In 991023 MMD Andrew Barrett wrote and Robbie commented:

> Second, I've read in the Encyclopedia of Automatic Musical Instruments
> about certain models of Aeolian Orchestrelle using pressure and a roll
> box.  I've never heard of a reed organ using pressure!

>[ A vacuum system is convenient for music rolls because the paper
>[ is sucked into contact with the tracker bar.  Some cardboard book
>[ systems operate with pressure, but a grooved roller is usually
>[ needed to keep the music in contact with the tracker bar.  Putting
>[ the tracker bar inside a pressurized box is a solution, but it's
>[ inconvenient...

Andrew,  There are two types of "pressure" systems.  The type used by
Aeolian on the Orchestrelles still has air passing _into_ the tracker
bar the same as a vacuum system.  Because the air pressure is elevated
above atmospheric, the spool box is enclosed and pressurized.

On a band organ using a pressure roll system, the air flows _out_ of
the tracker bar and therefore a metal weight is applied so that the
paper roll does not blow off of the tracker bar.  Once you understand
this, the internals are all standard pneumatics with valves, pouches,
bleeds, etc.

Bob Conant
Endicott, NY