> What I was able to find out is that the atmospheric pressure at 8500
> feet is about 11 pounds per square inch absolute (psia)* as opposed to
> 14.7 psia at sea level.  Theoretically, this means that the pressure
> 'pushing' on the surface of the bellows cloth and pouches is reduced.
> And, since the air is thinner (or less dense), it would seem that it
> would be easier to create vacuum.

Yes.  But it's important to recall that a player piano, or any air-
operated device, gets its power from the pressure difference between
some inner chamber and the outside air.  That's why Mr. Rhodes mentions
'differential' pressure in his comment:

>[ 1 atmosphere = 14.7 pounds per square inch, which lifts a water
>[ column approximately 33 feet.  The typical player piano functions
>[ on a (differential) pressure of 1 foot water column or less.

It is the difference in pressure between the inside of the bellows and
that of the ambient air that pushes your pneumatics and pouches and gong
beaters.  And, at one inch, that pressure difference is rather small.

What I don't know is whether the lower viscosity of the rarer air at
high altitudes would exacerbate leaks through seals and through the
various bellows.

Mark Kinsler
Lancaster, Ohio USA
http://www.mkinsler.com/