results from the vibrations of solid bodies, and the air only serves as a vehicle for transmitting them. In wind instruments, on the contrary, when the sides of the tube are of adequate thickness, the enclosed column of air is the sonorous body. In fact, the substance of the tubes is without influence on the primary tone with equal dimensions it is the same whether the tubes are of glass, of wood, or of metal. These different materials simply do no more than give rise to different harmonics, and impart a different timbre to the compound tone produced.

If tubes were simply blown into, there could be no sound; there would merely be a continuous progressive motion of the air. To produce a sound, by some means or other a rapid succession of

condensations and rarefactions must be produced, which are then transmitted to the whole column of air in the tube.

Hence the ne

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Mouth Instruments.

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so shaped that the air enters in an intermittent, and not a continuous manner. From the arrangement made use of to set air in vibration, wind instruments are divided into mouth instruments and reed instruments.

179. Mouth instruments.—In mouth instruments all parts of the mouthpiece are fixed. The pipes are either of wood or metal, rectangular or cylindrical, and are always long as compared with the diameter. Fig. 145 represents a wooden rectangular organ pipe; fig. 146 gives a longitudinal section by which the interval details are seen. The lower part P, by which air enters, is called the foot; it emerges through a narrow slit, and, on the opposite side, is a transverse aperture called the mouth; a and b are the lips, the upper one of which is bevelled.

The current of air arising by the mouth, strikes against the upper lip, is compressed, and by its elasticity reacts upon the current and stops it. This, however, only lasts for an instant, for, as the air escapes at ab, the current from the foot continues, and so on for the whole time.

In this way, pulsations are produced, which, transmitted to the air in the pipe, make it vibrate, and a sound is the result. In order that a pure note may be produced, there must be a certain relation between the form of the lips and the magnitude of the mouth; the tube also ought to have a great length in comparison with its diameter. The number of vibrations depends in general on the dimensions of the pipe, and the velocity of the current of air.

The mouthpiece we have described is used in organs. Fig. 147 represents another modification much in use in organ playing, and fig. 148 gives a longitudinal section. The letters indicate the same parts as in fig. 146. Fig. 149 shows the mouthpiece of a flageolet and whistle. In the German flute the mouthpiece consists of a small lateral circular aperture in the pipe. By means of his lips the player causes the current of air to graze against the edge of this aperture.

180. Reed instruments. In reed instruments the air is set in vibration by means of elastic tongues or plates, which are called reeds, and which are divided into free reeds and beating reeds.

Beating reed. This consists of a piece of wood or metal, a (fig. 151), which is grooved like a spoon. It is fixed to a kind of stopper, K, perforated by a hole, which connects the cavity with a long pipe, T. The groove is covered by a brass plate, /, which is called the tongue. In its ordinary position this is slightly away from the edges of the groove, but being very flexible, readily

approaches, and closes it. Lastly, a curved wire, br, presses against the tongue, and can be moved up and down.

The vibrating part of the tongue can thereby be shortened or lengthened at will, and the number of vibrations thus regulated. By means of this wire, reed pipes are tuned.

The reed is fitted to the top of a rectangular pipe KN, called the wind channel. This is closed everywhere except at the bottom,

where it can be fitted on a bellows. In models of reed pipes used in illustrating lectures, the sides of the upper part of the tube are made of glass, so as to show the construction of the reed. This arrangement is represented in fig. 150.

When air arrives in the wind channel, it first passes between the tongue and the groove, and escapes by the pipe T; but as the velocity increases, the tongue strikes against the edge of the groove, and closing it completely, the current is stopped. But, in virtue of its elasticity, the tongue reverts to its original position, and thus by a

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Bellows.

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series of alternate openings and closings, the same series of pulsations are produced as in mouth instruments; hence is formed a sound which is higher the more rapid the current of air.

the

Free reed. Grenié invented in 1810 a kind of reed called a free reed, because the tongue, instead of striking against the edges of groove, like the reed described above, grazes them so as to oscillate backwards and forwards. The groove consists, in this case, of a small wooden box, ac, the front of which is of brass plate. In the middle of this is a longitudinal slit, in which is applied the tongue, which can oscillate freely backwards and forwards so as to allow air to pass, which it closes each time it grazes the edges of the slit. In this case also a wire, r, regulates the length of the vibrating part of the tongue.

Between the four legs of a table there is a pair of bellows, S (fig. 153), which is worked by means of a pedal, P. D is a reservoir of flexible leather, in which is stored the air forced in by the bellows. If this reservoir is pressed by means of weights on a rod, T, moved by the hand, the air is driven through a pipe, A, into a wind chest, mn, fixed on the table. In this chest there are small holes closed by leather valves, s (fig. 154). These can be opened by pressing on keys, a, in front of the box. Below the valve is a spring, r, which raises the valve when the key is not depressed. The syren or sounding pipe is placed in one of these holes.

182. Nodes and loops.—Experiment shows, that when a pipe is sounded, the column of air is subdivided into equal parts, vibrating in unison, and separated by surfaces where the velocity of air is null. These fixed parts are called nodes; and the parts between the nodes where the column of air is in a state of vibration is called a loop, or a ventral segment.

It will be seen afterwards, that one and the same pipe may be made to yield several sounds, and that the nodes and ventral segments are then displaced. When a pipe closed at one end, a stopped pipe, is made to yield its fundamental sound, that is, the deepest one, the bottom is always a node, and the mouthpieces a ventral segment. An open pipe when sounded has a ventral segment at cacn end; and if it yields the fundamental sound, there is a single node in the middle.

When an aperture is opened in the side of a sounding pipe, the sound does not change if the aperture corresponds to a loop; but if it corresponds to a node, the sound is altered, for this node then becomes a loop. This property is used in wind instruments like the flute, the clarionet, along which holes are made which can be closed by the fingers, or by the aid of keys.

The formation of nodes and loops is far from being restricted to sounding tubes. On strings, plates and membranes, when they vibrate, exhibit parts which are fixed, and parts which are very mobile, that is to say, nodes and loops.

182a. Laws of the vibration of air in pipes.—The vibrations of air in pipes present two cases according as they are open or stopped. Laws of stopped pipes. When having placed a stopped pipe on the bellows, air is slowly passed, the deepest note, the fundamental sound, is produced. If, then, we denote by 1 the corresponding number of vibrations, when the current of air is forced, we suddenly get the sound corresponding to 3; and if the wind be still more