This a GARRETT COMPOUND ENGINE from c.1880. this is a 1/6 scale model of the full size engine casting and plans are available from CotswoldHeritage @ www.modelsteamenginesuk.com/ Compound means that it uses 2 cylinders a High Pressure (smaller) and a Low Pressure (larger cylinder). The steam from the High pressure cylinder is exhausted into the Low Pressure Cylinder and to compensate for the drop in pressure due to the fact that the steam has cooled somewhat the Low Pressure cylinder is larger in Diameter. This is the same principle that is used on Steam Locomotives. Link to the last of the GARRETT's http://www.abc.net.au/news/2014-07-25/massive-steam-train-restored-in-canberra/5624482