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FLUE-PIPES AND REEDS.
99
As the type of instruments of this class, we may take an ordinary organ pipe. Such pipes are constructed either of metal or wood; the former being an alloy of tin and lead, or for large pipes, zinc; the latter, pine, cedar, or mahogany. Fig. 50 represents a wooden, and fig. 51 a metal pipe, both in general view and in section. They may be closed, or open at the upper end. Fig. 52 shows an enlarged section of the lower part. The air from the wind chest enters at (a) and passes into the chamber (c), the only outlet from which is the linear orifice at (d). The air rushing from (d) in a thin sheet, strikes against the sharp edge (e), and the column of air in the pipe is set in vibration. The precise way in which this sheet of air acts is not quite clear. Helmholtz says '' The directed stream of air breaking against the edge, generates a peculiar hissing or rushing noise, which is all we hear when a pipe does not speak, or when we blow against the edges of a hole in a flat plate instead of a pipe. Such a noise may be considered as a mixture of several inharmonic tones of nearly the same pitch. When the air chamber of the pipe is brought to bear upon these tones, its resonance strengthens such as correspond with the proper tones of that chamber, and makes them predominate over the rest, which this predominance conceals." On the other hand, this thin sheet of air has been compared by Hermann Smith to an ordinary reed, and called by him an " aeroplastic reed." His theory is, that in passing across the embouchure (ed) the aeroplastic reed momentarily produces an exhaustive effect tending to rarify the air in the lower part of the pipe. This, by the elasticity of the air, soon sets up a corresponding compression, and these alternate rarefactions and condensations reacting upon the lamina, cause it to vibrate, and to communicate its vibrations to the air within the pipe.
The pitch of the fundamental tone given forth by a pipe, depends upon its length; the longer the pipe the deeper the note. The reason of this has been already fully explained in Chapter VII. To recapitulate what is there stated and proved: The vibration number of the sound produced by an open pipe, may be found, by dividing the velocity of sound by twice the length of the pipe; that of a stopped pipe, by dividing by four times its length. In the latter case the internal length must be measured, as "length