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the increase of the direct expansion and of a motion nearly rectilinear, it may easily
larger and larger quantity of matter, as it An umbrella, held in a proper position proceeds to diverge every way from its over the head, may serve to collect the centre. The actual velocity of the particles force of a distant sound by reflection, in of the medium transmitting it appears to the manner of a hearing-trumpet; but its diminish simply in the same proportion as substance is too slight to reflect any sound the distance from the centre increases : perfectly, unless the sound fall on it in a consequently their energy, which is to be very oblique direction. The exhibition of considered as the measure of the strength of the Invisible Girl is said to depend on the sound, must vary as the square of the disreflection of sound; but the deception is tance; so that at the distance of ten feet really performed by conveying the sound from the sounding body the velocity of the through pipes artfully concealed, and open- particles of the medium becomes one-tenth ing opposite to the mouth of the trumpet, as great as at the distance of one foot, and from which it seems to proceed.
their energy, or the strength of the sound, When a portion of a pulse of a sound is only one-hundredth as great. separated by any means from the rest of the An echo is a reflection of sound striking spherical or hemispherical surface to wbich against some object, as an image is reflected it belongs, and proceeds through a wide in a glass: but it has been disputed what are space, without being supported on either the proper qualities in a body for thus reside, there is a certain degree of diver- fecting sounds. It is in general known, gence, by means of which it sometimes that caverns, grottoes, mountains, and becomes audible in every part of the me- ruined buildings, return this reflection of dium transmitting it: but the sound thus sound. We have heard of a very extraordiverging is comparatively very faint. dinary echo, at a ruined fortress near LouHence, in order that a speaking-trumpet vain, in Flanders. If a person sung, he may produce its full effect, it must be di- only heard his own voice, without any rerected in a right line towards the hearer; petition; on the contrary, those who stood and the sound collected into the focus of a at some distance, heard the echo, but not concave mirror is far more powerful than at the voice; but then they heard it with sur. a little distance from it, which could not prising variations, sometimes louder, somehappen, if sound, in all cases, tended to times sotter, now more near, then more spread equally in all directions. It is said distant. There is an account in the Methat the report of a cannon appears many morts of the French Academy, of a simitimes louder to a person towards whom it lar echo near Rouen. It has been already is fired, than to one placed in a contrary observed, that every point against which direction. It must, says Dr. Young, have the pulses of sound strike, becomes the occurred to every one's observation, that a centre of a new series of pulses, and sound sound, such as that of a mill, or a fall of describes equal distances in equal times ; water, has appeared much louder after therefore, when any sound is propagated turning a corner, when the house or other from a centre, and its pulses strike against obstacle no longer intervened. Indeed the a variety of obstacles, if the sum of the right whole theory of the speaking-trumpet would lines drawn from that point to each of the fall to the ground, if it were demonstrable obstacles, and from each obstacle to a second that sound spreads equally in all directions. point, be equal, then will the latter be a In windy weather it may be often observed, point in which an echo will be heard. Thus that the sound of a distant bell varies almost let A, fig. 4, be the point from which the instantaneously in its strength, so as to ap- sound is propagated in all directions, and pear twice as remote at one time as an- let the pulses strike against the obstaother. Now if sound diverged equally in cles C, D, E, F, G, H, I, &c. each of all directions, the variation produced by these points becomes a new centre of pulses the wind would not exceed one tenth of the by the first principles, and therefore from apparent distance; but on the supposition each of them one series of pulses wių pass VOL. I.
through the point B. Now if the several as such a number of reflected pulses may
of the right lines AC FCB, arrive at the same time at the ear as may AD + DB, AE 7EB, AG+GB, be sufficient to excite a distinct perception.
