The Angelus Artrio system employs thirteen expression open ings, all leading from the interior to the exhaust pumps. Five of these are in the controlling governor, and the others in the treble and bass sections. They vary in size from tiny vents to a large "melody" opening, and all air must pass through one or more openings on its way from tracker-bar to main exhaust. In addi tion, there is an "in-leak" as on the Ampico, but the Angelus system works it from the tracker-bar. When a perforation appears against the second hole from the left-hand side, the "in-leak" is closed and an increase of power results.
The Duo-Art (Aeolian Company) is essentially a theme and accompaniment expression arrangement, the former being at a higher dynamic power than the other. Each has a knife-valve control of the passage-way, operated by a set of four pneumatics, varying in their amount of possible collapse. In this way six vari ants of power are obtained. There is also an "in-leak," automati cally closed by another knife-valve so as to get maximum power when the passage-way is already nearly full open.
The Welte-Mignon is worked by suction fan instead of the usual pumpers. Its expression unit consists of a governor bellows, held open by a coiled spring. This controls the position of a conical valve in the passage-way, and is itself automatically re sponsive to the amount of work being done at any moment within the pneumatic system. The expression pneumatic, which also works the same conical valve, is operated from the tracker-bar, and can act slowly or quickly according to the perforations in operation. (S. A. H.) BIBLIOGRAPHY.- S. Grew, Tke Art of the Player-Piano (1922), The Book of the Player-Pianist (1925) ; D. M. Wilson, The Plover-Piano: its Construction, how to play (1923).