Polly is one of Tubal Cain's designs, as published in "Building Simple Model Steam Engines".
The vertical boiler is 54mm diameter x approx 73mm tall, made of copper. It has a single flue to the chimney and a safety valve. There is no burner, as such, the intention being that solid fuel tablets are placed on the base under the boiler. This, of course, can only be done with tablets and a burner will have to be provided for use with any other fuel.
The steam feed goes directly to the back of the port face. The single acting oscillating cylinder is 7.5mm bore x 18mm stroke. The crankshaft has a disk crank. The flywheel is 37mm diameter, turned in brass incorporating a small pulley.