[pg1] Grove defines the organ as "a wind instrument, the basic principle of which is that its tones are produced by means of a number of pipes, each pipe producing only one note." The organ consists of a number of departments. Those connected to the manuals (keyboards) are known as choir, great, swell, (and more rarely) solo, and echo organs, (in keyboard order, from bottom to top), while the department connected to the pedals is known as the pedal organ. Organs are designated as one, two, three-manual instruments etc. according to their number of manuals.

Within each department are found stops (ranks of pipes) of various pitches and tones. These pitches, identified according to the approximate length of the lowest pipe (CC) run in octaves (32', 16', 8'-fundamental pitch, 4', and 2') as well as at other intervals (5 1/3-fifth diatonic note above, and 2 2/3'-twelfth diatonic note above.) Whereas 8' is the basic manual pitch, 16' is the basic pedal pitch. In order to reinforce the upper partials of the prime note, stops known as "mixtures" are used, imparting brilliance to the ensemble tone of the instrument. These stops contain more than one rank of pipes (two, three, sometimes five or more) which usually speak some upper diatonic note such as 12. 15. 17. 19, 21 and others. As this reinforcing is most desirable in the lowest part of the compass, these stops "break" back to lower intervals and fewer notes as they ascend.

Flue stops are [pg2] classified according to the acoustic principle on which they operate, the flue or the reed. Flue stops are classified tonally as diapason (fundamental organ tone), flute or string tone, while reed stops are either chorus or solo stops, the chorus reed being part of the fundamental tonal architecture while the solo reed is an imitation of some orchestral instrument.

In addition to contrast between stops there is also contrast between manuals. This includes contrast of volume and tone. Thus the great organ possesses diapason tone in ranks of 16', 8', 4', and 2' pitch for its basic tone. The swell organ is more subdued in volume yet its character besides the softer combinations is essentially that of brilliance and "fire" from flue "upperwork" and chorus reeds. This department is always enclosed in a box, which by means of a "louvre" front enables the player to achieve various degrees of expression without "registration" (or stop) changes.

The choir organ which is also sometimes enclosed, is a department consting of light accompanying stops and does not usually contain diapason work, its foundation more often being flute tone. From the foregoing description it will be readily seen that an elaborate mechanism is necessary to place such a variety of tone and pitch under the control of the performer.

[pg3] THE MECHANISM OF THE INSTRUMENT

The mechanism of the organ may be divided into four parts:

• the bellows
• the key action
• the draw-stop action
• and the couplers

The bellows which supply the wind for the pipes consist of a reservoir supplied with wind from the feeder. From the reservoir the wind is conveyed through a "wind trunk" of either metal or wood to the wind chest, a substantial box extending the whole length of the sound-board, about equal to it in depth and about two-thirds its width. Here we find the sound-board pallets. The key action is the mechanism enabling the player to open these pallets. The type of key (and also stop) action used in most of the instruments of this period is "tracker" action. This type of action consists of a key, sticker, roller and tracker connecting with a pull-down attached to the pallet.

On pressing down the key - working on a metal pin - the further end rises, lifting the vertical sticker. This in turn, lifting the front arm of the horizontal roller, causes the roller to revolve slightly and the back arm, at the opposite end of the roller to descend. This back arm is attached to the tracker made to any length necessary to reach from the back roller arm to the pull-down. Cloth discs and bushings ensure silence during operation.

To lighten the tracker action as much as possible, "relief" pallets are used to reduce the resistance at the pallet. There are two types: the "jointed" pallet in which two or three inches of the fore part move first and then the remainder, [pg4] perhaps for nearly a foot in length; and the "double" pallet in which a small valve is placed on the back of the large one, opening first.

The second system of mechanism is the draw-stop action which enables the player to operate a "slider" which lies beneath each rank of pipes. Thus, when in the "on" position, holes drilled in the slider allow the wind to pass through similary positioned holes in the wind chest and upper boards. The slider is connected to its respective draw-stop by a moveable "trundle" and trace rod. When the draw-stop is drawn forward, the trundle literally "turns the corner", partly revolving and moving the trace rod. The lower end of the lever is drawn inwards causing the upper end to move outwards and take the slider end with it. This action is reversed when the draw-stop is pushed in.

The action of a manual coupler consists of a set of levers ("backfalls") acting on each key. As the front of the backfall lifts, the far end descends and presses down a "sticker" resting on the back end of the T-shaped backfall of the manual-action which is then set in motion. The octave and suboctave couplers consist of splayed backfalls which extend from any given key to the tracker of its octave or suboctave. When the draw-stop is put in, the backfalls rise from the stickers at the front end. The pedal couplers work in a similar manner.

There are two other forms of coupler: the "ram" and the "tumbler". The tumbler consists of a "stock" which turns on [pg5] a centre when the coupler is on. The tumbler lies in the vertical position between the two sets of keys while it is horizontal when "off".

The swell box in the organs of this period are operated by either a trigger lever which is held in certain fixed positions by a notched rod (the "fixed" positions of the particular organs in this project being, with few exceptions, only two; "open" or "closed") or by a balanced swell pedal which when operated by pressure on either toe or heel will remain fixed in any one position. Both pedals operate the louvres by means of rods which work on the leverage principle.

Mention must be made of supplementary actions designed to overcome the physical effort required to operate the tracker mechanism. The development of supplementary actions was the outstanding contribution of the builders of this period (French, Cavaille-Coll as well as British) to the mechanism of the organ. This development arose through the need for an easier type of action to operate the larger organs then coming into vogue. In this respect, the development also embodied stop and coupler mechanism.

	Barker Lever Pneumatic Action

In the Barker lever action, depression of the key admits heavy pressure wind to a small external motor (something like a feeder) causing it to distend or inflate, and operate an ordinary tracker action leading to the pallet. Upon releasing the key, a small valve is opened, emptying the motor, which then collapses [pg6] allowing the pallet spring to close the pallet.

	Pressure Pneumatic Action

Tubular-pneumatic action is of two kinds. In the supply, charge or pressure system, depression of the key admits wind to a tube conveying it to further pneumatic work, which in turn exhausts an inflated motor subject to wind pressure within and without, causing it to collapse and pull down the tracker attached to it.

	Exhaust Pneumatic Action

In the exhaust system, depression of the key exhausts the tube and motor from the key end of the tube, opening the pallet as before.

Further developments of the period were the early experiments and patents in electric action by Gauntlett, Barker and Bryceson showing still further the keen interest and desire for mechanical development.

Next: [pg12] THE PRACTICE OF TONAL PRODUCTION