A contrivance for gradually varying the force of the tone in an organ or harmonium (also in the harpsichord and some early pianos), consisting of a…

a kind of swell-box in an organ.

a mechanism for creating crescendo and diminuendo effects on an organ by opening and closing the upper shutters on a swell box; cf. door-swell n. at…

an organ-swell having the front constructed like a Venetian shutter.

a form of swell in an organ operated by a valve in the wind-trunk.