Quints, twelfths, and sesquielleras, all take the sizes and lengths of their proper tones in the open diapason. Trumpet and Oboe stops, being what are called unison stops, also take their lengths from the open diapason. The clarion, an octave stop, is measured by the principal. The belle of all should be as large as their places in the organ will admit of.
Middle c in the dulciana is an inch and a quarter in diameter, and about two feet two inches long. A large size stop-diapason is seldom pure and musical. An inch and eighth wide and an inch and three eighths deep, with thirteen inches in length, is a good scale for a middle c pipe in wood. Scales may be taken of larger or smaller dimensions, if desired, in which case the larger must be shorter and the smaller must be longer. Where stops are repeated, two or three of the same name, it is thought better to have them of different diameters, from an opinion that two or more unison-pipes of the same diameter affect the ear only as one pipe. There is something In this opinion, but how much we will not venture to determine.
Fig. 5 is a section of a showing some part of the mechanism. A is the by which the pipes are held in upright position ; a shows the ends of the slides ; c, that part of the sound board which contains the channels for conveying the wind to the pipes; D, the pallet or valve which is opened by the finger of the organist, through the key, the lever, the pull-down, and tho roller (see Fig. 7); E is the spring which keeps the pallet in its place when not in use ; F is used by the organist in changing a stop or elide ; A is the draw-stop ; a is the trundle ; C is the lever ; D is the slide. The draw-stop is linked to the trundle at E, the trundle is linked to the lever at F, and the lever is fitted to the slide at o. Their centres are all at n. In con nection with the above, there are also pedals called composition pedals, three, four, and five to an organ.
Fig. 7 represents certain small parts of the action. A is the wire by which the pallet is opened ; n is the pull-down attached to it; a is the pallet-end of the roller, with its fixing to the board ; D is an end view of a roller, with a pullslown and lever, all in their relative connection ; r is the pull-down, and r is the lever, to which the key follows ; a is a common form of a vinare or centre, by which the trackers are united.
7'ruckers are pieces of pine wood, I of an inch wide, and from d to a j thick ; they are of different lengths, and may be used to any extent The Binningham organ is said to have four miles and a half of trackers in it. In the cathedral organ at Canterbury, they are employed to remove the keys to a distance from the instrument, which is done to the extent of ninety feet n is the form of a pedal-key ; t is the sharp, and u is the Rasura/. The length is about two feet; the distance from centre to centre of each pedal is two inches and a half.
Fig. 8 represents an anemometer, or wind-gauge. It is a glass tube, bent as in the figure : the dark part is supposed to•be water. w is the place to which it is applied in the organ, which is generally the hole for the pipe used as the pitch a or c. The pressure of the wind at si drives the water up to c, and the measure of their difference is what is called the wind. Some organs have more, and sonic less ; two inches and a half is the common measure.
Fig. 9, represents a percussion valve. It may be said to be a small bellows pressed by a spring. a is the wind-trunk, to which it is fixed ; n is the valve itself, and o is the spring. The use of this part of an organ is to steady the wind, which is sometimes disturbed by sudden transitions or very rapid performance : the spring of the valve is regu lated so as to give way on such occasions and to moderate their effects.
Fig. It) is a section of a common barrel-organ. n is the handle ; n the worm and crank, which move the barrel and bellows; a is the barrel on which the tunes are set. The setting is effected by braes pins and staples driven into the barrel at proper distances, accordingly as the notes are longer or shorter. D is the key, the rising of which over the pins causes it to press down the sticker (z) into the wind-chest (r), and thus to open the pallet for the wind to enter the pipe above ; o is the bellows ; a stop-diapason pipe. This diagram shows only one barrel, which in the common plan, but Messrs. Gray and Co. have introduced three, by which great risk to the mechanism is avoided and much trouble is saved.