sound passes from light to heavy air, a portion of it is reflected. This explains a singular effect which was observed by the celebrated traveller Humboldt. Being stationed some miles distant from the great Falls of the river Orinoco in South America, he found that during the night the sound of the waterfall was so loud that he could imagine himself close beside it. During the day, the sound was much feebler. You will perhaps think that this was quite natural owing to the greater stillness of the night, but the fact was actually far otherwise. In those regions the night is far more noisy than the day. Under the noonday sun, the forest beasts cease their yelling and roaring, and retire to sleep, while the innumerable swarms of insects, which fill the air with their humming during the night, are all stilled. Now pay attention to the true explanation. A large plain stretched between the place where M. de Humboldt was stationed and the waterfall, this plain being covered partially with grass, through which, however, a great number of rocks protruded. During the day these rocks became very hot— much hotter than the grass, and the consequence was, that over each rock during the day there was a column of light air—for you know that air swells and becomes light when heated. Hence the sound of the waterfall in passing through the atmosphere over the plain, crossed perpetually from heavy to light, and from light to heavy air. At each passage a small portion of the sound was reflected, and this occurred so often, that before it reached the place where M. de Humboldt was stationed, the sound was greatly enfeebled. At night the rocks became cooled, there was no longer that great difference of temperature between them and the grass; the atmosphere was more homogeneous, and the sound passed through it without reflection: the consequence was that the roar of the cataract was much louder during the night than during the day. SECT. 4. In the first section I explained to you how a single pulse of sound was transmitted through the atmosphere, and what it did in the ear. I have said that the tympanum is thrown into motion by the shock. Now, every motion in nature, when once excited, takes time to subside. In the case of the tympanum the motion subsides very speedily, but still it requires time; and if you cause two shocks to follow each other with sufficient speed, the last of them may reach the ear before the motion excited by the first has been extinguished, and thus a prolonged sound may be produced. Here I have to announce to you a most interesting fact, a musical sound is a sound which is prolonged in this way. It is produced by a series of impulses which strike the ear at regular intervals, and in quick succession. In producing a musical sound, therefore, we make use of a body which is capable of sending a succession of waves to the ear,—a vibrating string or belt; a vibrating tongue, as in the Jew's harp and the concertina ; a vibrating column of air, as in a flute or organ-pipe. The organs of voice also are capable of being thrown into vibration, like the reed of a clarionette, by the air passing from the lungs. But now I have to draw your attention to a peculiarity of these musical sounds or notes. They differ in pitch-some notes are high and others low; and the height or pitch of the note depends solely upon the number of impulses which the tympanum receives in a second. The greater the number of impulses per second, the higher the note. A string which vibrates 500 times in a second, produces a higher note than one which vibrates only 400 times a second. The shorter a string is, the more quickly it vibrates, and the higher the note that it produces. In like manner, the shorter the organ-pipe or the flute, —and you really shorten a flute when you take your fingers off its holes the quicker are its vibrations, and the higher its note. space permitted, I might state to you the relative lengths of the strings, or of the organ-pipes, necessary for producing all the notes of the gamut. I will content myself by saying, that when one string is half the length of another, it vibrates twice as quickly, supposing both to be screwed up equally tight, and the note it produces is the octave of that produced by the longer string. Thus it is that by judiciously varying the lengths of a few strings, by pressing upon them with his fingers, a violin player is able to produce a great variety of notes. If A succession of taps, if they only follow each other speedily enough, will produce a musical note. When a slate-pencil, held loosely in the hand and perfectly upright, is drawn along a slate, every boy knows that a jumping motion of the pencil, and a dotted line upon the slate, are produced. A series of distinct taps of the pencil is also heard, but the sound is a mere rattle. By pressing upon the pencil, these taps can be caused to succeed each other more quickly, until finally a musical note is produced. Most people, it is true, shut their ears against this melody, and D complain that it gives them the toothache; but it is nevertheless a good illustration of our present subject. If a card be held against the circumference of a toothed wheel, it is struck by the teeth as they pass, and the distinct taps are heard; but if the wheel rotate rapidly enough, the separate taps are