Thus AH+HB, AT+IB, &c., be all equal
a person often hears the echo of his to each other, it is obvious that the pulses stand at least 63 or 64 feet from the reflect
own voice; but for this purpose he should propagated from A to these points, and again, from these points to B, will all
ar. ing obstacle, according to what has been
said hefore. rive at B at the same instant, according to the second principle; and, therefore, if the
If a bell, a, fig. 5, be struck, and the unhearer be in that point, his ear will at the dulations of the air strike the wall cd in a same instant be struck by all these pulses. Hected back in the same line; and if a per
perpendicular direction, they will be reNow it appears from experiment, that the ear of an exercised musician can alone dis- son be situated between a and c, as at x, he tinguish such sounds as follow one another of the undulations as they went to the wall,
would hear the sound of the bell by means at the rate of 9 or 10 in a second, or any and he would hear it again as they came slower rate: and therefore, for a distinct back, after the reflection, which would be perception of the direct and reflected sound, the echo of the sound. So a person standthere should intervene the interval of sth of a second; but in this time sound de ing at x might, in speaking in the direction 1142
of the wall cd, hear the echo of his own scribes or 127 feet nearly. And voice. But in both cases the distance c x therefore, unless the sum of the lines drawn
must be 63 or 64 feet. If the undulations from each of the obstacles to the points A strike against the wall obliquely, they will and B exceeds the interval AB by 127 be reflected off obliquely on the other side ; feet, no echo will be heard at B. Since if
, for instance, a person stand at m, and the several sums of the lines drawn from the
there be any obstacle between that place obstacles to the points A and B are of the and the bell, so as to prevent him hearing same magnitude, it appears that the curve the direct sound, he may nevertheless hear passing through all the points, C, D, E, F, the echo from the wall c'd, provided the diG, H, I, &c. will be an ellipse. Hence alí rect sound fall in that sort of oblique direc. the points of the obstacles which produce tion so as to force the reflected undulations an echo, must lie in the surface of the ob- along the line cm. long spheroid, generated by the revolution At the common rate of speaking, we do of this ellipse round its major axis. See not pronounce above three syllables and a Conic SECTIONS. As there may be several half, that is, seven half syllables in a second; spheroids of different magnitudes, so there therefore, that the echo may return just as may be several different echoes of the same soon as three syllables are expressed, twice original sound. And as there may happen the distance of the speaker from the reto be a greater number of reflecting points flecting object must be equal to 1000 feet; in the surface of an exterior spheroid than for as sound describes 1142 feet in a second, in that of an interior, a second or a third gths of that space, thiat is 1000 feet nearly, echo may be much more powerful than the will be described while six half, or three first, provided that the superior number of whole, syllables are pronounced ; that is, reflecting points, that is, the superior num the speaker must stand near 500 feet from ber of reflecting pulses propagated to the the obstacle. And, in general, the distance ear, be more than sufficient to compensate of the speaker from the echoing surface, for for the decay of sound which arises from its any number of syllables, must be equal to being propagated through a greater space. the seventh part of the product of 1142 feet This is finely illustrated in the celebrated multiplied by that number. In churches echoes at the lake of Killarny, in Kerry, we never hear a distinct echo of the voice, where the first return of the sound is much but a confused sound, when the speaker inferior in strength to those which immedi- utters his words too rapidly; because the ately succeed it. From what has been laid greatest difference of distance between the down it appears, that, for the most powerful direct and reflected courses of such a numecho, the sounding body should be in one ber of pulses as would produce a distinct focus of the ellipse, which is the section of sound is never in any church equal to 127 the echoing spheroid, and the hearer in the feet, the liinit of echoes. But though the other. However, an echo may be heard in first reflected pulses may produce no echo, other situations, though not so favourably; both on account of their being too few in
number, and too rapid in their return to the one, the next but two, or more of the direct ear; yet it is evident, that the reflecting pulses. The appropriate notes of a room surface may be so formed, as that the may readily be discovered by singing the palses which come to the ear after two re. scale in it; and they will be found to defections or more, may, after having de- pend on the proportion of its length or scribed 127 feet or more, arrive at the ear breath to 1130 feet. in sufficient numbers, and also so nearly at By altering our situation in a room, and the same instant, as to produce an echo, expressing a sound, or hearing the sound of though the distance of the reflecting sar- another person, in different situations, or face from the ear be less than the limit of when different objects are alternately placed echoes. This is confirmed by a singular in the room, that sound may be heard echo in a grotto on the banks of the little londer or weaker, and more or less disbrook called the Dinan, about two miles from tinct. Hence it is, that blind persons, who Castlecomber, in the county of Kilkenny. are under the necessity of paying great atAs you enter the cave, and continue speak- tention to the perceptions of their sense of ing loud, no return of the voice is per- hearing, acquire the habit of distinguishing ceived; but on your arriving at a certain from the sound even of their own voices, point, which is not above 14 or 15 feet whether a room is empty or furnished; whefrom the reflecting surface, a very distinct ther the windows are open or shut, and echo is heard. Now this echo cannot arise sometimes they can even distinguish whefrom the first course of pulses that are re- ther any person be in the room or not. A flected to the ear, because the breadth of great deal of furniture in a room checks, in the cave is so small, that they would re a great measure, the sounds that are proturn too quickly to produce a distinct sen
duced in it, for they hinder the free comsation from that of the original sound: it munication of the vibrations of the air from therefore is produced by those pulses, which, one part of the room to the other. The after having been reflected several times fittest rooms for declamation, or for music, from one side of the grotto to the other, and
are such as contain few ornaments that obhaving run over a greater space than 127
struct the sound, and at the same time have feet, arrived at the ear in considerable yum
