Fig. 1 is a front elevation; Fig. 2, a lateral elevation; and Fig. 3, a longitudinal section. The boiler, which is of wrought iron, is 25 feet 6 inches in length, 19 feet in breadth, and 8 feet 6 inches in height. There are eight rectangular tubes b b running lengthways of the boiler. The fire is placed in the upper part of each of these, upon the bars c c in the section. At the farther end of the tubes is a transverse one d, extending the whole breadth of the boiler, which communicates with every one of the tubes containing the fire at each end of d. On the top, a return tube e e carries off the smoke and fire into another transverse tube f, out of the centre of which the chimney g rises. The cocks hh h are for ascertaining the height of water in the boiler. As an additional precaution, there are two cocks i i, which are placed, the one considerably above, and the other as much below, the assumed level of the water; these cocks communicate with a vertical glass tube j, of sufficient strength to endure the force of the steam.
On the cocks i i being opened, water enters into the lower cock, and steam into the upper one; and the pressure being the same in the boiler, the water stands at the same level, and thereby indicates at all times whether it be too high or too low in the boiler.
Recently, steam of high pressure having come somewhat extensively into use, in order to carry out as far as possible the expansive system of action, a corresponding modification in the construction has become necessary, and tubular boilers, variously arranged, have been employed. The annexed figure represents an arrangement for which Mr. D. Napier obtained a patent, and which he has introduced into several vessels under his management.
aa is a cylindrical chamber, with a dome top, constituting the outer shell,or outer casing of the boiler; and b b is a smaller cylinder, with a flat top c, placed concentrically within the chamber a, and constituting the fire-box, whilst the space included between the two cylinders forms the water-chamber. Within the fire-box, and on a level with the upper part of the fire-door, is a flat circular vessel d, which is connected to the annular part of the water-chamber by a neck e, and to the upper part of the chamber by several concentric rows of pipes f f. The vessel d is somewhat less in diameter than the fire-box, so that there is an annular space g between its circumference and that of the firebox, which forms a flue or passage through which the smoke and heated gases from the furnace pass, and thence traversing the interstices between the pipes f f, escape by the chimney h, which rises from the roof of the fire-box, and passes out through the dome of the boiler. In order to protect the chimney as much as possible from the effects of the fire, that portion of it which is within the boiler is surrounded by a water casing k, which is open at top.
The feed-water for continuing the supply of the boiler enters this casing near the bottom by the feed-pipe m, and overflows at the top; and in order to maintain a free circulation, a wide channel n is formed on the outside of the casing, which, being farther removed from the fire-box than any portion of the annular chamber, a descending current is maintained therein, whilst an ascending one takes place in the pipes and the annular water-chamber, as is indicated by the arrows. o is the fire-door, and p the ash-pit.