L

A

Fig. 88.

B

surrounds the furnace or fire-box, and fills this casing to the same level as that which it has in the boiler. When the engine is at work, the boiler is kept about half filled with

Fig. 89.

F

water; and, consequently, the casing surrounding the furnace is completely filled. The steam which is generated in the water contained in the casing finds its exit through the pipe D, and escapes into the upper part of the boiler. A section of the engine, taken at right angles to its length, is represented at fig. 89. Through the lower part of the boiler pass a number of copper tubes of small size, which communicate at one end with the fire-box, and at the other with the chimney, and form a passage for the heated air from the furnace to the chimney. The ignited fuel spread on the grate at the bottom of the fire-box disperses its heat by radiation, and acts in this manner on the whole surface of the casing surrounding the fire-box; and thus raises the temperature of the thin shell of water contained in that casing. The chief

part of the water in the casing, being lower in its position than the water in the boiler, acquires a tendency to ascend when heated, and passes into the boiler; so that a constant circulation of the heated water is maintained, and the water in the boiler must necessarily be kept at nearly the same temperature as the water in the casing. The air which passes through the burning fuel, and which fills the fire-box, is carried by the draught through the tubes which extend through the lower part of the boiler; and as these tubes are surrounded on every side with the water contained in the boiler, this air transmits its heat through these tubes to the water. It finally issues into the chimney, and rises by the draught. The power of this furnace must necessarily depend on the power of draught in the chimney; and to increase this, and at the same time to dispose of the waste steam after it has worked the piston, this steam is carried off by a pipe L, which passes from the cylinder to the chimney, and escapes there in a jet which is turned upwards. By the velocity with which it issues from this jet, and by its great comparative levity, it produces a strong current upwards in the chimney, and thus gives force to the draught of the furnace. In fig. 89. the grate-bars are represented at the bottom of the fire-box There are two cylinders, one of which works each wheel; one only appearing in the drawing fig. 88., the other being concealed by the engine. The spokes which these cylinders work are placed at right angles on the wheels; the wheels being fixed on a common axle, with which they turn.

at F.

In this engine, the surface of water surrounding the firebox, exposed to the action of radiant heat, amounted to twenty square feet, which received heat from the surface of six square feet of burning fuel on the bars. The surface exposed to the action of the heated air amounted to 118 square feet. The engine drew after it another carriage, containing fuel and water; the fuel used was coke, for the purpose of avoiding the production of smoke.

(189.) The Sanspareil of Mr. Hackworth is represented in fig. 90.; the horizontal section being exhibited in fig. 91.

The draught of the furnace is produced in the same manner

the cylinder into the chimney; the boiler, however, differs considerably from that of the Rocket.

A recurved tube

passes through the boiler, somewhat similar to that already described in the early engine of Messrs. Trevethick and Vivian. In the horizontal section (fig. 91.), D expresses the opening of the furnace at the end of the boiler, beside the chimney. The grate-bars appear at A, supporting the burning fuel; and a curved tube passing through the boiler, and terminating in the chimney, is expressed at B, the direction

of the draught being indicated by the arrow; c is a section of the chimney. The cylinders are placed, as in the Rocket, on each side of the boiler; each working a separate wheel, but acting on spokes placed at right angles to each other. The tube in which the grate and flue are placed diminishes in diameter as it approaches the chimney. At the mouth where the grate was placed, its diameter was two feet; and it was gradually reduced, so that, at the chimney, its diameter was only fifteen inches. The grate-bars extended five feet into the tube. The surface of water exposed to the radiant heat of the fire was sixteen square feet; and that exposed to the action of the heated air and flame was about seventy-five square feet. The magnitude of the grate, or sheet of burning fuel which radiated heat, was ten square feet.

(190.) The Novelty, of Messrs. Braithwaite and Ericson, is represented in fig. 92.; and a section of the generator and boiler is exhibited in fig. 93.; the corresponding parts in the two figures are marked by the same letters.

A is the generator or receiver containing the steam which works the engine; this communicates with a lower generator B, which extends in a horizontal direction the entire length of the carriage. Within the generator A is contained the furnace F, which communicates in a tube c, carried up through the generator, and terminated at the top by sliding shutters, which exclude the air, and which are only opened to supply fuel to the grate F. Below the grate the furnace

by a tube E, with a bellows D; which is worked by the engine, and which forces a constant stream of air, by the

tube E, through the fuel on F, so as to keep that fuel in vivid combustion. The heated air contained in the furnace F is driven on, by the same force, through a small curved tube marked e, which circulates like a worm (as represented in fig. 93.) through the horizontal generator or receiver; and, tapering gradually, until reduced to very small dimensions, it finally issues into the chimney G. The air in passing along this tube, imparts its heat to the water by which the tube is surrounded, and is brought to a considerably reduced temperature when discharged into the chimney. The cylinder, which is represented at K, works one pair of wheels, by means of a bell-crank, the other pair, when necessary, being connected with them.

In this engine, the magnitude of the surface of burning fuel on the grate-bars is less than two square feet; the surface exposed to radiant heat is nine and a half square feet; and the surface of water exposed to heated air is about thirtythree square feet.

The superiority of the Rocket may be attributed chiefly to the greater quantity of surface of the water which is exposed to the action of the fire. With a less extent of grate-bars than the Sanspareil, in the proportion of three to five, it exposes a greater surface of water to radiant heat, in the proportion of four to three; and a greater surface of water to heated air, in the proportion of more than three to two. It was found that the Rocket, compared with the Sanspareil, consumed fuel, in the evaporation of a given quantity of water,