A device for the gradation of volume in keyboard instruments.
Peter Williams, revised by Nicholas Thistlethwaite
The Swell organ is that manual department of an organ whose chest and/or pipes are enclosed on all sides by a box, one side of which incorporates a device (lid, flap, shutters, sashed panel, etc.) that can be opened and closed by connection with a foot-lever or pedal. A stop or half-stop may be thus enclosed, or several departments (Choir organ, Solo organ) or even the whole organ (Samuel Green, St George's Chapel, Windsor, 1790). The connection from foot-lever to swelling device can be mechanical, pneumatic, electrical, etc. and may be so made that fine gradations in the degree of closure are possible.
Some examples of the small Brustwerk of the 16th century may have had doors that could be opened; most authenticated examples before about 1700, however, have semi-fixed fretwork doors. The idea of foot-operated movable doors or, in chamber organs, flaps, occurred occasionally to builders (T. Mace, ...