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see the flash of a gun until we hear the report, we shall have a pretty correct estimate of the distance of the gun from us. The same principle applies to thunder and lightning. These also are really simultaneous, but we usually see the lightning first. If the thunder-cloud is far off, there is a long interval between them, because the sound takes long to reach us. But when the peal follows close upon the flash, the thunder-cloud must be near, and is therefore dangerous.

As the elasticity of air is reduced, it conveys sounds more and more feebly. Thus, on the top of a high mountain one must, as it were, try to speak louder than usual, in order to be distinctly heard. It is remarkable, however, that it is only the intensity of sound which is thus diminished in a rarefied medium; its velocity remains the same. It is also worthy of notice, that, in the same medium, sounds of all kinds are propagated with equal speed. The report of a cannon travels no faster than the soft notes of a guitar.

Though air is the most common, it is by no means tho best conductor of sound. The velocity of sound-waves in water is not much less than a mile per second. In elastic solids, they travel still faster; in glass and steel, for example, upwards of three miles per second. Sounds are also heard louder when conveyed to the ear by solid media. Thus, the scratching of a pin at one end of a long piece of timber, may be heard by an ear applied near the other end, though it could not be heard at the same distance through the air. On the same principle, the American Indian stoops with his ear close to the ground, in order to catch the sound of distant footsteps. If a piece of ribbon or garter be tied round the top of a poker, and the ends pressed one on each ear by a finger, while the poker hangs freely down, a gentle knock on the lower end of the poker will be heard like the tolling of a large bell. This is an experiment which any boy may try, and which is sure to astonish him.

When a sound-wave strikes against a wall, a mountain, or any other obstacle, it is reflected, and produces what we call an echo. If it then meet another obstacle, it will be reflected a second time, and this will give rise to a double echo. In certain peculiar situations, especially in mountainous countries, this process is repeated again and again, and the effect is sometimes most fantastic and surprising. The clouds themselves reflect sound, and produce indistinct echoes. It is to this that we owe the rolling of thunder, which is generally nothing more than the reverberation of a single sound, reflected from the clouds to the earth, and from the earth back again to the clouds. Hence the grandeur of a thunder-storm among mountains, where each mountain repeats the sound, till it gradually dies away. This is beautifully described by Lord Byron, as witnessed by him in the Alps:

"Far along,
From peak to peak, the rattling crags among,
Leaps the live thunder; not from one lone clond,
But every mountain now hath found a tongue,
And Jura answers, through her misty shroud,
Back to the joyous Alps, who call to her aloud."

MUSICAL SOUNDS.

Ir it be wonderful that a vibratory motion, when communicated to the ear, should give us the sensation of sound, how can we sufficiently admire the still more wonderful variety of that sensation! All sounds, as we have seen, travel through the same medium with equal velocity, but there must be some difference between the sound-waves of a whisper, a shriek, a huzza, and a peal of thunder. The nature and causes of that difference have not yet been fully ascertained. There is, however, one great and leading distinction between different sounds which admits of a simple and most remarkable explanation. Almost every one knows what is meant by a musical sound, and can distinguish it from a mere noise. If there be some who cannot, it is impossible to help them by a verbal description, for the difference cannot be expressed in words. Now one cause at least of this difference lies in the uniformity and regularity of the vibrations by which the sound is produced.

Suppose a card to be held close to the circumference of a toothed wheel which is made to revolve slowly, each tooth striking the edge of the card as it passes. The sound heard will be a sort of rattle, consisting of a series of taps or beats, all of the same loudness, and succeeding each other at equal intervals. But if the wheel be made to revolve with sufficient rapidity, the ear will no longer be able to distinguish the separate taps, and the sound will therefore be continuous. The reason is that the motion communicated to the organism of the ear by any particular tap, bas not time to subside before the next impulse arrives. As the taps, however, though no longer heard separately, are still regular and equal, the result will be a musical sound. The same singular fact is illustrated by a little experiment which boys often perform, without thinking of the instruction to be gathered from it. Who has not tried to produce a screak by holding his slate-pencil upright between his fore-finger and thumb, and running the point of it along his slate? Well, try it again, and notice more particularly what happens. First, hold the pencil loosely, and you will find that it does not screak, but simply rattles, the point jumping along, and making a dotted line as it goes. Now, press gently, and the dots become closer, the taps succeed each other more rapidly. Increase the pressure, and the separate taps will no longer be distinguishable. The sound will be a continuous one, certainly far from agreeable, but nevertheless musical. Its intolerable harshness may perhaps be partly owing to a want of perfect regularity in the motion of the pencil. We do not know all the causes which make a sound pleasing or displeasing to the ear. But this at least is certain, that all vibrations, in proportion as they are regular, uniform, and equal, produce sounds proportionably more agreeable and musical.

