| From R and W draw the central rays RC P, W CQ, through the centre C of the lens; then the rays of the conical pencil, proceeding from the point R to every point of the nearer surface of the lens, are refracted in such a manner by the lens, that they all emerge in directions parallel to the central ray R CP; but of the whole refracted pencil only a small portion enters the eye, namely the pencil A m n a, limited by the size of the pupil mn; and the head A of the arrow, whence this pencil proceeds, appears to lie in the direction of the pencil E A R' at R'. It is shown exactly in the same manner, that the point W will appear in the direction E B W' at W'. The enlarged image of the small arrow R W is therefore R' W'. The proportion in which the image is enlarged will be easily ascertained thus :-The triangles E R'W', CRW, are similar, and therefore the ratio of R'W' to R W, is that of E R' to C R, or of EM to CM; that is, as the least dis-image eighty feet behind it-then the size of the image tance E M of distinct vision, to the focal length C M of the lens. If, therefore, the least distance of distinct vision be divided by the focal length of the lens, the quotient will be its magnifying power. If E M be reckoned 6 inches for small objects, and if the focal length C M be 2 inches; then, since 6, divided by 2, gives 3 for a quotient, the magnifying power is 3 times. If CM were one quarter of an inch, then 6, divided by, gives 24 for a quotient, and the magnifying power would in this case be 24 times. A more simple explanation may be attempted as follows:-Turn to fig. 9, representing the lens with three faces on one side and flat on the other. There it is observed that the vision travels in the direction of the ray from the object, as it passes through the glass, and therefore sees an appearance of three objects. Now, in the above case of a magnifying lens, the vision in the same manner travels from the eye at E in the direction of the angle of refraction; it goes on to R and W', and thus the actual object being drawn out, as it were, to meet these points of vision, or seemingly expanded by the bent rays, we of necessity see an apparently larger object. If the glass were cut in faces, instead of being smooth, the object would not appear drawn out, but would be multiplied in as many points as there are faces. The inversion of the image by a lens may be illustrated by the diagram, fig. 15. ABC is an arrow, with the point uppermost, placed beyond the focus at F, of á double convex glass de f. In virtue of the refractive power of the lens, the rays which proceed from A meet at Z, and form an image of the arrow-point inverted; while the rays from C meet at X, and form a similarly inverted image of the feather part of the arrow. The rays proceeding from B unite at b. Here, only rays from A, B, and C, are represented, for the sake of clearness, but in point of fact rays from all parts of the object proceed through the lens, and hence an entire image is formed in an inverted position. Should the object A B C be brought nearer the lens, the image will be removed to a greater distance, because then the rays are rendered more divergent, and cannot so soon be collected into corresponding points beyond. To procure a distinct image, the object must be removed farther than the focal point F from the glass. In this exemplification, the object seems to be diminished; but if we make the small arrow the object, the larger one will be the image of it magnified. In order to explain the power of lenses in magnify. ing distant objects, and bringing them near us, let us suppose an object placed at one hundred feet distance from the eye of a spectator. Let us place a convex glass of twenty-five feet focal distance half way between the object and the eye; then, as has been previously observed, an inverted image of the object, and of the same size, will be formed fifty feet behind the lens. If this picture is looked at six or eight inches behind it, it will be very distinctly seen, and nearly as well as if the object itself had been brought to within six or eight inches of the eye of the spectator. If, however, instead of a lens of twenty-five feet focal length, a lens of a shorter focus is made use of, and so situated with respect to the eye and the object that its conjugate foci are at the distance of twenty and eighty feet from the lensthat is, the object is twenty feet before the lens, and its will be four times that of the object. If the eye, therefore, looks at this magnified image six inches behind it, it will be seen with great distinctness. In this case the image is magnified four times directly by the lens, and 200 times by being brought 200 times nearer the eye; so that its apparent magnitude is 800 times larger than before. At distances less than the preceding, the rule for finding the magnifying power of a lens, when the eye views the image which it forms at six inches' distance, is, according to Sir David Brewster, as follows:"From the distance between the image and object in feet, subtract the focal distance of the lens in feet, and divide the remainder by the same focal distance. By this quotient divide twice the distance of the object in feet, and the new quotient will be the magnifying power, or the number of times that the apparent magnitude of the object is increased. When the focal length of the lens is quite inconsiderable, compared with the distance of the object, as it is in most cases, the rule becomes this :-Divide the focal length of the lens by the distance at which the eye looks at the image; or, as the eye will generally look at it at the distance of six inches, in order to see it most distinctly, divide the focal length by six inches, or, what is the same thing, double the focal length in feet, and the result will be the magnifying power." THE