I

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 1 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 produce 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

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

Fig. 122.

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 в, represented in the transverse vertical section, fig. 122. This

Fig. 123.

A

H

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

dampers, by means of which the furnace of each boiler can be regulated independently of the other, and by which each boiler may be separately detached from communication with the chimney. The letters of reference in the horizontal section, fig. 121., correspond with those in the transverse vertical section, fig. 122., E representing the commencement of the flues, and c their termination.

A longitudinal section of the boiler made by a vertical plane extending from the front to the back is given in fig. 123.,

where F, as before, is the furnace, & the grate-bars sloping downwards from the front to the back, H the fire-bridge, c the commencement of the flues, and A the chimney. An elevation of the front of the boiler is represented in fig. 124., showing two of the fire-doors closed, and the other two removed, displaying the position of the grate-bars in front. Small openings are also provided, closed by proper doors, by which access can be had to the under side of the flues between the foundation timbers of the engine for the purpose of cleaning them.

Each of these boilers can be worked independently of the others. By this means, when at sea, the engine may be worked by any two of the three boilers, while the third is being cleaned and put in order. In all sea-going steamers multiple boilers are at present provided for this purpose.

In the boilers here represented the flues are all upon the same level, winding backwards and forwards without passing one above the other. In other boilers, however, the flues,

after passing backwards and forwards near the bottom of the boiler, turn upwards and pass backwards and forwards through a level of the water nearer its surface, finally terminating in the chimney. More heating surface is thus obtained with the same capacity of boiler.

The most formidable difficulty which has been encountered in the application of the steam-engine to sea-voyages has arisen from the necessity of supplying the boiler with seawater instead of pure fresh water. The sea-water is injected into the condenser for the purpose of condensing the steam, and it is thence, mixed with the condensed steam, conducted as feeding water into the boiler.

(209.) Sea-water holds, as is well known, certain alkaline substances in solution, the principal of which is muriate of soda, or common salt. Ten thousand grains of pure sea-water contain two hundred and twenty grains of common salt, the remaining ingredients being thirty-three grains of sulphate of soda, forty-two grains of muriate of magnesia, and eight grains of muriate of lime. The heat which converts pure water into steam does not at the same time evaporate those salts which the water holds in solution. As a consequence it follows, that as the evaporation in the boiler is continued, the salt, which was held in solution by the water which has been evaporated, remains in the boiler, and enters into solution with the water remaining in it. The quantity of salt contained in sea-water being considerably less than that which water is capable of holding in solution, the process of evaporation for some time is attended with no other effect than to render the water in the boiler a stronger solution of salt. If, however, this process be continued, the quantity of salt retained in the boiler having constantly an increasing proportion to the quantity of water, it must at length render the water in the boiler a saturated solution-that is, a solution containing as much salt as at the actual temperature it is capable of holding in solution. If, therefore, the evaporation be continued beyond this point, the salt disengaged from the water evaporated instead of entering into solution with the water remaining in the boiler will be precipitated in the form of sediment; and if the process be continued in the