A parallel motion invented by Dr. Cartwright in the year 1787. The toothed wheels, C, C, have equal diameters and numbers of teeth ; and the cranks, A, A, have equal radii, and are set in opposite directions, and consequently give an equal obliquity to the connecting-rods during the revolution of the wheels. The cross-head on the piston-rod being attached to the two connecting-rods, the piston-rod is caused to move in a right line.