bottom of the vessel. The front part of the boiler is fastened to a cast-iron frame, shown at J. This frame is bolted to the framing, and also forms the ash pit N. Since the flat sides of the furnaces and shells would bulge on account of the pressure, they must be braced or stayed. This is accomplished by the screw stays s, s. Similar screw stays are also employed to connect the combustion chamber with the rear head, strengthening them both against bulging; this kind of stay is also used between the combustion chamber and outer shell except at the top, which is strengthened by the girder stays n, supported by the sling stays 9, 9. The top of the furnace is stayed by the toggle braces t, t, attached to rings, shown at r, r, made of angle iron and riveted to the shell. Toggle braces p are used to stay the flat surfaces of the rear head and of the uptake. At K the steam pipe is attached; at L, the safety valve; the furnace door is shown at P; the grate at M.

690. The gases of combustion pass from the furnaces A through the flues c to the combustion chamber B, whence they pass through the tubes T to the front connection C and up the smokestack S. As shown in this figure, the water legs only extend down a little below the grate, the ash pit being formed by the frame J. Sometimes there is a water space below the ash pit; that is, the furnace and ash pit are entirely surrounded by water; a boiler constructed in this manner is known as a wet-bottomed boiler. The type illustrated in Fig. 150 is called a dry-bottomed boiler. Vertical boilers are found but very seldom, and have, in this country, gone almost entirely out of use for marine purposes.

In the fire-box boiler shown, the heating surface is formed by the inside of the furnaces above the grate, the inside of the flues c, the sides, top and bottom of the combustion chamber, deducting, of course, the spaces taken up by the flues c, c and the door D', the inside of the tubes, and the sides and bottom of the uptake. Only in case of a wet uptake is its surface to be taken as heating surface, and then only up to the water-line.

WATER-TUBE BOILERS.

691. The boilers previously described are known as fire-tube boilers. The gases of combustion pass through the tubes, the water being in contact with the outside of the tubes. Since all fire-tube boilers are of great weight (which weight is increased by the large volume of water contained by them), and, since they also occupy considerable space, their use is objectionable in small vessels. To overcome the above disadvantages, a distinct class of boilers, known as water-tube boilers, has been designed. These boilers have come into extensive use during the past few years, and their use is gradually extending to larger vessels. In a water-tube boiler the water to be evaporated is contained within the tubes, the gases of combustion being in contact with the outside of the tubes.

THE ROBERTS WATER-TUBE BOILER.

692. A Roberts water-tube boiler is shown in Fig. 151. The construction of the boiler is as follows: A cylindrical steam drum A forms a receptacle for a small body of water, the space above the water forming the steam space; it is made of a steel or iron plate, and closed by flanged heads riveted to the shell, having the heads stayed by the stayrods shown at m. Two side pipes E (only one is shown in the figure) are connected to the front and back of the steam drum by means of the down-flow pipes C, C, the angular down-flow pipes B' and the cross pipes B. It will be noticed that on each of the T's, D D is a flange, to which are bolted the angle irons carrying the jacket G, G. A plate, on which the ends of the grate bars X, as well as the firebrick lining L', are supported, extends across the furnace in the front and rear, the plates being bolted to the flanges of the bottom T's. The grate is composed of two lengths of grate bars, which are held up at the center of the furnace by the bearing bars x, x, which in turn are supported by studs screwed into the bottom of the side pipes. The firebrick lining L rests on the side pipes. Connected to the H. M. I.-21

top of the side pipes are the up-flow coils c and d; only two are shown in full, the rest being shown broken off; they start alternately from the right and left side pipes. These coils are composed of pipes and return bends, with the return bends tapped on a spread; " that is, so as to give all the pipes an upward inclination. This is clearly shown in the front elevation, which also shows at c' the point at which the up-flow coil centers the steam drum. The coil d enters the steam drum at a similar point at the opposite side. Two feed coils u and v are placed one on each side of the drum, and are supported on the small pipes o, o. Superheating coils.

n and n' are placed one on each side of the furnace, outside of the up-flow coils. The boiler is enclosed at the four sides and the top by a sheet-iron jacket G G, lined with some non-heat-conducting material. An opening is provided in the top of the jacket, through which the smokestack/connects with the space inside of the jacket, forming the combustion chamber, the space directly over the grate X being the furnace. At W the water column, mounted with a glass water-gauge and three gauge-cocks, is shown. The steam-gauge is shown at S; the furnace door at K, and the ash pit at Y.

The feed-water enters at s and is divided into two streams by a partition cast with the T's. One stream passes through the pipe r into the feed coil u; the second stream passes through a similar pipe into the feed coil v. The feed-water is heated to a high temperature before leaving the coils. On leaving them, the water passing through the coil u, enters the steam drum at w', and the water passing through v enters at w. In operation, the entering feed-water is discharged above the water-line to allow the steam which may have formed in the feed coils to rise to the top of the drum, and the water to fall to the water level. The horizontal layers of the feed coils are separated by cross pipes, shown at e, placed there to prevent sagging of the free ends of each layer of pipe composing the coils. In order to show the coil u more clearly, these pipes have been omitted in the figure. There is no water in these cross pipes.

693. The boiler being filled with water until the steam drum is about one-quarter full vertically, and the fire started, the water expands and becomes lighter much faster in the up-flow coils c and d than in the down-flow pipes C and C; this is due both to the fact of the small pipes absorbing a greater amount of heat, owing to their being directly over the fire, and also to their being of a smaller diameter, because then there is more heating surface for a given volume of water in the pipes. The result is that the water at once commences to rise in the small up-flow coils and to fall in the large down-flow pipes. As soon as steam bubbles commence to form, this movement of the water becomes more rapid, as water holding these bubbles is much lighter than solid water, even if their temperatures are the same. To assist the upward flow of the water, the pipes c and are given the upward inclination previously mentioned.

The up-flow coils now begin to throw currents of water mixed with steam bubbles into the drum. The steam bubbles break, the steam rises to the top of the drum, and the solid water flows out of each end into the cross pipes leading to the down-flow pipes. The lower part of the bottom T, D, of each down-flow pipe forms a mud, or sediment, pocket; that is, it provides a quiet place for the deposit of foreign matter held in suspension in the feedwater. This deposit may be drawn off by means of the cocks g, g. Just above the mud pockets the downward currents make a turn at right angles into the side pipes, part of the water flowing into the up-flow coils as it passes along underneath them. The two currents, one from each end of each side pipe, meet in the center of the side pipes and there form an eddy, the only non-circulating water in the boiler excepting that in the mud pockets. The steam at the top of the drum passes into a spray pipe, or dry pipe, a, running from one head to the other, and drilled full of small holes on top for about half its length in the center. By locating the holes near the center, no water can enter the dry pipe when the vessel is pitching. The superheating