Organ pipes are of two general descriptions, MOUTH PIPES, and REED-PIPES : each of these are of several different species.
Mouth-pipes are either of wood or metal, which differ something in shape, but which are formed on the same principle.
A wooden mouth-pipe consists of a right angled tube, Plate CCCCXLVI. Fig. 14. AA, BB, formed by four plane boads glued together, and usually sized with glue on the inside. The cross section of the pipe is not square, but oblong, one side being to the other as 5 to 4. The pipe is at one end, either quite open, or closely stopped, as at AA, by a plug or tomnion C, Fig. 15 co vered with leather, so as to be quite air-tight. The other end of the pipe is closed by the block- D, on which three of the walls are glued, except at the narrow aper ture across the liont wall, called the mouth a. The mouth is formed by the front Ivall being cut off a little above the upper part of the block, and at the same time bevelled from the front surface at b, so as to form a sharp edge with the back or inner surface of the wall at a, called the upper lip or wind cutter. The side walls and back wall of the pipe extend beyond the block D, and again embrace another block, leaving a narrow space between the two c. This lowest block is pierced to re ceive the foot E, which is a hollow cylinder open at both ends, glued into the lowest block, and forme(' at the othei end to be received inco a hole in the upper board, to be afterwards described. The cap F, Fig. 17. is a piece of mahogany, or other hard wood, somewhat thicker than the walls of the pipc, wnich covers that part of the front of the pipe from the mouth downwards. It is hollowed out, lea% ing edges to be glued upon those of the side walls, and upon the block into which the foot is inserted, but leaving the aperture c quite free. The up per edge of tile hollow c is a straight line, but retired a little front the level of the edges, so that when the cap is applied, there is a thin aperture between it and the block D, called the plate of wind. From the line e, the inner part of the cap is formed into a curved surface, so that it grows considerably deeper as it descends, leaving the greatest interval opposite the opening c. When
therefore the cap is glued on, Fig. 13. and the pipe is set in its place, the wintl is blown into the aperture d, passes between the block and the cap, and is lorced in a thin stream against the upper lip or wind cutter a; and being thus thrown into a vibratory motion, communicates this motion to the column of air within the pipe, and makes it sound ; just as the drawing the bow across the strings of a violin causes them to vibrate, and conse quently, to speak their respective notes.
To produce this effect, however, at least in the hest manner, so as to give a good quality of sound, the pipe must bc properly voiced. We have described the curved wedge-shaped hollow in the cap, and the thin aperture called the plate of wind. Now, in voicing the pipe, be fore the cap be put on, the upper. corner, or edge of the block, is slightly pared away, opposite the upper lip a, so as to direct the wind against the edge of it, or rather more inwaidly. Experience teaches the degree in which the corner is to be pared off. If it be done too much, or too little, the pipe will not speak, or will speak ill, or will be slow to speak. At the same time the up per part of the block is, by the edge of an angular file, toothed by thin cuts or lines running parailel to each other, but not at right angles to the under lip; deepest and widest at the mouth, and wearing gradually out as they recede from it, till they are lost in the plane sur face of the block. This voicing is shown fn Fig. 14. The use of it is to carry smoothly, and at the same time to divide the wind in its passage into the pipe. If the plate of wind were suffered to strike against the upper lip in an entire body, the tone would be imperfect, and accompanied with a disagreeable chirping at the com mencement of the pipes speaking. The nice voicing, therefore, is of capital importance, and on the manner of doing it depends the quality of the organ.