>> Thanx for the tips on low pressure steam Alec.  But whether steam
>> or air, we still got the problem of making a piston with a big
>> enough cross section to do anything useful...
 
> Day,
> The very first steam engines had a piston that was several FEET in
> diameter, and were used to pump water from coal mines in England. Speed
> was only a few strokes per minute; the earliest ones had MANUAL valve
> gear.
 
> And they worked at '0' PSI steam pressure.
 
"0" can be measured. If you have "0" PSIG, then you don't have steam. Now 
pressure measured in inches of water column, (in.W.C.) is a different story. 
Takes roughly 27.7 in.W.C. to equal 1 PSIG, so even 5 in.W.C. spread over a 
large area will yield significant force. Force being pressure applied times 
the surface area feeling the pressure. Quick calculation in the head gives 
around 18 PSIG for a 5 in.W.C. pressure applied to 100 sq. in. surface area.
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