This model was of small dimensions, standing little more than a foot and a half high, though it was sufficiently large to demonstrate the soundness of the principle on which it was constructed.

It was supported on three wheels, and carried a small copper boiler, heated by a spirit lamp, with a flue passing obliquely through it.

The cylinder, of 3/4 inch diameter and 2-inch stroke, was fixed in the top of the boiler, the piston-rod being connected with the vibratory beam attached to the connecting-rod which worked the crank of the driving-wheel.

This little engine worked by the expansive force of steam only, which was discharged into the atmosphere after it had done its work of alternately raising and depressing the piston in the cylinder.
Mr.Murdock's son, while living at Handsworth, informed the present writer that this model was invented and constructed in 1781; but, after perusing the correspondence of Boulton and Watt, we infer that it was not ready for trial until 1784.

The first experiment was made in Murdock's own house at Redruth, when the little engine successfully hauled a model waggon round the room,--the single wheel, placed in front of the engine and working in a swivel frame, enabling it to run round in a circle.
Another experiment was made out of doors, on which occasion, small though the engine was, it fairly outran the speed of its inventor.