In response to Nicholas Simons' posting regarding varying the output
of organ feeders in proportion to the changing air demands of the
pipes, this idea was implemented by the mid-1910s not in a musical
instrument, but in a central vacuum cleaning system installed in one
of the beautiful old mansions in Colorado Springs!

The suction source for the central vac is a large crankshaft-driven
reciprocating pump located in the basement, resembling a giant Western
Electric coin piano pump.  However, there are rocking levers with a
movable fulcrum connected between the crank rods and feeder rods.

The vacuum reservoir is mounted vertically at the opposite end of the
pump, as in a large Wurlitzer Mandolin PianOrchestra pump.  The open
end of the reservoir (opposite the hinge) has two long wooden rods
connecting it to the moveable fulcrum for the rocking levers.  When the
vacuum reservoir pulls shut, it changes the leverage between crank and
feeders, causing the feeders to move very little.  When demand is
greater and the reservoir opens, the feeders receive full thrust from
the crank.

While this is clever and intriguing, it is also mechanically much more
complicated than simple connecting rods and takes up more room.  If its
merits had outweighed its disadvantages in hand-cranked organs, it
probably would have been implemented a hundred years ago.

Another simpler mechanism is found in certain Philipps orchestrion
pressure pumps, in which the pressure feeders are all mounted next to
each other (with none opposing each other).  The stationary board of
each pressure feeder has a pallet valve next to its internal flap
valve.  The pallet valves are connected to the moveable board of the
pressure reservoir with adjustable gut cords.

When the pressure reservoir reaches the desired level of inflation, it
pulls the pallets open, and the pressure rushes from one feeder back
into the other.  There is no external spill valve.  Presumably, less
energy is spent blowing air back and forth than forcing it through an
external spill valve.  Unfortunately, the internal wind noise is just
as great as that produced by a normal spill valve.

Art Reblitz