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the increase of the direct expansion and velocity of the 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 forward, and thus increase their velocity. An umbrella, held in a proper position over the head, may serve to collect the force of a distant sound by reflection, in the manner of a hearing-trumpet; but its substance is too slight to reflect any sound perfectly, unless the sound fall on it in a very oblique direction. The exhibition of the Invisible Girl is said to depend on the reflection of sound; but the deception is really performed by conveying the sound through pipes artfully concealed, and opening opposite to the mouth of the trumpet, from which it seems to proceed. When a portion of a pulse of a sound is separated by any means from the rest of the spherical or hemispherical surface to which it belongs, and proceeds through a wide space, without being supported on either side, there is a certain degree of divergence, by means of which it sometimes becomes audible in every part of the medium transmitting it: but the sound thus diverging is comparatively very faint. Hence, in order that a speaking-trumpet may produce its full effect, it must be directed in a right line towards the hearer; and the sound collected into the focus of a concave mirror is far more powerful than at a little distance from it, which could not happen, if sound, in all cases, tended to spread equally in all directions. It is said that the report of a cannon appears many times louder to a person towards whom it is fired, than to one placed in a contrary direction. It must, says Dr. Young, have occurred to every one's observation, that a sound, such as that of a mill, or a fall of water, has appeared much louder after turning a corner, when the house or other obstacle no longer intervened. Indeed the whole theory of the speaking-trumpet would fall to the ground, if it were demonstrable that sound spreads equally in all directions. In windy weather it may be often observed, that the sound of a distant bell varies almost instantaneously in its strength, so as to appear twice as remote at one time as another. Now if sound diverged equally in all directions, the variation produced by the wind would not exceed one tenth of the apparent distance; but on the supposition WOL. I.
of a motion nearly rectilinear, it may easily happen that a slight change in the direc. tion of the wind shall convey a sound, either directly or after reflection, in very different degrees to the same spot. The decay of sound is the natural conse
quence of its distribution throughout a .
larger and larger quantity of matter, as it proceeds to diverge every way from its centre. The actual velocity of the particles of the medium transmitting it appears to diminish simply in the same proportion as the distance from the centre increases: consequently their energy, which is to be considered as the measure of the strength of sound, must vary as the square of the distance; so that at the distance of ten feet from the sounding body the velocity of the particles of the medium becomes one-tenth as great as at the distance of one foot, and their energy, or the strength of the sound, only one-hundredth as great. An echo is a reflection of sound striking against some object, as an image is reflected in a glass: but it has been disputed what are the proper qualities in a body for thus reflecting sounds. It is in general known, that caverns, grottoes, mountains, and ruined buildings, return this reflection of sound. We have heard of a very extraordinary echo, at a ruined fortress near Louvain, in Flanders. If a person sung, he only heard his own voice, without any repetition; on the contrary, those who stood at some distance, heard the echo, but not the voice; but then they heard it with sur. prising variations, sometimes louder, sometimes softer, now more near, then more distant. There is an account in the Memons of the French Academy, of a similar echo near Rouen. It has been already observed, that every point against which the pulses of sound strike, becomes the centre of a new series of pulses, and sound describes equal distances in equal times; therefore, when any sound is propagated from a centre, and its pulses strike against a variety of obstacles, if the sum of the right lines drawn from that point to each of the obstacles, and from each obstacle to a second point, be equal, then will the latter be a point in which an echo will be heard. Thus let A, fig. 4, be the point from which the sound is propagated in all directions, and let the pulses strike against the obstacles C, D, E, F, G, H, I, &c. each of these points becomes a new centre of pulses by the first principles, and therefore from each of them one series of pulses will pass
through the point B. Now if the several sums of the right lines A CTC B, AD + DB, A E + EB, A G + GB, A H + HB, AI + I B, &c., be all equal to each other, it is obvious that the pulses propagated from A to these points, and again, from these points to B, will all arrive at B at the same instant, according to the second principle; and, therefore, if the hearer be in that point, his ear will at the same instant be struck by all these pulses. Now it appears from experiment, that the ear of an exercised musician can alone distinguish such sounds as follow one another at the rate of 9 or 10 in a second, or any slower rate: and therefore, for a distinct perception of the direct and reflected sound, there should intervene the interval of oth of a second; but in this time sound deo, or 127 feet nearly. And therefore, unless the sum of the lines drawn from each of the obstacles to the points A and B exceeds the interval AB by 127 feet, no echo will be heard at B. Since the several sums of the lines drawn from the obstacles to the points A and B are of the same magnitude, it appears that the curve passing through all the points, C, D, E, F, G, H, I, &c. will be an ellipse. Hence all the points of the obstacles which produce an echo, must lie in the surface of the oblong spheroid, generated by the revolution of this ellipse round its major axis. See CoN1c Sections. As there may be several spheroids of different magnitudes, so there may be several different echoes of the same original sound. And as there may happen to be a greater number of reflecting points in the surface of an exterior spheroid than in that of an interior, a second or a third echo may be much more powerful than the first, provided that the superior number of reflecting points, that is, the superior number of reflecting pulses propagated to the ear, be more than sufficient to compensate for the decay of sound which arises from its being propagated through a greater space. This is finely illustrated in the celebrated echoes at the lake of Killarny, in Kerry, where the first return of the sound is much inferior in strength to those which immediately succeed it. From what has been laid down it appears, that, for the most powerful echo, the sounding body should be in one focus of the ellipse, which is the section of the echoing spheroid, and the hearer in the other. However, an echo may be heard in other situations, though not so favourably:
