Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/895

ORGAN 829 forms the top of the grooves. closed below with leather, except the opening left in each, which is closed by the key-valve or pallet. The sliders are connected with the draw-stops or stop- handles, which are covered in with The grooves are generally FIG. 2. A section of a groove, with the table, wimlchest, and pallet. c stout upper boards, on which the pipes stand. The stop- handles are pulled out, and holes are then bored straight down through the upper boards, sliders, and table to admit the wind from the grooves to the pipes. When the sliders are shifted by pushing in the handles, the holes no longer correspond, and the pipes are silenced. Pipes are divided first in- Fm . 3 ._ A bcc t ion at right angles to to flue-pipe* and reed-pipes. ng- - Flue-pipes are blown by a wind mouthpiece characteristic of the organ, while in reed-pipes the wind acts on a metal tongue vibrating on a reed, and the motion of the tongue determines the speech of the pipe. Pipes are made either of wood or of metal. Wood flue-pipes are generally of the form of a rectangular parallelepiped, metal flue- I pipes of a cylindrical <s Via IIP T?pprl iiinp arp FIG. 4. A portion of the table as it appears fenape need -pipes are from abo ^ with the ]i]aces for the gg-^ Conical Or pyramidal, and of the stops; the small circles show the widen towards the top. holes for tlie wiml - Some flue-pipes are made with stopped ends ; these as a rule sound a note about an octave lower than the corre sponding open pipes of the same length. Such are the stopped dia pason, bourdon, and stopped flute. The general ele mentary theory of the resonance of a pipe is tolerably simple. The effec tive length of the pipe is determined by measuring from the upper lip to the open end in open pipes, and from the upper lip to the stopper and back again in j rp Fio. o. <i, an open diapason ; b, a stopped diapason ; Stopped pipes. 10 C; an oboe ; and if, a trumpet, c and ct being forms this is added an of reed-pipes. allowance for the effect of each opening, since the condition of perfect freedom from constraint does not subsist at the opening itself. The corrected length is traversed twice (backwards and forwards) by sound, in the time of one vibration of the resultant note. This describes in a rough and general manner the way in which any disturbance gives rise to the note of the pipe ; but the theory of the mouth- Fio. 0. Mouthpieces in somewhat greater detail. pieces is a much more difficult matter, into which we cannot here enter. In reed-pipes which are simply conical the resonance of the body is nearly the same as that of an open pipe of the same length. Where the form is irregular no simple rule can be given. But the resonance of the body of the pipe is generally the same as the note produced. The tongue of a reed-pipe alter nately opens and closes the aperture of the reed. In this way it admits pulses of wind to the body of the pipe ; these, if they recur at the proper intervals, maintain its vibration, which takes place when the note produced cor responds to the resonance of the pipe. The reed itself has its vibrating length determined by a wire which presses against it. The free end of this wire is touched with the tuning tool until a satisfactory note is produced. The pitch of the different stops is commonly denoted by Foot- the conventional approximate length of the pipe sounded length, by C, the lowest key of the manual. Even in incomplete ltch) stops which have no bass, the length of the pipe which C would have if the stop were extended down serves to indicate the pitch. The conventional length of the C-pipe for stops having the normal pitch of the keys is 8 feet ; a pipe having twice this length sounds the octave below, a pipe having half that length the octave above, and so on. Thus stops which sound the octave below the normal pitch of the keys are spoken of as 16-foot stops. Even where the pipes are stopped so that the actual length is only 8 feet, they are spoken of as having "16-foot tone." Similarly 32 -foot stops sound two octaves below the normal pitch of the keys. But if these notes are produced by stopped pipes, whose actual length is only 16 feet, they are spoken of as having " 32-foot tone." Sixteen-foot and 32-foot stops are specially characteristic of the pedal, where the names also signify the length of the open pipe which would sound the note actually produced by the lowest C. In old organs, where the modern compass was not adopted, it was not unusual to find stops spoken of as of 12 feet or of 24 feet. In these cases the lowest note was frequently F. Old English organs, however, more often had G for their lowest note. The designation of the stops in these cases had be come rather anomalous, and need not be entered into. Of stops higher than the normal pitch of the keys, the octave is denoted by 4 feet if made with open pipes, 4-foot tone if stopped ; the twelfth is commonly spoken of as 2, the fifteenth or double octave as 2 feet. Higher-sounding stops are occasionally used, but these generally form part of "mixtures," and the foot-lengths of the separate ranks are not usually given. The true or accurate lengths of the pipes vary within considerable limits. The base of the scales (dimensions) varies according to the standard of pitch, and the voicing and the complicated natural laws of pipes produce other deviations from simple relations, so that the conventional dimensions can only be regarded as a simple means of classifying the stops according to their pitch -relations. For this purpose they are essential ; they are continually appealed to in discussion and description ; and they are almost invariably marked on the stop -handles in all countries, so that a moderate knowledge of foreign nomen clatures, combined with the habit of seizing the meaning