piston-rods cross pieces are attached of greater length than the diameter of the cylinders, so that their extremities shall project beyond the cylinders. To the ends of these cross pieces are attached by joints the rods of a parallel motion: these rods are carried downwards, and are connected with the ends of two beams below the cylinder, and placed on either side of it. The opposite ends of these beams are connected by another cross piece, to which is attached a connecting rod, which is continued upwards to the crank-pin, to which it is attached, and which it drives. Thus the beam, parallel motion, and connecting rod of a marine engine, is similar to that of a land engine, only that it is turned upside down; and in consequence of the impossibility of placing the beam directly over the piston-rod, two beams and two systems of parallel motion are provided, one on each side of the engine, acted upon by, and acting on the piston-rod and crank by cross pieces.

The proportion of the cylinders differs from that usually observed in land engines, for like reasons. The length of the cylinder of land engines is generally greater than its diameter, in the proportion of about two to one. The cylinders of marine engines are, however, commonly constructed with a diameter very little less than their length. In proportion, therefore, to their power their stroke is shorter, which infers a corresponding shortness of crank and a greater limitation of play of all the moving parts in the vertical direction. The valves and the gearing by which they are worked, the air-pump, the condenser, and other parts of the marine engines, do not materially differ from those already described in land engines.

These arrangements of a marine engine will be more clearly understood by reference to fig. 119.*, in which is represented a longitudinal section of a marine engine with its boiler as placed in a steam-vessel. The sleepers of oak, supporting the engine, are represented at x, the base of the engine being secured to these by bolts passing through them

This cut is taken from the plate of the engine of the Red Rover, manufactured by Boulton and Watt, given in the last edition of Tredgold on the Steam Engine.

and the bottom timbers of the vessel; s is the steam-pipe leading from the steam-chest in the boiler to the slides c, by which it is admitted to the top and bottom of the cylinder. The condenser is represented at B, and the air-pump at E. The hot well is seen at F, from which the feed is taken for the boiler; L is the piston-rod connected by the parallel motion a with the beam H, working on a centre K, near the base of the engine. The other end of the beam I drives the connecting rod м, which extends upwards to the crank which it works upon the paddle-shaft o. QR is the framing by which the engine is supported. The beam here exhibited is shown on dotted lines as being on the further side of the engine. A similar beam similarly placed, and moving on the same axis, must be undertood to be at this side connected with the cross head of the piston in like manner by a parallel motion, and with a cross piece attached to the lower end of the connecting rod and to the opposite beam. The eccentric which works the slides is placed upon the paddle shaft o, and the connecting arm which drives the slides may be easily detached when the engine requires to be stopped. The section of the boiler, grate, and flues, is represented at w u. The safety-valve y is enclosed beneath a pipe carried up beside the chimney, and is inaccessible to the engine-man; h are the cocks for blowing the salted water from the boiler; and II the feed-pipe.

The general arrangement of the engine-room of a steamvessel is represented in fig. 120.

The nature of the effect required to be produced by marine engines does not render either necessary or possible that great regularity of action which is indispensable in a steamengine applied to the purposes of manufacture. The agitation of the surface of the sea will cause the immersion of the paddle-wheels to be subject to great variation, and the resistance produced by the water to the engine will undergo a corresponding change. The governor, therefore, and other parts of the apparatus, contrived for giving to the engine that great regularity required in manufactures, are omitted in nautical engines, and nothing is introduced save what is

necessary to maintain the machine in its full working efficiency.

To save space, marine boilers are constructed so as to pro

duce the necessary quantity of steam within the smallest pos

sible dimensions. With this view a more extensive surface in proportion to the capacity of the boiler is exposed to the action of the fire. The flues, by which the flame and heated air are conducted to the chimney, are so constructed that the heat may act upon the water on every side in thin oblong shells or plates. This is accomplished by constructing the flues so as to traverse the boiler

backwards and forwards several times before they terminate

in the chimney. Such an arrangement renders the expense of the boilers greater, but their steam-producing

B

Fig. 122.

power is proportionally augmented, and experiments made by Mr. Watt, at Birmingham, have proved that such boilers with the same consumption of fuel will produce,

as compared with common land boilers, an increased

evaporation in the proportion of about three to two.

The form and arrangement of the water-spaces and flues in marine boilers may be collected from the sections of the boilers used in some of the government steamers, exhibited in figs. 121, 122, 123. A section made by a horizontal plane passing through the flues is exhibited in fig. 121. The furnaces F communicate in pairs with the flues E, the air following the course through the flues represented by the arrows. The flue E passes to the back of the boiler, then returns to the front, then to the back again, and is finally carried back to the front, where it communicates at c with the curved flue B, represented in the transverse vertical section, fig. 122. This

Fig. 123.

A

H

G

curved flue B finally terminates in the chimney A. There are in this case three independent boilers, each worked by two furnaces communicating with the same system of flues; and in the curved flues B, fig.122., by which the air is finally conducted through the chimney, are placed three independent