There is probably no substance which is not in some measure a conductor of sound; but sound is much enfeebled by passing from one medium to another. If a man, stopping 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 th e opposite end df the stick, or of a piece of timber, be it ever so long, the beating of the watch will be distinctly heard ; whereas, in the usual way, it can scarcely be heard at the distance of 15 or 18 feet. The same effect will take place if he stops •both his ears with his hands..
and rests his teeth, his temple, or the care tilaginouspart of one of his ears, against the end_ofthe stick. Instead of a watch, a gentle scratch may.he made at one end of a pole or rod, and the person who keeps the ear in close contact with the other end of the pole will hear it very plainly. Thus, persons who are dull of •hearing may, by applying theirteeth to some part of an harpsichord, or other sounding body, hear the sound much better" other wise.
lf a person tie a poker oraily other piece of metal on to the middle of a strip of flan neFabout a yard long. then press with his thumbs or fingers the ends of the flannel. into his ears, while he swine the poker against any obstacle, as an iron or steel fender, he will hear a sound very like that of a large church bell.
Sound, like light, after it re flected from several places, may be col lected in one point, as, into a Thetis; and' it will be there more audible thanyin any other part, even that at the plat whence it proceeded. On this nciPle it is that a whispering gallery is ontnict cd. The form of a whispering gallery nnist be that of *concave hemisphere, as ABC, plate Acoustics, fig. 2. and if a low sound orwhisper be uttered at A, the vi brations expanding them sel v'esi every way Will impinge on the points D, D, D, &c. and from thence he reflected to E, E, F., and from thence to the pointsyand G, till at last they all meet in 0-, where the sound will be the most The augmentation of sound,by meMlefspeake in trumpets, is usually illustrated in the following manner : Let ABC, fig. 3. be the tithe, BD the axis, and B the mouth-piece for conveying the voice to the tube. Then itis evident, when a person speaksjat Bin the trumpet, the whole force of his voice is spent upon the air contained in the tube, which will be agitated through its whole length, and, by various re fleceons from the side of the tube to theaxis, the air along the middle part of the tube will be greatly condensed, and its momentum propor tionably increased, so that when it comes to agitate the air at the orifice of the tube AC, its force will he as much greaterthan what it would have been without the tube, as the surface of a sphere, whose radius is equal to the length of the tube,is great er than the surface of the segment of such sphere, whose base is the orifice of the tube. Fora person speaking at II, with out the tube, will 'nave the force . of his voice spent in exciting concentric inper fieiesefair all round the point Ili and when those superficies or pulses of air are dif fused as far as D every way, it is plain the force of the voice will there be diffused through the whole superficies of a sphere whose radius is BD; but in the trumpet • it will be so confined, that at its exit it will be diffitsed through so much of that spherical surface of air as corresponds to the orifice of the tube. But since the force is given, its intensity will be always inversely as the number of particles it has to move ; and therefore inthe tube it will he to that without, as the superficies of such a sphere to the area of the large end of the tube nearly. But it is obvious, Dr.
Ti. Young observes, that the confinement of the voice can have little effect in in creasing the strength of the sound, as this strength depends on the velocity with which the particles move. Were this rea soning conclusive, the voice should issue through the smallest possible orifice ; cy lindrical tubes would be preferable to any that increased in diameter; and the less the diameter, the greater would be the effect athe instrument ; because the plate or mass of air to be moved would, in that case, be less, and consequently the effect of the voice the greater ; all which is con tradicted by experience. The cause of the increase of sound in these tubes must therefore be derived from some other principles : and among these we shall pro bably find, that whatthe ingenious Kircher has suggested is the most deserving of our attention. He tells us, that "the augmen tation of the sountl depends on its reflec tion from the tremulous sides of the tithe ; which reflections, conspiring in propa gating the pulses in the same direction, must increase its intensity." Newton also seems to have considered this as the prin cipal cause, in the scholium of Prop. 50, P. II. Princip. when he says, "We hence see why sound% are so much increased in stentorop'honic tubes, for every reciprocal motion is, in each return, increased by the generating cause." Farther, when we speak in the open air, he effect on the tympanum of a distant auditor is produced merely by a single pulse. But when we use a tube, all the pulses propagated from the mouth, except those in the direction of the axis, strike against the sides of the tube, and every point of impulse becoming a new centre, from whence the pulses are propagated in all directions, a pulse will arrive at the ear from each of those points. Thus, by the use of a tube, a greater num ber of pulses are propagated to the ear, and consequently the sound increased. The confinement too of the voice may have a little effect, though not such as is ascrib ed to it by some; for the condensed pul ses produced by the naked voice freely expand every way; but in tubes, the late. ral expansion being diminished, the direct expansion will be increased, and conse. quently the velocity of the particles, and the intensity of the sound. The substance also of the tube has its effect; for it is found, by experiment, that the more elas tic the substance of the tube, and conse quently the more susceptible it is of these tremulous motions, the stronger is the sound. If the tube be laid on any non elastic substance, it deadens the sound, because it prevents the vibratory motion of the parts. The sound is increased in speaking-trumpets, ifthe tube be suspend ed in the air ; because the agitations are then carried on without interruption. These tubes should increase in diameter from the mouth-piece, because the parts vibrating in directions perpendicular to the surface will conspire in impelling for ward the particles of air, and consequent ly, by increasing their velocity, will in. crease the intensity of the sound: and the surface also increasing, the number of points of impulse and of new propaga tion will increase proportionably. The se veral causes, therefore, of the increase of sound in these tubes, Dr. Young concludes to be, 1. The diminution of the lateral, and consequently the increase of the di rect expansion and velocity ofthe included air. 2. The increase of the number of pulses by increasing the points of new propagation. 3. The reflections of the pulses from the tremulous sides of the tube, which impel the particles of air for ward, and thus increase their velocity.