no longer distinguishable, but melt into a continuous musical note. A series of puffs can also produce a musical note. If a locomotive could send out its puffs quickly enough, we should have a musical sound of deafening intensity. Instruments have been made for the express purpose of producing taps or puffs, and such instruments are provided with machinery which tells us the exact number of puffs or taps accomplished in a second. By means of such instruments we can tell the exact number of vibrations produced by the organs of a singer. We have only to bring the instrument and the voice to the same pitch; the number of puffs there recorded by the instrument is the number of vibrations accomplished by the singer. In the same way the number of times a bee flaps its wings in a second can be accurately determined from the hum of the insect. In this way, indeed, it has been ascertained that gnats sometimes flap their little wings fifteen thousand times in a second! How wonderful all this is, my boys, and how well worthy of your attention! And how beautiful does the arrangement appear, that Nature should possess such wonders, and that man should possess the power of investigating and understanding them! LIGHT. VELOCITY OF LIGHT-REFLECTION-REFRACTION-THE EYE-COLOUR. SECT. 1. Some of the ancients believed that light issued from the eye. This is wrong; the eye sees bodies by the light which it receives from them. Some bodies, such as the sun, a candle, or a glowing coal, are themselves sources of light; other bodies again are visible because of the light shed upon them by luminous bodies, and have no light of their own. The moon is an example. When you look into your companion's face, too, you see it, not because it is luminous, but because it is illuminated by light from some other source, which light is sent from the face to your eyes. The eyes themselves are not luminous. Even a cat's eyes, which shine in the night, are not luminous; for if the animal be placed in perfect darkness, its eyes will not shine. The human eyes can, by proper means, be made to glow like a red-hot coal; but this is done by throwing light into them, and is not due to light which they themselves possess. Light moves with an immense velocity. Our earth, you know, travels round the sun at an average distance from him of ninety-five millions of miles. There is another planet, called Jupiter, which also travels round the sun at a far greater distance from him than the earth. It is found, by the most accurate observations, that the light from Jupiter requires eight minutes more time to reach the earth, when the earth and Jupiter are on opposite sides of the sun, than when they are both on the same side of it. These eight minutes are the time required by the light to travel across the earth's orbit, and as this orbit is 180 millions of miles in diameter, you may find, by an easy calculation, that the velocity of light through space is about 192,000 miles a second. You now see the reason why the flash of a gun reaches you before its report, a subject to which I have drawn your attention in the article on Sound. Like sound, also, light is reflected. All bodies reflect light, but rough bodies scatter it irregularly in all directions. Smooth bodies, such as glass, polished metal, or the surface of tranquil water, reflect it in one certain direction. The law of reflection will be understood from Fig. 1. Let R E be a section of the reflecting surface, say a piece of looking-glass; and let the line A B be B R E L A A FIG. 1. perpendicular to the surface. A beam of light falling upon the surface along the line A B will be reflected back along the same line. But if the beam of light fall obliquely upon R E, like L B, it will be reflected obliquely along the line B L'; and it will be so reflected as to make the angle at i equal to the angle at r. The former of these angles is called the angle Those of you of incidence, and the latter the angle of reflection. who know a little of Euclid will, I am sure, be able to prove that the beam of light falling from L upon B, and reflected from B to L', pursues the shortest path possible. If you can get into a darkened room, into which a sunbeam enters through a small aperture, and try to verify this law for yourselves with a bit of looking-glass, you will be able, I am sure, to obtain a very solid knowledge of the reflection of light. R A SECT. 2. Let us now apply the law of reflection in one or two instances. Suppose that a lady wished for a looking-glass in which she should be able to see her entire figure, and that she asked you how high the looking-glass must be to enable her to do this, what would be your reply? I daresay you would, on the spur of the moment, say that the glass must be as high as the lady; but if she acted on this advice she would be put to very unnecessary expense. Let us try to get at the truth. Let the line A B (Fig. 2) represent the height of the lady, and let R F be a looking-glass of the same height. The light from the lady's eye at A, which strikes the looking-glass perpendicularly at R, is reflected back along the same line, and the lady sees the image of her eye at A' just as far behind the E F B looking-glass as the real eye is before it. The light from the lady's foot at B reaches the eye by first striking the looking-glass |