as such a number of reflected pulses may arrive at the same time at the ear as may be sufficient to excite a distinct perception. Thus a person often hears the echo of his own voice; but for this purpose he should stand at least 63 or 64 feet from the reflecting obstacle, according to what has been said before. If a bell, a, fig. 5, be struck, and the undulations of the air strike the wall ca in a perpendicular direction, they will be reflected back in the same line; and if a person be situated between a and c, as at ar, he would hear the sound of the bell by means of the undulations as they went to the wall, and he would hear it again as they came back, after the reflection, which would be the echo of the sound. So a person standing at a might, in speaking in the direction of the wall ca, hear the echo of his own voice. But in both cases the distance ca: must be 63 or 64 feet. If the undulations strike against the wall obliquely, they will be reflected off obliquely on the other side; if, for instance, a person stand at m, and there be any obstacle between that place and the bell, so as to prevent him hearing the direct sound, he may nevertheless hear the echo from the wall cil, provided the direct sound fall in that sort of oblique direction so as to force the reflected undulations along the line cm. At the common rate of speaking, we do not pronounce above three syllables and a half, that is, seven half syllables in a second; therefore, that the echo may return just as soon as three syllables are expressed, twice the distance of the speaker from the reflecting object must be equal to 1000 feet; for as sound describes 1142 feet in a second, $ths of that space, that is 1000 feet nearly, will be described while six half, or three whole, syllables are pronounced; that is, the speaker must stand near 500 feet from the obstacle. And, in general, the distance of the speaker from the echoing surface, for any number of syllables, must be equal to the seventh part of the product of 1142 feet multiplied by that number. In churches we never hear a distinct echo of the voice, but a confused sound, when the speaker utters his words too rapidly; because the greatest difference of distance between the direct and reflected courses of such a number of pulses as would produce a distinct sound is never in any church equal to 127 feet, the limit of echoes. But though the first reflected pulses may produce no echo, both on account of their being too few in number, and too rapid in their return to the ear; yet it is evident, that the reflecting surface may be so formed, as that the pulses which come to the ear after two reflections or more, may, after having described 127 feet or more, arrive at the ear in sufficient numbers, and also so nearly at the same instant, as to produce an echo, though the distance of the reflecting surface from the ear be less than the limit of echoes. This is confirmed by a singular echo in a grotto on the banks of the little brook called the Dinan, about two miles from Castlecomber, in the county of Kilkenny. As you enter the cave, and continue speaking loud, no return of the voice is perceived; but on your arriving at a certain point, which is not above 14 or 15 feet from the reflecting surface, a very distinct echo is heard. Now this echo cannot arise from the first course of pulses that are reflected to the ear, because the breadth of the cave is so small, that they would return too quickly to produce a distinct sensation from that of the original sound: it therefore is produced by those pulses, which, after having been reflected several times from one side of the grotto to the other, and having run over a greater space than 127 feet, arrived at the ear in considerable numbers, and not more distant from each other in point of time than the ninth part of a second. M. De la Grange demonstrated that all impressions are reflected by an obstacle terminating an elastic fluid with the same velocity with which they arrived at that obstacle. When the walls of a passage, or of an unfurnished room, are smooth and perfectly parallel, any explosion, or a stamping with the foot, communicates an impression to the air, which is reflected from one wall to the other, and from the second again towards the ear, nearly in the same direction, with the primitive impulse: this takes place as frequently in a second, as double the breadth of the passage is contained in 1130 feet; and the ear receives a perception of a musical sound, thus determined its pitch by the breadth of the passage. On making the experiment, the result will be found accurately to agree with this explanation. If the sound is predetermined, and the frequency of vibrations such as that each pulse, when doubly reflected, may coincide with the subsequent impulse, proceeding directly from the sounding body, the intensity of the sound will be much increased by the reflection; and also, in a less degree, if the reflected pulse coincides with the next but