EYE-VISION. Having, in our ACCOUNT OF THE HUMAN BODY, described the anatomical construction of the eye, we shall here confine ourselves to the actual process of vision. As mentioned in the article referred to, the eye, in front, consists of the iris or variously coloured ring, which has the property of contracting or expanding to regulate the admission of light through the little dark spot in the centre called the pupil. Immediately behind the iris and pupil, there is a transparent substance, resembling in shape a double convex glass, which is thence called the crystalline lens. The use of this lens is to collect and refract the rays of light, so that they may converge to a point beyond; in other words, cause them to fall on the back part of the eye, called the retina. Such are the main instruments of vision; and the sense of seeing is produced by certain nerves which convey intelligence of the image on the retina to the brain. If these nerves be injured, the image will still be pictured on the retina, but the mind will possess no power of recognising their presence. It will be understood from these explanations, that the main instrument of vision is the crystalline lens, which collects the rays and brings them to a focus on the retina. If the lens be perfectly transparent, and of the proper convexity, the light is enabled to act with due effect on the retina, and the representation of the object looked at will be correctly pictured to the mind. But if the transparent coating of the eye be dull, or the lens be either too flat or too convex, every object will appear dim. Two kinds of defective vision are more common than any other, and they are known by the name of longsightedness and short-sightedness. Long-sightedness, or Fig. 16. L B which the crystalline lens is too flat. CA is the cornea or outer covering, b is the crystalline lens, and d is the retina behind: B is the object looked at. We may observe, that in consequence of the flatness of the lens b, the rays proceeding from the object are not sufficiently refracted, but proceed to a focus as far back as R; in other words, the focus would be at R, if the retina would permit; but as the retina is in the way, the rays, from not being focalised upon it, cause imperfection in the vision. To remedy this, we interpose an artificial convex lens, or glass of a pair of spectacles, L L, and by its aid the rays, represented by dotted lines in the figure, are brought to a focus on the retina at d. Thus, by selecting spectacles of a proper focalising power in relation to the eyes, one kind of imperfect vision is very happily remedied. Short-sightedness arises from a cause the reverse of that just alluded to, being produced by too great a degree of convexity in the crystalline lens and cornea. In this case, the rays come to a focus too soon within the eye, and do not reach the retina, unless the object is brought quite close to the organs of vision. We offer a representation of this condition in fig. 17. In Fig. 17. B consequence of the projecting globularity of the cornea CA, and the too great refracting power of the crystalline lens, the rays from the object B fall short of the retina at R. To remedy this, we interpose a double concave lens, L L, by which the rays are rendered more divergent before they reach the eye, and are brought to a focus, where they should be, on the retina. We have said above, that, in short-sighted persons, the rays do not reach the retina unless the object is held close to the eyes. The effect produced by this is similar to that of employing concave spectacles; because the nearer we hold an object to our sight, the angle of the rays from it is the wider; the rays are more expanded before they enter the eye-that is, more divergent. This may be illustrated by fig. 18. E Fig. 18. The extreme rays from a point to the pupil of the eye make a greater angle at o, than those from a point of a more distant object a make at a; that is, the rays from o are more divergent on entering the eye than the rays from a, and thus nearness of an object is equivalent to seeing it at a greater distance through a concave lens. So when the object a is farther distant than o, the rays from a have a less divergence, which is Fig. 19. viewed at different distances, depends on the size of what is called the visual angle-that is, the angle formed at the eye by the rays from the extremities of the object. We may exemplify this by the fig. 19. An eye is looking at an object a b, and another object c d, at double the distance. It is evident that the rays from ab are more expanded, or cause a larger angle on the eye, than the rays from cd. Various familiar phenomena are explained from the law of the visual angle under which an object is seen; the apparent size being less always in proportion as the distance of an object is greater. Hence the principles of perspective in drawing, by which objects are made to appear at a great distance in the background of a picture, although in reality they are as far forward as the objects in front.