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ACOUSTICS.

ACOUSTICS1 is the science which treats of sound. Sound is the sensation produced when the vibrations of some sounding body are conveyed directly or indirectly to the organ of hearing. That a body producing sound is in a state of vibration at the time is seen in any stringed musical instrument, the prongs of a tuning-fork, the lip of a bell, &c. These vibrations must be communicated to the ear through some medium connecting the sounding body with the ear, the ordinary medium being the air. This is proved by the fact, that when a bell is struck in a chamber from which the air has been withdrawn, no sound is produced, and that the less dense the air becomes, the fainter are the sounds heard in it, so that on high mountains, where the air is very rare, what would elsewhere be loud talking is heard like whispers. Air, however, is not the only medium, the vibrations being conveyed through water, wood, and other substances. If the head be held under water, any noise at a distance, such as that caused by two stones being knocked against each other, is heard with great distinctness. Or if the ear be placed at one end of a log of wood, the slightest scratch at the other end is heard very distinctly, the vibrations being transmitted through the particles of the wood. Bodies denser than air are better conductors of sound, but, as air is the ordinary medium, the general principles of sound have reference to it. Vibrations are the wave-like motion which is communicated to the air by any shock, such as a shot, the blow of a hammer, or an explosion. The air is not made to change its place, but the compression of its particles caused by the shock is passed along like the undulations of a rope held at one end and moved up and down. These undulations pass outward from the body producing them, in all directions, exactly like those caused on the surface of water by a stone thrown into it, and, like them too, they diminish in force as they proceed outward.

Loudness of Sound. The loudness of any sound depends upon the force of the concussion of the air, and the force with which this concussion is conveyed to the ear; so that, from the decreasing force of the sound-waves, in a certain ratio, the intensity or loudness of sound diminishes with the distance of the hearer from the object causing it. But a great many other things besides the distance have to be taken into account as affecting the intensity of sound. Thus, sounds at a distance are heard much better at one time than another: this arises from various causes. As was mentioned above, the denser the air is the better are sounds heard; a dense state of the atmosphere would therefore account for sounds being heard

1 From Greek akoustikos, relating to hearing or sound, from akouō, to hear.

distinctly at a great distance at a particular time. Sounds are also better heard in a dry atmosphere, as moisture seems to interfere with the propagation of sound-waves. Although a simple current of air does not of itself produce sound (it only does so when it comes in contact with obstacles in its way, as trees, houses, &c., by which vibrations are produced), yet it will carry sound-waves in any direction; sounds are therefore heard more distinctly when there is a breeze blowing from the source of the sound towards the hearer.

Velocity of Sound.-The velocity with which sound-waves are propagated can be calculated very exactly, owing to the extreme rapidity with which light travels. The velocity of light is so great that for any distance within the limits to which sound could be heard, its passage may be considered as instantaneous. This being the case, the blow, shot, explosion, &c. causing any noise, may be seen to take place before the noise is heard; and the time that elapses between seeing the cause and hearing the noise is the time that it takes for the propagation of the sound-waves to the distance of the hearer. If a man be observed at a distance striking heavy blows on anything, the blow will be seen to descend some time before the sound of it is heard; so with the firing of a gun, the flash is seen before the report is heard. It has been found by experiment that sound travels at the rate of about 1142 feet per second, or of a mile in about four seconds and a half. The velocity is somewhat less, however, when the temperature is lower, being only 1090 feet at the freezing-point. In water, sound travels about four times, and in solids, from ten to twenty times more quickly than in air.

Reflection of Sound-Echo.-We are now to consider what takes place when a sound-wave meets a large obstacle to its progress. The vibration is to a small degree communicated to the solid, just as light is transmitted through a new medium; but the principal effect is to reflect the wave, just as a wave of the sea is thrown back from a rock. When a sound-wave is thrown back in this way from a flat surface, the sound is carried back to the point where it was produced, and this return-sound is called an echo. It may be produced by a perpendicular wall of rock, a wood, or other obstacle. If a sound be produced between two faces of rock, for instance, it will first be echoed by both; and then those echoed sounds (the sound being now doubled) will be thrown back from one to the other again and again, becoming fainter each time; so much so, that a shot in such a situation may be repeated forty or fifty times. The best echo is produced by a concave surface, such as the roof of a cave, by which the sound-wave is reflected to a focus, as in the case of rays of light. If one happen to stand so as to bring one's ear into the focus, the whole of the sound, concentrated at that point, falls upon the ear, and the effect is astounding.

The ears of animals are admirably adapted to receive impressions from the vibrations of the air caused by so-called sounding bodies. The external ear collects part of the sound-wave, and turns it into the passage which conducts it to the membrane from which the vibrations are conveyed to the auditory nerves. The ear-trumpet, which is a trumpet-shaped instrument, having a wide mouth at the end of a tube, is simply a contrivance by which a greater volume of the sound-wave is collected and conveyed into the ear.

