Elements of natural philosophy (Volume 2-3) . A portion of the diverging rays of sounds proceeding from a mouthat the focus F, will be reflected by the trumpet indirections parallel to the axis A F; and the living forcesof the aerial molecules which, without the trumpet,would have been diffused over that portion of the spheri-cal surface on the outside of a cone of which FE andFR are the most diverging elements, become, by itsuse, concentrated within the limits of a circle whosediameter M N, is equal to that of the trumpets mouth,and superposed upon the living forces arising from theaction of
Elements of natural philosophy (Volume 2-3) . A portion of the diverging rays of sounds proceeding from a mouthat the focus F, will be reflected by the trumpet indirections parallel to the axis A F; and the living forcesof the aerial molecules which, without the trumpet,would have been diffused over that portion of the spheri-cal surface on the outside of a cone of which FE andFR are the most diverging elements, become, by itsuse, concentrated within the limits of a circle whosediameter M N, is equal to that of the trumpets mouth,and superposed upon the living forces arising from theaction of the direct sound. The axis of the trumpet be-ing directed upon a person at a distance, sounds of audi-ble intensity may thus be conveyed to him, which hecould not hear from the unassisted organs of speech. § 83. The Hearing Trumpet,which is intended to assist per-sons who are hard of hearing,is similar to the speaking trum- HearingtrumPet;Pet; but tlie operation is re-versed. The rays of sound en-ter this instrument at the larger Fig. ELEMENTS OF ACOUSTICS. 91 opening and are so reflected as to become united at the Constructionsmaller end, which is inserted into the ear. and use; § 8i. Whispering Galleries, so called from the fact whisperingthat the faintest whisper uttered at one point may be dis-gallcrios;tinctly heard at another and distant point, without its be-ing audible at intermediate positions, depend upon theoperation of the same principle, to wit, the convergenceof the rays of sound by reflexion. The best form forEest form-these galleries is that of the ellipsoid of revolution. Insuch a chamber two persons, one in either focus, couldkeep up a conversation with each other which wouldbe inaudible at other points. The ear of Dionysius in ancient history; it was a grotto cut outof the solid rock at Syracuse, in which a person placedat one point could hear every word, however faintlyuttered, in the grotto. It was doubtless of a parabolic
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