Page:Popular Science Monthly Volume 40.djvu/661

 pneumatic bellows (N), which, being pressed down by the force of compressed air from the wind-chest, opens the large valve (O). This supplies all the pipes in that chest with wind. The swell (B) and solo (D) key-boards also operate their respective wind-chests on the same principle.

The couplers are operated through the medium of lever (P), which is controlled through a block glued on the tracker (J). When the performer desires to couple the choir to the great organ, the draw-stop pulled out has the effect of pushing the square (G) up against the tracker (K), when the desired result is attained. Again, when he wishes to couple the swell to the great, the pulling out of the draw-stop brings up the lever (R) against the block on the tracker in that connection. The coupling of the swell octave is attained by the levers (S), which are placed at an angle so as to pull the tracker of the swell one octave higher, by an ingenious movement. In coupling the solo organ to the great, the lever (T) is moved up to the block on the tracker of that manual. A little study of the plan, and the points given, will explain the general system of action very clearly. In this instrument a set of bells of three octaves (U) are operated from the great-organ key-board by a mechanism of the square piano order; pneumatic agency is also used here, as the pressure of the key will not of itself furnish the dynamic impulse necessary to put them in vibration. This is attained by a draw-stop, which puts lever (V) and tracker (J) in connection by pushing the former up against the latter. To play any of the key-boards by pedals, a "cam" serves to raise the levers (W) to the tail of the keys, thus establishing a connection.