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FLUE-PIPES AND REEDS.
Ill
current of air is forced through the reeds by means of a bellows. In the American organ, the bellows so act, as to form a partial vacuum below the reeds, the external air being thus drawn through them.
The pitch of the sounds, obtained from such reeds evidently depends upon the vibration rate of the reed itself. This again depends upon the size and thickness of the reed, and the elasticity of the material of which it is composed. Harmonium reeds are usually sharpened by gently filing or scraping the free end, and flattened by applying the same operation to the part of the tongue near the fixed end. A rise of temperature, diminishes the rate of vibration, as the tongue expands and its elasticity is diminished. The pitch is also somewhat affected by the force of the wind.
The tones obtained from reeds such as the above, are very rich in overtones. All the series of partials up to the sixteenth, or even higher, may be distinctly recognised in any of the lower notes of the harmonium; in fact, the undue prominence of the higher partials is one of the drawbacks of this instrument.
In order to understand this wealth of partials in reed tones, we must turn back to Chap. Vlll. "We saw there, that Fourier has proved mathematically, that every form of wave may be analysed into a number of simple waves, whose lengths are inversely as the numbers 1, 2, 3, 4, 5, &c. Now it is plain, that the more abrupt or discontinuous the compound wave, the greater will be the number of its constituent simple waves. The compound sound wave resulting from the vibration of a reed, is highly discontinuous; since the individual pulses must be separated by complete pauses during the closing of the apertures. Hence the number of its con­stituent simple waves will be correspondingly large, that is, the compound tone produced by a vibrating reed is made up of a very large number of partial tones. The harder and more unyielding the tongue, and the more perfectly it fits its aperture, the more discontinuous will be the pulses, and consequently the more intense and numerous the overtones.
The compound tone of the reed, being thus overburdened by the intensities of its upper partials, it becomes an advantage to soften these latter, or what comes to the same thing, to strengthen the fundamental, without at the same time strengthening the overtones. This can be done, by placing the reed at the mouth of an open pipe, the fundamental tone of which is of the same pitch as the fundamental tone of the reed. This latter tone will then be greatly reinforced by the resonance of the pipe. The other partial tones of