By altering our situation in a room, and expressing a sound, or hearing the sound of another person, in different situations, or when different objects are alternately placed in the room, that sound may be heard louder or weaker, and more or less distinct. Hence it is, that blind persons, who are under the necessity of paying great attention to the perceptions of their sense of hearing, acquire the habit of distinguishing from the sound even of their own voices, whether a room is empty or furnished; whether the windows are open or shut, and sometimes they can even distinguish whether any person be in the room or not. A great deal of furniture in a room checks, in a great measure, the sounds that are produced in it, for they hinder the free communication of the vibrations of the air from one part of the room to the other. The fittest rooms for declamation, or for music, are such as contain few ornaments that obstruct the sound, and at the same time have the least echo possible. A strong and continued sound fatigues the ear. The strokes of heavy hammers, of artillery, &c. are apt to make people deaf for a time: and it has been known that persons who have been long exposed to the continued and confused noise of certain manufactories, or of water-falls, or other noisy places, can hear what is spoken to them much better in the midst of that noise than elsewhere. We shall conclude this article with an experiment or two for the amusement of the younger part of our readers. Experiment 1. Place a concave mirror, AB, fig. 6, of two feet in diameter, in a perpendicular direction, and at the distance of about five or six feet from a partition EF, in which there is an opening equal in size to the mirror; against this opening must be placed a picture painted in water-colours, on a thin cloth, that the sound may easily pass through it. Behind the partition, at the distance of a few feet, place another mirror GH, of the same size as the former, and directly opposite to it. At the point C is to be placed the figure of a man seated on a pedestal, with his ear exactly in the focus of the first mirror; his lower jaw must be made to open by a wire, and shut by a spring. The wire must pass through the figure, and under the floor, to come up behind the partition. Let a person properly instructed be placed behind the partition, near the mirror; any one may now whisper into the ear of the image, with the assurance of being answered. The deception is managed by giving a signal to the person behind the partition, who by placing his ear to the focus I, of the mirror GH, will hear distinctly what the other said, and moving the jaw of the statue by the concealed wire, will return the answer directly, which will be heard distinctly by the first speaker. Ex. 2. Let two heads of plaster of Paris be placed on pedestals, on opposite sides of a room. A tin tube of an inch in diameter must pass from the ear of one head through the pedestal under the floor, and go up to the mouth of the other. When a person speaks low into the ear of one bust, the sound is reverberated through the length of the tube, and will be distinctly heard by any one who shall place his ear to the mouth of the other. The end of the tube which is next the ear of the one head should be considerably larger than that end which comes to the mouth of the other. If there be two tubes, one going to the ear, and the other to the mouth of each head, two persons may converse together, by applying their mouth and ear reciprocally to the mouth and ear of the busts, while other people standing in the middle of the room, between the heads, will not hear any part of the conversation. Ex. 3. Fig. 7 is a representation of the Eolian harp, which was probably invented by Kircher. This instrument may be made by almost any carpenter; it consists of a long narrow box of very thin deal, about five or six inches broad, and two inches deep, with a circle in the middle of the upper side of an inch and a half in diameter,
in which is drilled small holes. On this side
seven, ten, or more strings of very fine gut are stretched over bridges at each end, like the bridge of a fiddle, and screwed up or relaxed with screw-pins. The strings are all tuned to one and the same note; and the instrument is placed in some current of air, where the wind can pass over its strings with freedom. A window, of which the width is exactly equal to the length of the harp, with the sash just raised to give the air admission, is a proper situation. When the air blows upon these strings with different degrees of force, it will excite different tones of sound; sometimes the blast brings
out all the tones in full concert, and sometimes it sinks them to the softest murInurs. There are different kinds of these instruments; one, invented by the Rev. W. Jones, has the strings fixed to a sounding-board, or belly, within a wooden case, and the wind is admitted to them through an horizontal aperture. In this forn, the instrument is portable, and may be used any where in the open air. The tension of the strings must not be great, as the air, if gentle, has not sufficient power to make them vibrate, and if it blows fresh, the instrument does not sing, but scream. See HARMoNics. ACQUITTAL, in law, is a deliverance or setting free from the suspicion of guilt ; as one who is discharged of a felony is said to be acquitted thereof. Acquittal is either in fact, or in law; in fact, it is where a person, on a verdict of the jury, is found not guilty; in law, it is when two persons are indicted, one as a principal, &c. the other as accessary: here if the former be discharged, the latter of consequence is acquitted. ACQUITTANCE, a discharge in writing for a sum of money, witnessing that the party is paid the same. A man is obliged to give an acquittance on receiving money; and a servant's acquittance for money received for the use of his master shall bind him, provided the servant used to receive his master's rents. An acquittance is a full discharge, and bars all actions, &c, ACRIDAE, in entomology, the name by which Linnaeus has distinguished the first family of the gryllus, or the cricket, properly so called: the characters of which are, that the head is conical and longer than the thorax, and the antemae ensiform, or swordshaped. Of this family there are eight species, none of which are found in Britain. The insects of this family feed on other insects. See GRYLLUs.
ACROCHORDUS, in natural history, a genus of the class Amphibia, and of the order Serpents. There are but three species, viz. A.javanicus, warted snake, brown, beneath paler; the sides obscurely variegated with whitish. It inhabits Java, chiefly among the pepper plantations; grows sometimes to seven feet long. The warts, by means of a magnifying glass, appear to be convex carinate scales, and the smaller ones are furnished with two smaller prominences, one each side the larger. Head somewhat flattened, hardly wider than the neck, body gradually thicker towards the middle,