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Musical Notes.-We have hitherto spoken only of a single shock transmitted through the atmosphere as producing a single sound. The sensation conveyed to the ear when blows are struck with a hammer is a succession of distinct sounds. When sounds are produced in such quick succession that the successive pulses of the air cannot be distinguished by the ear, a musical note is produced. For example, if a toothed-wheel, with a plate of some thin elastic substance resting on the teeth, be turned slowly, the shock produced by the plate falling off each tooth is conveyed separately to the ear, and separate sounds are heard; but when it is turned more rapidly, the shocks all blend into one, and the sound heard is a continued whir;' and the more rapidly the wheel is turned the more sharp and shrill the whir' becomes. It is on this principle that notes are produced from the strings of musical instruments. When a string stretched between two objects is pulled aside in the middle, its elasticity causes it to return to its proper position; but the momentum it has meantime acquired carries it past to about the same distance on the other side. Its elasticity here comes into play again, and it is again carried past the middle straight position almost as far as it was pulled aside at first: the same thing is repeated; and this goes on, the vibrations on each side gradually becoming less and less, till the string at last comes to rest. When this is done with a slack string, the vibrations are slow; but when the string is tight, the vibrations are so quick that, as in the case of the toothed-wheel when turned rapidly, the separate pulses are blended into a continuous note; and the tighter the string is pulled, the sharper is the sound produced by its vibrations. A good illustration of successive vibrations blending to cause a continuous sound, is the 'buzz' of a fly, which is supposed to be not the voice of the insect, but the sound produced by the rapid vibrations of its wings. It is on this rapidity of vibration that the difference of sounds depends. We observed that the 'whir' of a toothed-wheel striking an elastic plate becomes more shrill the quicker the wheel is turned, and that the sound produced by a vibrating string is sharper the quicker the vibrations are made. Now, a short string will evidently vibrate more quickly than a long one; therefore, every degree of sharpness of sound can be produced from strings by making them of different lengths and of

different tightness. A string half the length of another, and of the same tension, vibrates twice as quickly, and the musical note produced by it has a fixed relation to that produced by the other. From this it will be understood why a fiddler places his fingers up and down on the strings as he plays. The string can only vibrate as far as the point on which he places his finger; by putting his finger on the string a good way up, he shortens the string, and a sharper sound is produced; thus, besides having the range of the four strings, which give different notes from being stretched to a different tightness (the bass-string being also loaded with copper wire, to make it vibrate more slowly), he has a range of notes on each string, by means of shortening them to different lengths.

Different notes are also obtained from pipes, according to their length. The sound produced by a pipe is due to the vibrations of the air in the pipe, which are caused by the shock communicated by blowing into it, or partly by the construction of a mouth-piece. The shorter the pipe, the greater is the shrillness of the note produced. Every one knows that the note of a large flute is soft and mellow, while that of a small one is shrill. In an organ, there is a pipe for every note, the different lengths being calculated with extreme care; while in a flute a similar effect is produced by opening and shutting holes along the length of the one pipe. Another system on which musical notes are produced is by placing a slender elastic plate over a slit or opening through which air is made to pass, by blowing, as in the musical toy of this description. The air causes the little pieces of steel to vibrate with great rapidity and thus produce a musical note. The human voice consists simply of musical notes produced by the vibration of two membranes at the top of the windpipe, with their free edges opposite each other, and a slit or opening left between them for the passage of the air by which they are vibrated. When a boy places a blade of grass between his thumbs, and, by blowing on the edge, produces a note by no means musical in one sense, the note is produced exactly on the principle of the human voice as produced by the larynx, being caused by the extremely rapid vibrations of the edge of the blade of grass. The voices of

children and women are shriller than those of men, because the membranes of the larynx are shorter in the former than in the latter, and thus produce sharper notes.

OPTICS.

The subjects treated of in the science of OPTICS 1 are Light and Vision. We learned in the chapter on ACOUSTICS, that the sensations of Sound are produced by vibrations in sounding bodies being communicated to the air, and by it transmitted to the nerves of the ears. It is now believed that both Light and Heat consist of a vibratory motion of the particles of light-giving and hot bodies, and that they are transmitted in a manner exactly similar to the transmission of sound. The medium, however, cannot be the air, since light and heat pass where there is no air; philosophers have accordingly come to the conclusion that all space is occupied by an infinitely elastic substance or fluid called ether. The vibrations of the particles of a luminous body are communicated to the ether; the pulses transmitted through it enter the eye, and strike upon the retina, and, being thence conveyed to the brain, produce the sensation of Sight. These vibrations pass from a luminous body in every direction, and for this reason Light is said to be composed of rays, from Latin radius, the spoke of a wheel, and these rays are said to be divergent. Light travels at the rate of about 194,000 miles in a second.

When light falls on the surfaces of bodies, some or all of the rays are reflected, or thrown back by the surface; or, some or all of the rays are transmitted, or pass through the body, according to the nature of the body, and the manner in which the light falls upon it. Rays which are transmitted from one substance into another, are bent out of the straight line, and are said to be refracted. The following lessons on Optics consist of a short statement of the Laws of Reflection and Refraction.

Reflection.

Let AB (fig. 24) be a reflecting surface, as a mirror, with a ray of light falling on it in the direction of ed; and let cd be a line perpendicular to AB: then, by the first law of reflection, the ray ed will be reflected from AB in the direction db, so that ed, dc, and db shall all be in the same plane; and by the second, the angle cdb is equal to the angle cde. A ray falling on a surface, as ed, is called the incident 2 ray, and d is called the point of incidence; cd, the perpendicular at the point of incidence, is called the normal; and db, the reflected ray.

1 From Greek optikos, relating to sight.

A

B

Fig. 24.

The first law of reflection may

2 From Latin incido, to fall upon.

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