> 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.
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