Musical sounds differ among themselves in various respects, and to their skilful combination we owe all those charming effects of melody and harmony which constitute music. Some of their differences cannot be easily explained, but one, the difference of pitch, has been shown to depend solely on the number of vibrations which strike the ear in a given time. It is this which determines whether a note is high or low in the musical scale. The reader is supposed to know what is meant by the pitch of a note, and to be able to distinguish between a high note and a low one. But it may be well to remind him, that a high note is not necessarily loud, pitch being altogether different from loudness or intensity. Now all notes of the same pitch, whether loud or not, are found to be produced by vibrations succeeding each other with the same rapidity. If the rate of vibration be increased, a higher note is the result.

Let us return for a little to the experiment of the card and wheel. The faster the wheel revolves, the higher will be the sound produced, and, by properly regulating the rate of revolution, we may obtain all the notes of the gamut from this simple apparatus. As the beat of each tooth against the card causes a separate vibration, and the number of such beats in a second can be easily ascertained, it is quite possible in this way to find how many vibrations per second are necessary to produce a sound of any given pitch. The result of such an inquiry need not here be given in detail. But it is interesting to know that doubling the rate of vibration makes the sound an octave bigher, and, in general, that all the notes of the scale correspond to rates of vibration bearing a simple numerical proportion to each other. The lowest note on a large piano-forte is produced by vibrations at the rate of 27 per second, the highest by vibrations at the rate of about 3500 per second. Strings vibrate with more or less rapidity according to their length and tightness. Hence it is that the violin player, by properly tuning his instrument, and placing his fingers on the strings, so as to vary the length of the vibrating portions, can produce such a wonderful variety of sounds. For a similar reason the pipes of an organ are made of unequal length, and the length of a flute is virtually altered by opeving or shutting the holes. The shorter a pipe or string is, the more rapid are its vibrations, and the higher its note. Most people fancy that the hum of a bee is, like the

song of a bird, a vocal sound. But the fact is, that it is produced by the creature's little wings, whose constant flapping throws the surrounding air into vibration. The same thing is true of other insects. The wings of a gnat, for example, give forth a note more than two octaves higher than the highest note of a good piano-forte, from which it follows that they must flap at the rate of 15,000 times per second! What an astounding discovery—how little to be expected-how difficult fully to realize—and how well calculated to fill our minds with admiring reverence for the Great Author of nature, who has made the least, as well as the greatest of His works to show forth His praise !

QUESTIONS FOR EXAMINATION.

Give examples of wave motion. In what respect is it deceptive ? Describe the nature of the motion which produces sound. At what rate does sound travel in air? in water? in glass? How may we know the distance of a thundercloud? Show that solids convey sound better than air. Explain the phenomena of echoes. What is necessary to the production of a musical sound ? Give two examples of a musical sound produced by a rapid succession of taps. What determines the pitch of a note? How may the rate of vibration corresponding to any note be ascertained? Give examples of different rates, Why has a flute holes? Why has an organ pipes of different length?

THE FIELDS.

CONSIDER what we owe merely to the meadow-grass, to the covering of the dark ground by that glorious enamel, by the companies of those soft, and countless, and peaceful spears. The fields! Follow but forth for a little time the thoughts of all that we ought to recognise in those words. All spring and summer is in them—the walks by silent, scented paths -the rests in noonday heat—the joy of herds and flocksthe power of all shepherd life and meditation-the life of sunlight upon the world, falling in emerald streaks, and falling in soft blue shadows, where else it would have struck

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