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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.
620. 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 sther respects to be a conductor of sound. Wood and the earth appear, also, to conductors of sound.
621. 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 comparatively instantaneous in its progress, the distance 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 echo, when addressed at a proper distance; but, if there are trees or other objects to the right or left, the various echoes destroy each other.
622. 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 expulsion 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 modifications of the mouth, tongue, and lips.
Singing is performed by a very delicate enlargement or contraction of the glottis, aided likewise by the mouth and tongue for articulation. 623. The natural music of birds, and the
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, as the pipe, &e.
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.
624. Sound is varied by the rapidity and momentum 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
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.
625. Hence, it was long since found, that if two strings of a harp were of equal lengths, they produced the same tone, or vibrated together, 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 626. It was, afterwards, found, that if one of these strings were accurately biseeted, the vibrations 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 unison or concordance between them: they harmonize or vibrate together for once in the long string or twice in the short
to the ear.
Hence, there is no jarring or discord; but they are said to be in concord; and, in regard to the intervening subdivisions, have been called octaves.
627. 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, 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 fifths 80, or two-thirds.
628. In like manner, another string might be divided, so as to correspond with every fourth yibration 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 5; 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.
629. 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 major sixth, 10 vibrations to 6 (or 5 to 3), 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 vibration, or an exact recurrence of vibrations after the nearest intervals.
630. The strings of a piano-forte, harp, or violin, are brought into accordance or successive octaves, or recurring tones, by the accuracy 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 proper number of vibrations in relation to the other strings.
631. These seven notes, then, are the basis of all music; and, with the addition of five half tones, are the alphabet of music, and fill all the concordant intervals of one octave.
Octaves may, however, rise upon each other in successive ratios or degrees, as in the piano-forte, which has 5 and even 7 octaves; or '5 sets of natural notes as above, and 5 semi-tones, or flats and sharps to each octave.
632. For the purpose of obtaining further variety in composing tunes or melodies, these several notes
may be played shorter or longer; and, in this respect, are divided as under.
2 minums make 1 semi-breve; 2 crotchets make 1 minum; 2 quavers make 1 crotchet; 2 semi-quavers make 1 quaver ; 2 demi-semi-quavers make 1 semi-quaver ; 32 demi-semi-quavers are to be played in the time of one semi-breve.
Again ; in regard to the tune itself, there are also two sorts of time, slow and quick, as common time and treble time.
633. When an agreeable succession of simple notes, having a perfect beginning and ending, is played or sung, it is called a tune, an air, a melody ; as a song, hymn, dance, or march, according to its several purposes.
When these notes, forming an air, are combined with corresponding notes, in different octaves, or on other instruments, and the whole is scientifically made to produce a concordant and agreeable effect, it is called Harmony.
The bass and treble of a piano-forte played at the same time with the left and right hand, constitute the most common practice of harmony.
Some of Handel's pieces have been played by 1000 instruments and voices, all sounding harmoniously together.
Obs.—The human soul may be moved in all its passions by music; and as a soother of the mind, and a source of exquisite pleasure, the practice on some instrument cannot be too strongly recommended as a branch of liberal eduoation te children of both sexes.
66 When thro' life, unblest we rove,
56 Losing all that made life dear,
" In days of boyhood, meet our ear;
" Wakening thoughts that long have slept,