The engine is a cross compound Corliss valve condensing steam.  Speed is 68rpm controlled by a Porter type governor and a Lumb regulator acting on the high pressure cylinder valves’ Dobson type trip gears.  The high pressure cylinder “James” has a bore of 17 inches with a stroke of 4 feet.  The low pressure cylinder “Mary Jane” has a 34 inch bore and a stroke of 4 feet.

Cross compound engines are so named because the cylinders and cranks are on either side of the flywheel and the steam crosses from the high to the low pressure cylinder, as compared to a Tandem Compound which is “in line”, the cylinders being one behind the other.

Steam from the boiler at 160 lbs per square inch is first expanded in the high pressure cylinder, and “James” turns the flywheel over. There is still more energy in the steam, so it is passed across to the low pressure cylinder “Mary Jane” and she re-uses the same steam to give the wheel a further turn.  The exhaust steam is then passed to the condenser in the basement to be turned back into re-usable water.

This mill engine was installed, and first started service in 1920, and ran a continuous reliable service through to 1978 when the mill was finally closed.

text & images © 2007 Bancroft Mill Engine Trust