Root's double-reciprocating or square piston engine. The “cylinder”, A, of this engine is of oblong square form and contains two pistons, B and C, the former working horizontally, and the latter working vertically within it; the piston, C, is connected with the wrist, a, of the crank 011 the main shaft, b. The ports for the admission of steam are shown black. The two pistons produce the rotation of the crank without dead points.