-(See DRAWING AND PERSPECTIVE.) Another important circumstance connected withi vision requires to be noticed. In consequence of the refractive power of the crystalline lens, the rays from an object fall upon the retina in such a manner that the image is there pictured upside down; and this inversion of the real appearance of things requires to be corrected by an act of the mind under the influence of experience. We beg leave to offer Dr Arnott's expla nations on this somewhat puzzling point :-" Because the images formed on the retina are always inverted as respects the true position of the objects producing them-just as happens in a simple camera-obscurapersons have wondered that things should appear upright, or in their true situations. The explanation is not difficult. It is known that a man with wry neck judges as correctly of the position of the objects around him as any other person, never deeming them to be inclined or crooked, because their images are inclined in relation to the natural perpendicular of his retina; and that a bedridden person, obliged to keep his head upon his pillow, soon acquires the faculty of the person with wry neck; and that boys who at play bend themselves down to look backwards through their legs, although a little puzzled at first, because the usual position of the images on the retina is reversed, soon see as well in that way as in any other. It appears, therefore, that while the mind studies the form, colour, &c., of external objects in their images projected on the retina, it judges of their position, not by the accidental position of the images on the retina, but by the direction in which the light comes from the object towards the eye, no more deeming an object to be placed low because its image is low in the eye, than a man in a room into which a sunbeam enters by a hole in the window-shutter, deems the sun low because its image is on the floor. A candle carried past a key-hole, throws its light on the opposite wall, so as to cause the luminous spot there to move in a direction the opposite of that in which the candle is carried; but a child is very young indeed who has not learned to judge at once of the true motion of the candle by the contrary apparent motion of the image. A boatman, who, being accustomed to his oar, can direct its point against any object with great certainty, has long ceased to reflect, that to move the point of the oar in some one direction, his hand must move in the contrary direction. Now, the seeing things upright, by images which are inverted, is a phenomenon akin to those which we have here reviewed." The same able writer on physics proceeds to a defi❘nition of another peculiarity in visual arrangements, will be affected. We need here only refer to the experience of our readers on this interesting point, and mention generally, that no satisfactory explanation has ever been given of the reason why the colours in the spectra differ from those which were actually seen. OPTICAL INSTRUMENTS. namely, why, from having two eyes, the object does not appear to us to be double:-"In answer to this, we shall only state the simple facts of the case. As in two chess-boards there are corresponding squares, so in the two eyes there must be corresponding points, and when on those points a similar impression is made at the same time, the sensation or vision is single; but if the impression be made on points which do not correspond, owing to some disturbance of the natural position of glasses, are instruments in the form of tubes, fitted up Telescopes. Telescopes, sometimes called spyingthe eyes, the vision becomes double. Healthy eyes are with lenses of different kinds and powers, and used for so wonderfully associated, that, from earliest infancy, examining distant objects. The word telescope is from they constantly move in perfect unison. By slightly the Greek, and signifies afar off, and to see. A telepressing a finger on the ball of either eye, so as to pre-scope of a simple construction, consists of a convex lens vent its following the motion of the other, there is im- placed at one end of a tube, which is termed the objectmediately produced the double vision; and tumours glass; and by it the light reflected by the objects in about the eye often have the same effect. Persons who front is collected and formed into images near the other squint have always double vision, but they acquire the end of the tube, where they are inspected by another power of attending to the sensation in one eye at a time. lens, of shorter focal length, called the eye-glass. This Animals which have the eyes placed on opposite sides lens is fixed in a smaller tube, which slides backwards of the head, so that the two can never be directed to and forwards, so as to admit of the focal distance being the same point, must have in a more remarkable degree adjusted to different eyes, &c. In telescopes with only the faculty of thus attending to one eye at a time. poses, with a convex eye-glass, the image is intwo lenses, such as those used for astronomical purverted-a circumstance of no importance in viewing the heavenly bodies. In fig. 20 we have a represen The corresponding points in the two eyes are equidistant and in similar directions from the centres of the retina, which centres are called the points of distinct vision, and at them the imaginary lines named the axes of the eyes terminate; but it is worthy of remark that these points, in being both to the right or both to the left of the centres, must be one of them on the inside of the centre, as regards the nose, and the other on the outside that is to say, a point of the left eye between the centre and nose has its corresponding point in the right eye between the centre and the cheek-and from this fact arise consequences meriting attention. When the two eyes are directed to any object, their axes meet at it, and the centres of the two retina are opposite to it, and all the other points of the eyes have perfect mutual correspondence as regards that object, giving the sensation of single vision; but the images formed at the same time, of an object nearer to or farther from the eye than the first supposed, cannot fall on corresponding points, for an object nearer than where the axes meet would have both its images on the outsides of the centres, and an object more distant would have both its images on the insides of the centres, and in either case the vision would be double. Thus, if a person hold up one thumb before his nose, and the other in the same direction, but farther off, by then looking at the nearest, the more distant will appear