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XXVIII. Acoustics and Music.

613. Sound is an effect of vibration, and is produced by diverging waves of the air. This is evident, from the vibration of stringed instruments; and from the effect on Water in musical glasses.

Sound, like heat, appears to depend on the reflection of the surrounding bodies, and also on the density of the air,

Aeronauts can scarcely hear each other speak, when high in the atmosphere; and the discharge of a pistol from an air-balloon produces scarcely any report, for want of reflecting bodies.

Obs. That bodies move, or tremble, when they produce sound, is evident in drums, bells, and other instruments, whose vibrations are distinctly perceptible; and it is equally clear, that a similar vibration is excited in the air; because bells, glasses, basins, and musical strings, will sound, merely by the action propagated from other sounding bodies, and will not sound in a vacuum.

614. The vibrations that produce sound, have been aptly compared to the circles produced by throwing a stone into the water; but judging by their effect on the water in a musical glass, the undulations are more pungent and decided.

A bell rung under water, gives the same tone as in air; and water is known in other respects to be a conductor of sound. Wood and the earth appear, also, to be conductors of sound.

615. Sounds, or their undulations, are found to travel at the rate of 1142 feet in a second, or about 13 miles in a minute.

Hence, as any corresponding light is compa

ratively instantaneous in its progress, the dis tance of the 'report of thunder, or of a piece of cannon, may be exactly calculated.

Sounds also are reflected like light; and hence we have echoes, which are like plain mirrors, and whispering galleries, and repeating caves, like so many concave mirrors.

Obs. Every building standing alone, is 'an écho, when addressed at a proper distance; but, if there are trees or other objects to the right or left, the various echtes destroy each other.

616. Speaking trumpets confine and give a limited direction to sound, independently of the mechanical effects of their reflection.

The human voice is produced by the expul sion of air from the lungs, and by the vibrations excited in that air, by a very small membrane called the glottis, in its passage through the trachea or windpipe; and by the subtle modifi cations of the mouth, tongue, and lips.

Singing is performed by a very delicate enlargement or contraction of the glottis, sided likewise by the mouth and tongue for alienlation.

617. The natural music of birds, and the power of singing or producing agreeable notes. by the human voice, led, in the course of ages, to the contrivance of stringed instruments, as the harp, lyre, &c.; and to the invention of wind-instruments, us the pipe, &c.

In stringed instruments, the air is struck by the string, and the vibrations of the air produce a corresponding sound in the ear; but, in pipes,

the air is forced against the sides by the breath, and its vibrations or tones are produced by the re-action of the sides,

618. Sound is varied by the rapidity and mo« mentum of the vibrating body; and this depends on the length, tension, and size of the string.

A short string vibrates quicker than a long one, and therefore produces the sharpest and highest tones; and a short and small pipe, from a like cause, produces sharp notes; and large pipes, grave and deep ones,

Savages discovered this; and they made, and still make, instruments which please themselves and their wild companions; but art and science go further; they ascertain the causes of their pleasure, and direct them so as to increase it.

619, Hence, it was long since found, that if two strings of a harp were of equal lengths, they produced the same tone, or vibrated toge ther, or in unison.

They produce the same number of vibrations exactly in the same time; their vibrations, if struck together, accord; hence, they produce the same sound to the ear.

620. It was, afterwards, found, that if one of these strings were acurately bisected, the vibra tions became half the length of the vibrations of the whole, and the note twice as acute; but as every other vibration of the half string corresponds with every vibration of the whole one, there is a constant waison or concordance bes tween them; they harmonize or vibrate together

for once in the long string or twice in the short

one.

Hence, there is no jarring or discord; but they are said to be in concord; and, in regard to intervening subdivisions, have been called oclaves.

621. But as a harp, composed of strings of only two lengths, would produce little variety of sound, it was justly considered, that if other strings could be contrived, whose vibrations corresponded even with less frequency than the octave, the compass and variety would be in

creased without discord.

Hence, as the number of vibrations of a string is 1, while that of its octave is 2; the next best division would be, to produce a string, which, while the original vibrated 2, the next should vibrate 3; this was done; and this note, which is two-thirds of the original, is called a fifth.

Obs.--If, then, the original string was 120 parts, the octave would be 60, and the fifth, 150, or two-thirds.

622. In like manuer, another string might be divided, so as to correspond with every fourth vibration of the original; and this would be of three-fourths of its length, or 90 parts of 120, and is called a fourth.

So on with others, whose vibrations accord 5 for every 4, and 6 for every 6; also 5 for every 3, and 5 for every 8, till seven melodious or according vibrations are made of the original chord.

A harp, constructed of strings, divided in this manner, produces an agreeable melody; the vibrations according and agreeing with one another at equal intervals, although the tones are different.

623. If a string consists of 120 parts (inches, or barley-corns), the octave will be two vibrations to 1, or 60 parts of 120.

The fifth, 3 vibrations to 2, or 80 parts.

The fourth, 4 vibrations to 3, or 90 parts. The major third, 5 vibrations to 4, or 96 parts.

The minor third, 6 vibrations to 5, or 100 parts.

The major sixth, 10 vibrations to 6 (or 5 to or 72 parts.

And the minor sixth, 16 vibrations to 10 (or 8 to 5), or 75 parts.

Obs. These divisions of a string, constitute the diatonic scale; the sole and simple objeet of which, is to produce the greatest variety of tones with unisons of vibiation, or an exact recurrence of vibrations after the nearest intervals.

&F+ 3* } 624. The strings of a piano forte, harp, or violin, are brought into accordance or successive octaves, or recurring tones, by the accu racy of the ear.

In the harp, &c., their lengths are exactly proportioned to the scale by the maker; but as the strings vary in their tension, owing to the weather and other causes; and as they cannot all have the same precise bulk, it is necessary, from time to time, to tune them; which means nothing more, than to make each perform its

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