ORGAN (Gr. organon, a contrivance requiring skill on the part of the user of it), a musical instrument played by finger-keys, and in general partly also by foot-keys, and consisting of a large number of pipes of metal and wood made to sound by a magazine of wind accumulated by bellows, and admitted at will by the player. The following description is necessarily restricted to the most furtdamental arrangements of this very complicated instrument. As met with in cathedrals and large churches, the organ com prises four departments, each in most respects a separate instrument with its own mechanism, called respectively the great-orgam, the choir-organ, the swell-organ, and the pedal-organ. Each has its own clavier or keyboard, but the different claviers are brought into juxtaposition, so as to be under the control of one performer. Claviers played by the hands are called manuals; by the feet, pedals. Three manuals, belonging to the choir, great, and swell organs respectively, rise above each other like steps in front of where the performer sits; while the pedal-board by which the pedal-organ is played is placed on a level with his feet. The condensed air supplied by the bellows is conveyed through wooden tubes or trunks to boxes, called wind-chests, one of which belongs to each department of the organ. Attached to the upper part of each Nvind chest is a sound-board, an ingenious contrivance for conveying the wind at pleasure to any individual pipe or pipes, exclusively of the rest. It consists of two parts, an upper board and an under board. On the upper board rest the pipes, of which a number of different quality, -ranged behind each other, belong to each note. In the under board is a row of parallel grooves, running horizontally backwards, corresponding each to one of the keys of time clavier. On any of the keys being pressed down, a valve is opened which supplies wind to the groove belonging to it. The various pipes of each key stand in a line directly above its groove, and the upper surface of the groove is perforated with holes bored upwards to them. Were this the whole mechanism of the sound-board, the wind, on entering any groove, would permeate all the pipes of that groove; there is, in the upper board another series of horizontal grooves at right angles to those of the lower board, supplied with sliders, which can, to a small extent, be drawn out or pushed in at pleasure by a mechanism worked by the drtiw-slops placed within the player's reach. Each slider is perforated with holes, which, when it is drawn out, com pletes the communication between the wind-chest and the pipes: the communication with the pipes immediately above any slider being, on the other hand, closed up when the slider is pushed in. The pipes above each slider form a continuous set of one particular
quality, and each set of pipes is called a stop. Each department of the organ is supplied with a number of stops, producing sounds of different quality. The great-organ, some. of whose pipes appear as show-pipes in front of the instrument, contains the main body and force of the organ. Behind it stands the choir-organ, whose tones arc less powerful ind more fitted to accompany the voice. Above the choir-organ is the swell-organ, whose pipes are inclosed in a wooden box with a front of louvre-beards like Venetian blinds, which may be made to open and shut by a pedal, with a view of producing crescendo and diminuendo effects. The pedal-organ is sometimes placed iu an entire state behind the choir-organ, and sometimes divided, and a part arranged on each side. The most usual compass of the manuals is froin C on the second line below the bass staff to D on the third space above the treble staff; and the compass of the pedals is from the same C to the D between the bass and treble staves. The real compass of notes is, as will be aeeu, much greater.
- Organ-pipes vary much in form and material, but belong to two great classes, known as mouth-pipes, (or flute-pipes) and reed-pipes. A section of one of the former is represented in the figure. Its essential parts are the foot a, the body b, and a flat plate c, called the language, extending nearly across the pipe at the point of junction of foot and body. There is an opening, de, in the pipe, at the spot where the language is -discontinuous. The wind Amitted into the foot rushes through the narrow slit at d, and, in impinging against e, imparts a vibratory motion to the column of air in the pipe, the result of which is a musical note, dependent for its pitch on the length of that column of air, and consequently on the length of the body of the pipe: by doubling the length of the pipe, we obtain a note of half the pitch, or lower by an octave. Such is the general principle of all month-pipes, whether of wood or of metal, subject to considerable diversities of detail. Metal pipes have generally a cylindrical section; wooden pipes a square or oblong section. A mouth-pipe may be stopped at the upper end by a plug called a tompion, the effect of which is to lower the pitch an octave, the vibrating column of air being doubled in length, as it has to traverse the pipe twice before making its exit.