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pressed by the first shock, then expands, comes again in contact with the current of air, and is thus made to vibrate periodically, which vibra

tions consist of condensations and

[graphic]

expansions.

The vibrations of the air consist of condensations and expansions, which follow each other with great rapidity, and a tone is always produced if the vibrations follow each other with periodical regularity. A tone can, therefore, be produced if a continuous current of air is interrupted with great rapidity and regularity. An instrument invented by Seebeck, in which this takes place, is called a Siren, and in its simplest form is shown in fig. 58. It consists of a disc, perforated with holes arranged in circles, and which can be set in rapid rotation. Air is blown through the tube B against the holes. The more numerous the holes, and the more rapidly the disc is made to revolve, the higher will be the tone produced. Each puff of air then produces a vibration of the air. The siren affords the means of determining the number of vibrations for any particular tone. If the disc A is provided with four series of holes, the number of which, counting from within outwards, is

Fig. 57.

respectively 8, 10, 12, 16, and if we blow upon them in this succession, we hear the recognised chord, which, starting from the note c, may be distinguished by c, e, g, c.

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20. Dan relave The quality remains the same for every rate of revolution, provided the rate is constant, only the absolute pitch and character of the tones change with the rapidity. If we had a disc which was provided with rows of holes, corresponding to an entire scale, we could then produce perfect melodies upon it.

In this instrument the tone is caused by the sudden expansion experienced by the current of air, which issues from the tube as soon as a hole comes before the opening of the tube, and by its sudden condensation ast soon as the hole is past. This action is then transmitted through the air to our ear in the form of waves of sound.

A more complete form of siren, constructed by Dove, is shown in fig. 59. A current of air is blown through the tube, B B, into the chamber, A A. The latter is provided with a lid, which is perforated by a number of

holes; close above it is a disc s s, which revolves upon a perpendicular axis, and is perforated by the same number of holes in an oblique direction. The holes of

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the lid, also, can be inclined in the opposite direction. The current of air in its passage through the instrument, strikes against the sides of the holes in the disc, and

sets the disc in rotation, the rapidity of which gradually increases. The interruption of the current of air causes a tone, the height of which gradually increases, and on this account produces the unpleasant impression of a howl. Since the number of revolutions can be read off, by means of a toothed wheel with a dial and pointers, t z z, as soon as the toothed wheel is put into gear with the screw, t, by the handle, h, we are then able to discover the number of puffs of air, that is to say, vibrations for each particular note.

There is another method of producing tones which is employed in music, i.e. by means of Reed-pipes, which are used in organs, clarionets, oboes, and bassoons. In the instruments which have been mentioned hitherto, the tone is produced either by a fixed body in a state of

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vibration, a string, a tuning-fork, etc., without any current of air reaching it, or by the interruption of a current of air, without it setting any elastic body in vibration-as in the siren. In reed-pipes, on the contrary, both methods are employed, and together they produce a peculiar note of their own. The solid vibrating body here consists of the so-called Tongue, fig. 60. An elastic strip of

metal z z, is fixed at one end and is allowed to vibrate The tongue

freely in the orifice b b, of a frame a a.
forms one side of a closed tube,
which is open above, and which can
be placed in a wind-chest (fig. 61).
The current of air which is blown
through the lower tube of the wind-
chest can only escape through the
orifice between the tongue and the
frame. The air first presses the
tongue into the tube, and escapes
by the upper opening, to which a
funnel-shaped tube should be fixed
to strengthen the sound. The tongue,
however, by reason of its elasticity,
springs back again, covers the open-
ing, and periodically interrupts the
current of air by its movement to
and fro. A tone is thus produced,
the pitch of which is determined by
the number of vibrations made by
the tongue. In some instruments the
tone can be changed by a moveable
wire, the position of which may be
changed so as to increase or diminish.
the length of the vibrating tongue.

Trumpets and horns are wind instruments, in which the lips of the performer play the part of a tongue.

[graphic]

Fig. 61.

The extended lips are set in vibration, while the tube of the instrument serves as a resonator, and considerably increases the intensity of the tone. There is, therefore,

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