double, and by looking at the more distant, the nearest will appear double. The reason for applying the term 'point of distinct vision' to the centre of the retina, is felt at once by looking at a printed page, and observing that only the one letter to which the axis of the eye is directed, is distinctly seen; and, consequently, that although the whole page be depicted on the retina at once, the eye, in reading, has to direct its centre successively to every part." The retina of the eye possesses such exquisite sensibility, that it retains the impression of the image of any bright object presented to it, for the space of the sixth of a second after the object has been withdrawn, or after the eye has been shut. Thus the burning end of a rapidly whirled stick will appear to form hoops of fire; and a fiery meteor or sky-rocket shooting rapidly through the air, will appear as a long line of light. The miud is in these and similar instances deceived, as the eye in reality sees only a point of fire at precisely the same time. The retina, for the same reason, retains for a time an impression of any vivid colour. When we look at the sun, the retina is so strongly affected as to be incapable for a time of seeing other objects distinctly. The most remarkable circumstance connected with these phenomena is, that when the eye is shut after such impressions, a spot of colour, different from the colour looked at, is apparently seen. A spot of this nature is in optics called a spectrum; and works of an extended character on the science embrace lengthened definitions of the various spectra with which the eye 60 M Fig. 20. tation of the manner in which a simple telescope with As the rays The magnifying power of such a telescope being limited, it became necessary to contrive an instrument in which the deficiency would be remedied. This has been accomplished by the construction of a telescope with a convex eye-glass, called the astronomical telescope. But this telescope inverts the image-a deficiency which is removed by constructing the instrument with four double convex lenses, as represented in M N Fig. 21. fig. 21. The rays from the object M N, are refracted by the glass A È B, and we have an inverted image n. The rays now pass through C D and E F, by which transit they bring the image upright at m, and by the glass G H they are made to enter the eye at E. This, and other instruments in which refracting lenses are employed, are called refracting telescopes, and they magnify or bring near in proportion as the focal distance of the object-glass is greater than the focal distance of the eye-glass. length where much power is required, and on that acRefracting telescopes require to be of considerable count reflecting telescopes are for many purposes preferred. The reflecting telescope was invented by Sir Isaac Newton, but has been much improved since his time. A view of the improved instrument is given in fig. 22. The peculiarity of this instrument is, that the image of the object is reflected from a concave mirror within the tube, and this image is again reflected from a small mirror to the eye. Referring to the figure, T is Fig. 22. the tube, and A B the object to be represented. At the end opposite from the object, there is a small tube t t. At the main end of the wide tube, there is a concave mirror D F, with a hole in the middle at P. The principal focus of this mirror is at IK; here the image m is inverted, and the rays, crossing each other at n, go on to the small reflector L. From this they are reflected in parallel lines through the hole P. At P they enter the plano-convex lens R, which causes them to converge at a b; but here the image requires to be magnified, which is done by means of the plano-convex lens S; in other words, the object is seen under the angle of d. In order to accommodate focal distances, the small mirror L can be removed to a greater distance or brought nearer, by the rods and screws communicating from X. carry off the smoke from the flame, it is provided with a tube T at the top. L is the light, and M N a concave mirror to give strength to the light, and send the rays through the tube AB in front. At A in this tube is a hemispherical illuminating lens, and there is a convex lens at B. In the middle of the tube there is a wide part CD, open at the sides, for the reception of slides. Microscope is a term compounded of two Greek words, These slides are slips of glass on which pictures are signifying to see what is small, and denotes that instru-painted, and the principle of the apparatus consists in ment employed to examine minute objects. Those forming a representation of the picture, in a magnified microscopes of greatest power, and termed compound, size, on a distant white wall or screen S. The slide approach to the telescope in their form. The diffe- being placed in one of the conjugate foci of the lens B, rence lies in this, that whilst in the telescope the ob- the image is consequently enlarged. By bringing the ject-glass forms the image of a distant object just as lantern nearer the screen, we diminish the representamuch smaller than itself as the distance of the image tion, because we cause the rays to strike the screen at from the glass is less, in the microscope, conversely, a point where they are less divergent. It is an ima small object, placed near the focus of the object-glass, provement in exhibiting the representations from the produces a more distant image, as much larger than magic lantern, to cause the images to fall on a piece of itself as the image is more distant. In both cases an distended and wetted muslin, behind which the spectaappropriate eye-glass is employed. The object-glass tors are placed. Lately, the mode of representing of a microscope is in general very small, that of a telescenes has been further