A and B represent the two cylinders, in the larger of which the steam is allowed to expand itself, after passing from the high pressure cylinder B. The steam, which in the first instance is of considerable elasticity, is admitted to the cylinder B, by the tube and valve E, and entering the cylinder above its piston, impels it to the bottom. When this is effected, a communication is opened between the upper part of the cylinder B, and the under side of the cylinder A. The communication between the cylinder B and the steam-pipe E, is now reversed, and the steam is made to press on the under side of the piston B, a communication being at the same time formed between the upper part of the cylinder A, and the pipe leading to the condenser which is seen beneath. So that if we suppose the two pistons connected by means of their rods with one end of an ordinary working beam, the upward and downward strokes of each will be performed at the same time. We have hitherto considered the steam as passing direct from the boiler to the cylinder B; this, however, is in reality effected by a more circuitous route, as it is in the first instance admitted to the steam-case of the larger cylinder by the pipe C, and passing round a similar case, encircling the cylinder B, it is then made to enter at E. The pipe at D is merely intended to form a communication for carrying back to the boiler any water that may be produced by condensation in the steam-case, before the engine arrives at a proper temperature for working. Having thus briefly examined the nature of Mr. Woolf's engine, it may now be advisable to revert to the boiler, by which he proposes to generate steam of sufficient elasticity for the use of the small cylinder, which requires elastic vapour of great expansive force. The boiler, represented by the diagram beneath, consists of a series of tubes, of cast-iron, connected by screw-bolts with the under side of a larger vessel A A, communicating with the engine. The upper boiler is furnished with four, and in some cases, with five apertures; the first of which is intended for the admission of water, to supply the waste which continually arises from evaporation. The safety valves, man-hole, and water-pipe are also shewn. [Illustration] The mode of setting this boiler is also of considerable importance, as it is advisable to give a long and waving course to the chimney. [Illustration] A A still represents the principal boiler, while the figures 1, 2, 3, &c. indicate the passage of the flame and heated air; a section of the chimney being shewn at O. The steel-yard safety-valve which was employed in all the early engines is simple, and the nature of its construction may readily be understood. A represents a portion of the upper part of the boiler; B the safety-valve or plug made to fit air-tight on the valve-seat beneath; C the lever working on its axis at D, and furnished with a moveable weight E, adjusted to balance the pressure of steam within the boiler.