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ORIGIN OF THUNDER.

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since the time of Franklin no one has ever doubted that the same agent is operating in both cases. We are accustomed to experiment on our laboratory tables with electricity distributed over a few square feet of tin-foil; in the atmosphere there are sometimes thousands of acres of electrified cloud. When the difference of quantity and also of intensity in the two cases is considered, there can be no surprise at the difference in amount, or violence of the effects produced by lightning over those resulting from common electricity. But although there is so evident an identity between many of the effects of ordinary and atmospheric electricity, there is one in which they have a great dissimilarity. When a Leyden jar or battery is discharged, a sharp, sudden noise is produced, which might perhaps be properly called a snap. When the atmospheric electricity is discharged, a deep rolling sound follows, best described by the expressive word thunder. The cause of this is readily explained. During a thunder-storm the atmosphere is generally loaded with dense clouds. Now supposing the sound to be produced from one point, and to be a single crash, as it diverges in every direction there will be repeated reflections, and the sound will be reverberated from some surfaces that are near, and others that are far distant, giving to the explosion that continuous, rolling sound, by which it is so peculiarly distinguished.

Sir John Herschel has proposed a very curious explanation of thunder, to which we cannot give consent. "Let us conceive," he says, "two flashes of lightning, each four miles long, both beginning at points equidistant from the auditor, but the one running out in a straight line directly away from him, the other describing an arc of a circle having him in its

centre.

Since the velocity of electricity is incomparably greater than that of sound, the thunder may be regarded as originating at one and the same instant in every point of the course of either flash. But it will reach the ear under very different circumstances in the two cases. In that of the circular flash, the sound from every point will arrive at the same instant, and affect the ear as a simple explosion, with stunning loudness. In that of the rectilinear flash, on the other hand, the sound from the nearest point will arrive sooner than from those at a greater distance; and those from different points will arrive in succession, occupying altogether a time equal to that required by sound to run over four miles, or about twenty seconds."

In this theory it is supposed that the sound follows the flash, and, diverging in every direction, must of course reach an auditor on the surface of the earth. If this supposition be true, there can be no doubt of the accuracy of the theory, The sound however is, we imagine, produced at the point of discharge, and from that diverges, as when a bell is rung, or a cracker is exploded. The electricity, in its passage, whether in a direction away from the auditor, or in an arc of a circle around him, will condense the atmospheric air; and, unless that be supposed the origin of new sounds, we cannot conceive how Sir John Herschel's supposition can be substantiated. We may err altogether in our conception of his theory, but it appears to us that the sound cannot be produced at any other than the point of discharge, and from this diverges in every direction, suffering reflection from every point, causing that roll which is the peculiar character of the sound. The reflected sound can never arrive first, as some

ECHOS IN BUILDINGS.

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persons have imagined; for if that could happen, it would be no longer true, that two sides of a triangle must necessarily be greater than the third. The greatest intensity may be at one or another place, according to circumstances; but this cannot, so far as we can understand the question, depend on the length or direction of the flash.

The application of the principles of reflection has been thought of great importance in the construction of buildings. All places intended for public speaking and the performance of music, should be constructed in that form, and with those provisions, known to give a ready passage to sound without the interference of reverberation. In small apartments the form is of but little importance, so far as regards the production of echo, for the incident and reflected sounds so rapidly follow each other, that there is no perceptible interval between them; in fact, they are brought to the ear as a single sound. In large buildings, on the other hand: "In churches, theatres, and concert-rooms," says the author to whom we have already frequently referred, "the echo is heard after the principal sound has ceased; and if the building is so constructed as to return several echos in very different times, the effect will be unpleasant. It is owing to this, that in cathedrals the service is usually read in a sustained, uniform tone, rather that of singing than speaking, the voice being thus blended in unison with its echo. A good reader will time his syllables, if possible, so as to make one fall in with the echo of the last, which will thus be merged in the louder sound, and produce less confusion in his delivery.

It is very difficult to direct the architect in the construction of a building best suited for sustaining sound. One effect

should certainly be sought, that of obtaining reflection; and one should be avoided, that of an echo from one sound blending with a note of a different pitch.

Everything that can stifle a sound should be avoided. Windows, deep recesses, carpets, and curtains, are in every respect injurious to the propagation of sound. They have the effect of preventing reflection, which should always be promoted, as a means of increasing intensity. Particular forms have been sometimes recommended as fit to reflect sounds; but if the laws governing the reflection of sound, are the same as those which influence the direction of light, they can be of little service. It would be easy to arrange reflecting surfaces of a particular form, in such a manner, that the speaker being in one focus and the hearer in the other, the sound would have great intensity; but under ordinary circumstances it is required to convey sound of great intensity over the whole of a building, and not to concentrate the effect upon any one point.

But while the architect aids the reflection of sound, he must be careful to prevent the possibility of an echo. This is especially necessary in concert-rooms, for as a number of notes may be struck in a short interval of time, it is possible that the echo of one may interfere with the original sound of another, and a constant discord would in this case afflict the ear of an auditor.

CHAPTER IV.

NOISES AND MUSICAL SOUNDS.

THE readiness with which the ear detects the varieties of sound is very remarkable. There are numerous instances in which the sensation discriminates, without putting us into possession of any means by which to communicate to another person the differences so readily detected by us. A friend is known or identified by his voice, and yet it would be impossible so to describe it, except in very remarkable instances, as to give a third person the means of distinguishing it from others. The Indian hunters are said to hear, at great distances, sounds that have so small an intensity as to be altogether inaudible to inexperienced ears, and can frequently determine what is the animal by whose step the sounds are produced. We have been frequently struck with the great facility with which we, in common it is imagined with all who have for any length of time resided in the metropolis, can detect, during the night, the nature of the vehicle that is passing; and especially how suddenly one is roused, even from a state of slumber, by the peculiar and yet indescribable roll of a fire-engine.

The eye is justly said to be the most excursive organ, but its liability to deception bears a large proportion to the extent of external influence. By the organ of sight we are admitted to a communion with nature of a more intimate character

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