improved by using two lanterns, scope large. An object-glass of a microscope having placed at equal distances; in this case, while the view one-eighth of an inch of focal distance, and so placed in one is being withdrawn, the view in another is comas that the image of the object is formed at six inches, ing on, and the eye is charmed with seeing, for exthe image will be of a diameter forty-eight times as ample, a scene in winter dissolve and assume the apgreat as the object; and when viewed through an eye-pearance of a similar scene in summer. Such is the glass of half an inch focus, it will appear magnified principle of the dissolving views, exhibited at the Polytwelve times more, or will appear 30,000 times larger technic Institution in London in 1841. than the object. A single or one-lens microscope, magnifies chiefly by allowing the eye to see the object nearer than it could do without the glass. A Camera Obscura or Dark Chamber is formed by placing a convex lens in an aperture made in the window-shutter of a darkened room. A glass of proper size and focal distance is chosen, and a screen or the wall of the chamber is properly prepared to receive the light, and by this means there is painted on it an accurate picture of all the objects seen from the window, every thing bearing an exact resemblance to the reality. Nothing can surpass the beautiful effects produced by this delightful instrument. The Camera Lucida is an instrument now frequently used in drawing landscapes, delineating objects of natural history, and copying and reducing drawings. The best form of the instrument consists of a piece of thick parallel glass, at one end of which there is a metallic mirror having a highly polished face. The rays from the object are made first to pass through the glass, when they are reflected back upon one of its sides by the mirror, and from the glass they are again reflected to the eye. The Magic Lantern.-When a small object is placed close to a lens, and the image reflected upon the wall of a dark chamber, at say one hundred times farther from the lens than the object is, there will be a greatly magnified representation of the object. It will only be seen, however, under ordinary illumination; and it is therefore necessary to have a very strong light, m ACOUSTICS. THE term ACOUSTICS is derived from two Greek words, which signify I hear and an art, and is applied to that branch of natural philosophy which treats of the nature of sound, and the laws of its production and propagation. Atmospheric vibration is allowed to be the cause of sound. For instance, a bell is struck by its clapper, the body of the bell consequently vibrates, as we may sensibly assure ourselves by applying our nail lightly to the edge: in its agitation, it beats or makes impulses on the air, which, yielding under the stroke or pressure, is compressed or condensed to a certain distance around. The compressed air instantly expands, and in doing so, repeats the pressure on the air next in contact with it; and thus each one of the original strokes of the vibrating metal sends out a series of shells of compressed air, somewhat like the waves dispersed over a lake from the dropping of a stone into its placid bosom, and like them always lessening in bulk and force. These shells are from two inches to thirty feet in thickness. The air, thus agitated, finally reaches the ear, where it gives a similar impulse to a very fine nervous membrane, and the mind then receives the idea or impression which we call a sound. With regard to the velocity with which the impulse | A will not hear B's gun until several seconds after he of sound advances, it appears, from the most accurate hears his own, because the sound will require that time experiments on the discharge of pieces of ordnance, to pass through the distance between them. And the and marking the interval between the flash and the re- same will be the case with B. One might at first suppose port, at a distance carefully measured, that, when the that if A should wait and fire at the moment he hears atmosphere is at the temperature indicated by 62° of the report from B, the two sounds would then be heard Fahrenheit's thermometer, sound travels at the rate together. A would hear them together, but the time that of 1125 feet per second, which is nearly equal to the must elapse after B had fired, before the sound from velocity of a cannon-ball the moment it issues from the A would come to him, would be greater than if they piece. The ball is very speedily retarded by the resist- fired at the same moment. For he must wait till the ance of the air, but sound advances with undiminished sound of his own gun had gone to A, and then until the velocity, though unequal intensity. It will travel a sound of A's discharge should return to him. It is thus mile in little more than four seconds and a half, or evidently impossible for two persons, standing at a distwelve and three-fourth miles per minute. On this tance from each other, to produce a sound which shall depends an easy method of determining in many cases be heard by both at the same time. our distance from objects, and which may often prove useful, particularly in thunder-storms. We have only to observe in seconds the interval between the flash and the report, and allow four seconds and a half to every mile, or 1125 feet to every second. It is remarkable, also, that all kinds of sounds, strong or weak, acute or grave, advance with the same velocity; and this arises from the circumstance, that all the oscillatory movements in the air, however minute or however extended, are performed each in the very same interval of time. For every degree of Fahrenheit above 62°, the velocity of sound is increased one foot and about a seventh (strictly 1 14-100th foot), and for every degree below 62°, it is