We have already noticed Mr. Gemmel's twin-boat in our second section. In the same patent (1837) he proposed to employ two pair of paddle-wheels of the ordinary construction to steam vessels generally, with a view to enable them to attain a speed adequate to the power of the engine employed, and that even when the breadth of the wheel is necessarily restricted, to enable the vessel to pass through narrow openings, such as the gates of canals, etc. One of his arrangements for these purposes is exhibited in the accompanying figures 1 and 2, the former being a longitudinal section of the vessel, with a side elevation of the machinery, and the, latter a plan of the same.
In a suitable metallic framing a a a, are fixed a pair of marine engines, working horizontally; b b are the working cylinders, e c the slide-valve cases, d d the eccentrics by which the valves are worked; but the rods are omitted to avoid confusion in the engraving; eeee piston-rods, proceeding from each side of the pistons, and working through stuffing-boxes in the two covers of each cylinder. To maintain the parallelism of the piston-rods, their cross head: move in horizontal guides, f f f f are the connecting rods, proceeding from the cross heads of the piston-rods to the cranks upon the two shafts, g and h, which carry the two pairs of wheels i i i i. The air-pump k is placed between the two engines, and is worked by a short crank /, formed in the middle of the shaft g; m is the boiler, and n the chimney.