It is not obvious from the illustration, but the crankshaft and the shaft carrying the cylinder are offset from each other. Therefore as the flywheel and the cylinder rotate the piston in the cylinder moves in and out. The only real advantage seems to be the compactness of the engine.
Once again it is not clear in the picture, but the cylinder is only supported on the side nearest the viewer. There must have been large sideways loads on the bearing.
Note the awkward two-belt drive to the flyball governor, which gives twice the chance of a broken belt leading to the engine racing. Presumably it was considered important to have the governor right on top of the control valve, to give a faster response to load changes as there is less volume of steam to expand if the load is reduced.
Many thanks to my correspondent Clive for bringing this to my attention.
Source: Practical Magazine p432 of Vol 2, issue 12, 1873. The original publication seems to have been in the Scientific American for April 5th 1873. The picture and text is the same.