lessened in the same measure; so that, when the temperature is at the freezing-point, the rate is only 1090 feet per second. That water is a vehicle of sound as well as the air, is proved by various circumstances, particularly by the fact, that a bell rung under water can be heard above; and if the head of the auditor be also under water, it will be still more distinctly heard. The sound which the sonorous body produces, however, is graver than that which it gives forth in the air. That the atmosphere is necessary for the transmission of sound is evident from the fact, that a bell rung in the exhausted receiver of an air-pump can scarcely be heard. Smooth bodies form favourable channels of sound, as, for example, the surface of ice, snow, water, or the hard ground. Savages, it is well known, are in the habit of putting their ear to the ground in order to discover the approach of enemies or beasts of prey. Tubes convey sounds with great accuracy and to great distances, and this property has been applied to various useful purposes. The most valuable of these purposes is that of examining the chests of persons supposed to possess pulmonary affections. This is done by means of the stethoscope, an instrument invented by Dr Laennec of Paris, and which resembles a small trumpet. The wide end of the instrument is applied to the body, and the other is held to the ear of the physician, who then has a very clear perception of the sounds caused by the action of the lungs, and can judge whether they be healthy or the reverse. A person of skill can exactly describe the condition of the lungs from the nature of the sounds which thus reach his ear. In a public exhibition in London, there has long been shown an apparatus consisting of a four-footed stand and several trumpet-mouthed tubes, from any one of which a spectator will receive a ready answer to a question. The answer is said to come from "the invisible girl," and the true explanation of the puzzle is, that a secret tube in the legs of the apparatus communicates the sounds to a girl placed in a neighbouring apartment. In consequence of sound requiring a certain length of time to travel, it is impossible for two sounds at any distance from each other, to be heard at the same moment by persons who are not at equal distances from both. "If two persons, A and B," says an American writer," are standing at the distance of one mile from each other, and each fires a gun at the same moment, The velocity here assigned to sound, is that given by Sir John rachel as the mean of the best experiments. It is on account of this principle, that in long ranks of soldiers, where two bands of music are placed at a considerable interval from each other, it is impossible for the two bands to keep time with each other. They may indeed play together, but each soldier will hear the nearest sounds quickest, and thus they will seem to be out of time. It is often noticed too, that if from an eminence we look upon a long column which is marching to a band of music in front, the various ranks do not step exactly together. Those in the rear are in each step a little later than those before them. This produces a sort of undulation in the whole column, which is difficult to describe, but which all who have noticed it will understand. Each rank steps, not when the sound is made, but when, in its progress down the column at the rate of 1125 feet per second, it reaches their ears. Those who are near the music hear it as soon as it is produced, while the others must wait till sufficient time shall have elapsed for it to have passed through the air to them. Should a commander stand at the distance of a fifth of a mile from his army, and command them to fire, they might all obey at the moment when the word of command reaches them; but the officer will hear the report of the guns from those at the side nearest him first, then those a little farther off, and so on to the most remote. Thus, though all might obey with equal alacrity, the sounds will not and cannot appear simultaneous, for the reports of the distant guns must be delayed long enough for the command to pass from the officer to the men, and then for the sound to return. All attempts, therefore, to make the firing appear exactly simultaneous from a long line must be in vain." An echo, or duplication of sound, is one of the most interesting phenomena in acoustics. The cause of it is precisely analogous to the reaction of a wave of water. When a wave of water strikes the precipitous bank of a river, it is thrown back in a diagonal direction to the side whence it came, and there again strikes on the bank. In the same manner, the pulses or waves of sound are reflected or thrown back from flat surfaces which interrupt them, and, thus returning, produce what we call an echo. It is evident that the smoother the surface which reflects the sound, the more perfect will be the reverberation. An irregular surface, by throwing back the wave of sound at irregular intervals, will so confound and distract it, that no distinct or audible echo will be reflected. On the contrary, a regular concave surface will reflect sound in such a manner, that at a certain point the reflections from each part of the concave surface will be concentrated into a focus capable of producing a very powerful effect. The velocity with which an echo returns to the spot where the sound originates, depends, of course, upon the distance of the reflecting surface; and since sound travels at the rate of 1125 feet in a second, a rock situated at half that distance will return an echo in exactly one second. The number of syllables which we pronounce in a second will in such a case be repeated distinctly, while the end of a long sentence would blend with the commencement of the echo. An echo may be double, triple, or even quadruple, |