The annexed figure represents a longitudinal section. At a is the foundation-plate, on which the engines are erected; it is supported upon two deep sleepers of wood, which cross the floor-timbers of the vessel, to which they are firmly bolted. A portion of the bed-plate is formed into a channel, nearly as deep as the sleepers, part of which channel forms the bottom of the condenser, and another part receives the foot of the air-pump; b the cylinder; c the slide-case, which is formed of three vertical compartments, connected at top and bottom by the apertures dd; the middle compartment forms the steam-chamber, and the side compartments are the eduction passages; e is the steam inlet; fthe slide-valve, formed of two short slides, connected by a rod, which is found preferable to a single long slide for large engines, as the latter is liable to warp; the slides, which on the back form about one-third of a circle, are pressed up to the seats by screws at the back, acting upon a block of metal, faced with a thick sort of mat, made for the purpose;/the valve lever, working upon the shaft of the parallel motion as a fulcrum; the motion of the eccentric is communicated by the intermediate lever g; h the parallel bar; j the condenser, cast in one piece, with k the hot-well, and bolted to the foundation-plate; i the injection pipe.

A tube, or cylindrical passage, is cast in the condenser, through which pass the gudgeons I of the working beam; these gudgeons are very securely wedged into bosses, cast on the sides of the condenser, and the brass bearings are fixed in a boss in the centre of the beam; m the air-pump; n the foot-valve; o the blow-through valve, through which the air and water are blown out of the condenser at starting the engine; p the delivery valve, through which the water passes into the hot-well; q the passage leading to the feed-pump; r the connexion with the relief valve; s the passage by which the waste water is carried off into the sea; t an air-vessel; v the beam; w the connecting rod; y the crank; z the blow-through cock, connecting the steam-chamber of the slide-case with the eduction passage; this cock is opened previously to starting the engines, for the purpose of expelling the air from the engines, or, as it is termed, blowing through.

The feed-pump and bilge-pump are worked from the cross-head of the air-pump, and could not be shown in the sketch. The annexed figure represents, on a larger scale, a section of the feed-pump, with its valves. a is the plunger, attached to the cross-head b, and working through a stuffing-box in the pump-barrel c; d is the valve-box attached to the side of the hot-well; e is the suction-valve, through which the water is drawn from the hot-well into the pump by the rise of the plunger. On the descent of the plunger the water is driven out at the valve f, along the feed-pipeg to the boiler, unless the regulating valves or cocks on the boiler should be quite closed, when it raises the loaded valve h, and returns to the hot-well by the aperture k. The load per square inch on the valve h must somewhat exceed the pressure per square inch of the steam in the boiler.

In addition to the bolts, by which the cylinder, condenser, and side-frames are screwed down to the foundation-plate, there are twelve strong bolts, called holding-down bolts, which are passed from the outside through the floor-timbers, sleepers, foundation-plate, and bosses on the cylinders, condensers and side-frames, and screwed as firmly as possible by strong nuts. These bolts are of copper, or iron tinned, in order to resist the corrosion of the sea-water.

The arrangement of the Beam Engine is, perhaps, that which is best adapted for general use in steam-vessels, as all the parts admit of easy access; the weight of the moving parts on each side of the beam nearly counterbalance each other, and the requisite pumps are easily attached; but their weight and the space they occupy have, in the progress of the improvements which marine engines have undergone, been deemed very serious drawbacks; and, at present, opinion seems to be setting in favour of what are termed "direct-action" engines, most of the leading engineers having adopted each some particular arrangement for dispensing with the side-beams. Several of these deviations from the common form we shall now proceed to describe, commencing with one which may be considered as intermediate between the Beam and the Direct-action Engines; it is commonly termed a "Lever Engine," the action of the piston being transmitted to the crank by a lever, whose fulcrum is at the extremity.