In 1781 an engineer named Hornblower proposed using the expan sive principle by means of a double cylinder, but was prevented from carrying out his plan by the comprehensive and jealously guarded patents of Watt and his partners. In 1804, however, Woolf brought this principle of the double-cylinder engine into use. The annexed figure (fig. 11) will explain the mode of its action with au improved slide-valve.
The steam enters through the passage p above the piston in the smaller cylinder a, at a considerable pressure : while the piston is des cending under its influence, the steam from beneath passes through the tube r above the piston in the large cylinder B, which is impelled downwards by its expansion, the steam which was previously under this piston having passed to the condenser by the passage t. When the stroke is completed, the slide is moved downwards by its rod o. The small plugs or pistons v and w pass below the openings r and t, and the slide below the orifices p and q, and the action is reversed.
But though possessing considerable advantages, the double-cylinder engine has not become common, unless in the case of large pumping engines, probably owing to the complication of its structure, and the increased effects of radiation from so large a surface, more than com pensating its merits; and the expansive principle, equally applicable to a single cylinder, is now principally employed in engines of the com mon construction.
To reduce the compass and weight of the engine sufficiently to render it portable, the cumbrous apparatus of the condenser, and Its attendant pumps and cisterns, bad to be discarded; and since the principle of condensation was consequently renounced, it was necessary to raise the steam to a pressure sufficient to overcome that of the atmosphere on the opposite side of the piston. To allow the steam to act alternately on both sides of the piston, that which had just acted on one side to drive the piston was expelled into the open air through an orifice, corresponding to that which would have con nected the cylinder with the condenser in an engine of the usual construction ; but unless this orifice were as large as the diameter of the cylinder, which obviously it never can be, the steam, retarded in its escape by the contraction of the passage, must diminish, by its resistance to compression, the effective force of that which is acting to impel the piston.
Such is the simple principle, and such the greatest defect, of the non-condensing engine ; but the saving in original cost and the paramount advantage of portability more than compensate for this defect ; so that the use of this kind of engine has become general, not only for the purposes of locomotion, but for a variety of others where the engine is stationary, and probably in many instances where its advantages are imaginary.
Since the pumps of the condensing engine are dispensed with in the non-condensing one, the beam may be so likewise; the piston-rod is made to move in a straight line, by having a cross-piece attached at its top, which slides between guides fixed on each side of the cylinder, the rod which works the crank of the fly-wheel being attached to the end of this cross-piece. A etill further simplification is effected by con necting the piston-red directly with the crank on the shaft of the fly wheel, the cylinder being mounted so as to oscillate as the willed revolves on the steam passage, and thus alternately to open and close the communication between the top and bottom of the cylinder. Such engines are termed vibratory, and are successfully used where space must be economised, as with marine engines, but the weight of the cylinder thus moved is so much to be deducted from the power of the engine, and further causes a rapid wear of the centres on which it turns, which consequently cannot be long preserved steam-tight, and require frequent renewaL '• ' • • ' Marine engines, or those used for propelling vessels, are in this country generally condensing engines, their situation admitting the abundant use of cold water. The principal peculiarity in the arrange ment of the marine engine Is the position of the beam, which, for the purpose of economising room, is placed lower than the cylinder, and is double, there being ono on each side ; a rod from one end of each of these is connected with a cross-piece at the top of the piston-rod, the rectilinear motion of which is produced either by guides, or by a crank arrangement, analogous in its action to the parallel motion. The other ends of the double beam are connected by a cross-piece, carrying in Its centre the " rod " to work the crank on the shaft of the paddles. In all vessels of any magnitude, there are two engines complete, so arranged that while the rod and crank of one are in their neutral position, those of the other are in that of greatest effect. Two engines aro necessary to equalise and continue the motion of the wheels; for in the marine engine, the paddles, instead of performing the part of fly-wheels to continue and control the motion of the piston, require the whole force of the engines to maintain their own motion, owing to the resistance they have to overcome. There is also this further advantage derived from two engines, that if one should be injured, the vessel may be still propelled by the other, and not be entirely dependent on her sails, as she would otherwise be.