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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 treuble, when they pro. duce sound, is evident in drums, bells, and other instru. ments, whose vibrations are distinctly perceptible; and it is equally clear, that a similar vibration is excited in the nir; because bells, glasses, basins, and musical strings, will sound, merely by the action propagated from other souvding bodies, and will not sound in a vacuum.

614. The vibrations that produce sound, bave 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, end whispering galleries, and repeating caves, like so many concave mirrors.

004.--- Every building standing alone, in 'an echo, when nadressed at a proper distance, but, if there are trers or other objects to the right or left, the various echoes de stroy each other.

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

The human voice in produced by the expulsiou of air froni the lungs, and by the vibrations excited in that nir, lng a very small membrane called the glottis, in its passage through the trachea or windpipe; and by the subtle modifi cations of the mouth, tonglie, and lips.

Singing is performed by a very delionte enlargement or contraction of the glottie, nider likewise by the mouth and tongue for unlieulation.

017. 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-inistriments, 18 the pipe, &c.

in stringed instruments, the air is struck by the string, and the vibrations of the air produce 1 corresponding sound in the car; but, in pipes,

the air is forced against the sides by the breath, and its vibrations or toues are produced by the reaction of the sides.

01. Sound in varied by the rapidity and me: mentum of the vibrating body and this depeudo on the length, tension, and size of the string,

A short string vibrates quicker thau a long one, and therefore produces the sharpent and highest tenen i and a short and small pipe, trom a live causeproduces sharp notes: ww large pipes, prave and deep ones,

Savagen discovered this and they made, and atill make, instruments which please themselves and their wild companions, but art and science an 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 we strings of harp were of equal lengths, they produced the same tone, or vibrated toge ther, or in unexom.

They produce the same number of vibrations exaetly in the same time, their vibrations, if struek together, accord; benee, they produce the same sound to the car.

(90. It was, alterwards, found, that If one of these strings were acurately bisceted, the vibration, 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 oor: responds with every vibration of the whole oue, there is a constant wnosno or aumeerdiner botween them; they harmonise or vibrute together

for once in the long string or twice in the short olie,

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

021. 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 increased 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. whichi, while the original vibrated 2, the next should vibrate 8; this was done ; and this note, which is two-thirds of the original, is called a Sith,

Obs.--If, then, the original atrine wn. 120 parts, the octuve would be 00, and the Njihu moi, or two-thirds,

622. Iu like manner, 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, aud is called a fourth.

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

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-corus), the octare will be two vibrations to 1, or

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 i, or 86 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 object of which, is to produce the greatest variety of tones with unisons of vibiation, or an exact recurrence of vibrations after the nearest intervals.

1 624. The strings of a piano forte, harp, or violin, are brought into accordance or succes sive 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 ihe 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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