the least echo possible. bers, and not more distant from each other A strong and continued sound fatigues the in point of time than the ninth part of a se
The strokes of heavy hammers, of arcond. M. De la Grange demonstrated that tillery, &c. are apt to make people deaf all impressions are reflected by an obstacle for a time: and it has been known that terminating an elastic Ruid with the same persons who have been long exposed the velocity with which they arrived at that ob- continued and confused noise of certain stacle. When the walls of a passage, or of an manufactories, or of waterfalls, or other unfurnished room, are smooth and perfectly noisy places, can hoar what is spoken to parallel, any explosion, or a stamping with them much better in the midst of that noise the foot, communicates an impression to than elsewhere. the air, which is reflected from one wall to
We shall conclude this article with an the other, and from the second again to- experiment or two for the amusement of wards the ear, nearly in the same direction, the younger part of our readers. with the primitive impulse : this takes place Experiment 1. Place a concave mirror, as frequently in a second, as double the AB, fig. 6, of two feet in diameter, in a breadth of the passage is contained in 1130 perpendicular direction, and at the disfeet; and the ear receives a perception of a tance of about five or six feet from a parmusical sound, thus determined its pitch tition EF, in which there is an opening by the breadth of the passage. On mak- equal in size to the mirror; against this ing the experiment, the result will be found opening must be placed a picture painted in accurately to agree with this explanation. water-colours, on a thin cloth, that the If the sound is predetermined, and the fre- sound may easily pass through it. Behind quency of vibrations such as that each pulse, the partition, at the distance of a few feet, when doubly reflected, may coincide with place another mirror GH, of the same size the subsequent impulse, proceeding directly as the former, and directly opposite to it. from the sounding body, the intensity of At the point C is to be placed the figure of the sound will be much increased by the a man seated on a pedestal, with his ear exreflection; and also, in a less degree, if the actly in the focus of the first mirror; his reflected pulse coincides with the next but lower jaw must be made to open by a
wire, and shut by a spring. The wire out all the tones in full concert, and somemust pass through the figure, and under the times it sinks them to the softest murfloor, to come up behind the partition. murs. Let a person properly instructed be placed There are different kinds of these instrubehind the partition, near the mirror; any ments; one, invented by the Rev. W. Jones, one may now whisper into the ear of the las the strings fixed to a sounding-board, or image, with the assurance of being an- belly, within a wooden case, and the wind swered. The deception is managed by is admitted to them through an horizontal giving a signal to the person behind the par- aperture. In this form the instrument is tition, who by placing his ear to the focus portable, and may be used any where in I, of the mirror GH, will hear distinctly the open air. The tension of the strings what the other said, and moving the jaw of must not be great, as the air, if gentle, has the statue by the concealed wire, will re not sufficient power to make them vibrate, turn the answer directly, which will be and if it blows fresh, the instrument does heard distinctly by the first speaker. not sing, but scream. See HARMONICS.
Ex. 2. Let two heads of plaster of Paris ACQUITTAL, in law, is a deliverance be placed on pedestals, on opposite sides of or setting free from the suspicion of guilt ; A tin tube of an inch in diame
as one who is discharged of a felony is said ter must pass from the ear of one head
to be acquitted thereof. through the pedestal under the floor, and
Acquittal is either in fact, or in law; in go up to the mouth of the other. When a fact, it is where a person, on a verdict of person speaks low into the car of one bust, the jury, is found not guilty; in law, it is the sound is reverberated through the length when two persons are indicted, one as a of the tube, and will be distinctly heard by principal, &c. the other as accessary: here any one who shall place his ear to the mouth if the former' be discharged, the latter of of the other. The end of the tube which is consequence is acquitted. next the ear of the one head should be
ACQUITTANCE, a discharge in writ. considerably larger than that end which ing for a sum of money, witnessing that the comes to the mouth of the other. If party is paid the same. there be two tubes, one going to the ear, A man is obliged to give an acquittance and the other to the mouth of each head,
on receiving money; and a servant's actwo persons may converse together, by ap- quittance for money received for the use of plying their mouth and ear reciprocally to his master shall bind him, provided the the mouth and ear of the busts, while other servant used to receive his master's rents. people standing in the middle of the room, An acquittance is a full discharge, and bars between the heads, will not hear any part all actions, &c, of the conversation.
ACRIDÆ, in entomology, the name by Ex. 3. Fig. 7 is a representation of the which Linnæus has distinguished the tirst faEolian harp, which was probably invented mily of the gryllus, or the cricket, properly by Kircher. This instrument may be made so called: the characters of which are, that by almost any carpenter; it consists of a
the head is conical and longer than the long narrow box of very thin deal, about thorax, and the antemæ ensiform, or swordfive or six inches broad, and two inches shaped. Of this family there are eight spedeep, with a circle in the middle of the up- cies, none of which are found in Britain. The per side of an inch and a half in diameter, insects of this family feed on other insects. in which is drilled small holes. On this side See GRYLLUS. seven, ten, or more strings of very fine gut ACROCHORDUS, in natural history, a are stretched over bridges at each end, like genus of the class Amphibia, and of the orthe bridge of a fiddle, and screwed up or re der Serpents. There are but three species, laxed with screw-pins. The strings are all viz. A.javanicus, warted snake, brown, betuned to one and the same note; and the neath paler; the sides obscurely variegated instrument is placed in some current of air, with whitish.
It inhabits Javil, chiefly where the wind can pass over its strings among the pepper plantations ; grows with freedom. A window, of which the sometimes to seven feet long. The wartz, width is exactly equal to the length of the by means of a magnifying glass, appear to harp, with the saşlı just raised to give the be convex carinate scales, and the smaller air admission, is a proper situation. When ones are furnished with two smaller promithe air blows upon these strings with diffe- nences, one each side the larger. Head somerent degrees of force, it will excite different what flattened, hardly wider than the neck, tones of sound; sometimes the blast brings body gradually thicker towards the middle,