is performed much slower in the production of sibilant sounds, than in the production of clear ones. . The semi vocal sounds are proJ~cecl by the stream of air having received quick vibrations, or clear sound, in passing through the larynx, or in the cavity of the mouth; but a part of it, as the outsides of this sonorous current of air, afterwards receives slower vibrations, or hissing sound, from son;e other passages of the lips or mouth, through which it then flows. Lastly the stops, or consonants, impede the current of air, whether sonorous or sibilant, for a perceptible time; and probably produce some change of tone in the act of opening and closing their apertures. There are other clear sounds besides those formed by the larynx; some of them· are formed in the mouth, as may be heard previous to the enunciation of the letters b, and d, and ga; or during the pronunciation of the semivocal letters, v. z. j. and others in sounding the liquid letters rand 1; these sounds· we shall term urisonance. The other clear sounds are formed in the nostrils, as in pronouncing the liquid letters m. n. and ng. these we shall term narisonance. Thus the clear sounds, except those above mentioned, are formed in the larynx along with the musical height or lowness of note; but receive afterward a variation of tone from the various passages of the mouth: add to these that as the sibilant sounds consist of vibrations slower than those formed by the larynx, so a whistling through the lips consists of vibrations quicker than those formed by the larynx. As all sound consists in the vibrations of the air, it may not be disagreeable to the reader to attend to the immediate causes of those :vibrati9ns. When any sudden impulse is given to an elastic fluid like the air, it acquires a progressive motion of the whole, and a condensation of the constituent particles, which first receive the impulse; on Analysis qf Articulate Sounds. 111 this account the currents of the atmosphere in stormy seasons are never regular, but blow and cease to blow by intervals; as a part of the moving stream is condensed by the projectile force; and the succeeding part, being consequently rarefied, requires some time to recover its density, and to follow the former part: this elasticity of the air is like,vise the cause of innumerable eddies in it; which are mu~h more frequent than in streams of water; as when it is impelled agamst any oblique plane, it results with its elastic force added to its progressive one. l-Ienee when a vacuum is formed in the atmosphere, the sides of the cavity forcibly rush together both by the general pressure of the superincumbent air, and by the expansion of the elastic particles of it; and thus produce a vibration of the atmosphere to a considerable distance: this occurs, whether this vacuity of air be occasioned by the discharge of cannon, in which the air is displaced by the sudden evolution of heat, which as suddenly vanishes; or whether the vacuity be left by a vibrating string, as it returns from each side of the arc, in which it vibrates; or whether it be left under the lid of the valve in the trumpet stop of an organ, or of a child's play trumpet, which continues perpetually to open and close, when air is blown through it; which is caused hy the elasticity of the currents, as it occasions the pausing gusts of wind mentioned above. Hence when a quick current of air is suddenly broken by any intervening body, a vacuum is produced by the momentum of the proceeding current, between it and the intervening body; as beneath the valve of the trumpet-stop above mentioned; and a vibration is in consequence produced; which with the great facility, which elastic fluids possess of forming eddies, may explain the production of sounds by blowing through a fissure upon a sharp edge in a common organpipe or child's whistle; which has always appeared difficult to resolve; for the less vibration an organ-pipe itself possesses, the more agreeable, I am informed, is the tone; .as the tone is produced by the vibration of the air in the organ pipe, and not by that of the sides of it; though the latter, when it exists, may alter the tone though not the note, like the belly of a harpsichord, or violin. When a stream of air is blown on the edge of the aperture of an |
