strong induction coil, which, by means of clockwork, gives an electric spark every second, which makes a hole in the blackened paper. By this means the number of vibrations in a second can easily be counted.

Physicists have agreed to understand by the word vibration, a forward and backward movement of the vibrating body, so that a single vibration is represented by a curve of the undulating line, first in an upward, and

then in a downward direction. The body makes an excursion or elongation from its position of rest first to one side, then swings past its position of rest to the other side, and then returns to its position of rest again, after which the movement is periodically repeated in the same manner. The time occupied by the entire vibration is called a period of vibration, and its extent the amplitude of the vibration.

In all musical instruments the tones are produced by some such action as this. A stretched string, which produces the tone in pianos and stringed instruments, vibrates in the manner shown in fig. 56, by springing first to one side and then to the other. A glass plate or a bell sounds when it is struck, in consequence of the vibrations, which its particles make at right angles to its

a

Fig. 56.

surface. Such vibrations, which are made in a direction perpendicular to the length of the body, or better, perpendicular to the direction of the propagation of the vibrations, are called transverse vibrations. Longitudinal vibrations, however, can also produce tones; for instance, metallic rods when struck upon their terminal surfaces, or wooden rods when rubbed in the direction of their length.

In the examples given the tone is caused by the vibrations of a solid elastic body. A tone, however, can be produced directly by vibrations of the air, when, for instance, we blow across the mouth of a hollow ball, of a bottle, or of a hollow cylinder. An instrument which rests on this principle is the mouth-pipe, which can be used in the organ, or as a flute. Fig. 57 represents two such organ-pipes. Air streams up from below into the chamber K from bellows, and is directed by the triangular piece of wood d, through a small cleft c, against the lip a b, by which it is made to blow against th n of air contained in the cylinder R R, and

to vibration. In this action the air is com

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

expansions.

R

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

F

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.

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 as 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

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