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a somewhat troublesome business, but results in much better steaming power, especially in cold weather. Air-holes must be cut round the bottom of the lining to give good ventilation.

A boiler of this size will keep a 1 by 1-1/2 inch cylinder well supplied with steam at from 30 to 40 lbs. per square inch.

A Horizontal Boiler.

[Illustration: FIG. 82.—Longitudinal section of large water-tube boiler.]

The boiler illustrated by Fig. 81 is designed for heating with a large paraffin or petrol blow-lamp. It has considerably greater water capacity, heating surface—the furnace being entirely enclosed—and water surface than the boiler just described. The last at high-water level is about 60, and at low-water level 70, square inches.

The vertical section (Fig. 82) shows 1/16-inch barrel, 13 inches long over all and 12 inches long between the end plates, and 6 inches in diameter. The furnace flue is 2-1/2 inches across outside, and contains eleven 1/2-inch cross tubes, set as indicated by the end view (Fig. 83), and 3/4 inch apart, centre to centre. This arrangement gives a total heating surface of about 140 square inches. If somewhat smaller tubes are used and doubled (see Fig. 84), or even trebled, the heating surface may be increased to 180-200 square inches. With a powerful blow-lamp this boiler raises a lot of steam.

Tubing the Furnace Flue.—Before any of the holes are made, the lines on which the centres lie must be scored from end to end of the flue on the outside. The positions of these lines are quickly found as follows:—Cut out a strip of paper exactly as long as the circumference of the tube, and plot the centre lines on it. The paper is then applied to the tube again, and poppet marks made with a centre punch opposite to or through the marks on the paper. Drive a wire-nail through a piece of square wood and sharpen the point. Lay the flue on a flat surface, apply the end of the nail to one of the poppet marks, and draw it along the flue, which must be held quite firmly. When all the lines have been scored, the centring of the water tubes is a very easy matter.

[Illustration: FIG. 83.-End of horizontal boiler, showing position of holes for stays and fittings.]

The two holes for any one tube should be bored independently, with a drill somewhat smaller than the tube, and be opened to a good fit with a reamer or broach passed through both holes to ensure their sides being in line. Taper the tubes—2-7/8 inches long each—slightly at one end, and make one of the holes a bit smaller than the other. The tapered end is passed first through the larger hole and driven home in the other, but not so violently as to distort the flue. If the tubes are made fast in this way, the subsequent silver-soldering will be all the easier.

[Illustration: FIG. 84.—Doubled cross tubes In horizontal boiler flue.]

The Steam Dome.—The large holes—2 inches in diameter—required for the steam dome render it necessary to strengthen the barrel at this point. Cut out a circular plate of metal 4 inches across, make a central hole of the size of the steam dome, and bend the plate to the curve of the inside of the barrel. Tin the contact faces of the barrel and "patch" and draw them together with screws or rivets spaced as shown in Fig. 85, and sweat solder into the joint. To make it impossible for the steam dome to blowout, let it extend half an inch through the barrel, and pass a piece of 1/4-inch brass rod through it in contact with the barrel. The joint is secured with hard solder. Solder the top of the dome in 1/8 inch below the end of the tube, and burr the end over. The joint should be run again afterwards to ensure its being tight.

[Illustration: FIG. 85.—Showing how to mark out strengthening patch round steam dome hole.]

The positions of stays and gauges is shown in Fig. 83.

Chimney.—This should be an elbow of iron piping fitting the inside of the flue closely, made up of a 9-inch and a 4-inch part. The last slips into the end of the flue; the first may contain a coil for superheating the steam.

A Multitubular Boiler.

[Illustration: FIG. 86.—Cross section of multitubular boiler.]

Figs. 86 and 87 are respectively end and side elevations of a multitubular boiler having over 600 square inches of heating surface—most of it contributed by the tubes—and intended for firing with solid fuel.

The boiler has a main water-drum, A, 5 inches in diameter and 18 inches long, and two smaller water-drums, B and C, 2-1/2 by 18 inches, connected by two series of tubes, G and H, each set comprising 20 tubes. The H tubes are not exposed to the fire so directly as the G tubes, but as they enter the main drum at a higher point, the circulation is improved by uniting A to B and C at both ends by large 1-inch drawn tubes, F. In addition, B and C are connected by three 3/4-inch cross tubes, E, which prevent the small drums spreading, and further equalize the water supply. A 1-1/2-inch drum, D, is placed on the top of A to collect the steam at a good distance from the water.

Materials.—In addition to 1-1/2 feet of 5 by 3/32 inch solid-drawn tubing for the main, and 3 feet of 2-1/2 by 1/16 inch tubing for the lower drums, the boiler proper requires 22-1/2 feet of 1/2-inch tubing, 19 inches of 3/4-inch tubing, 2-1/4 feet of 1-inch tubing, 1 foot of 1-1/2-inch tubing, and ends of suitable size for the four drums.

[Illustration: FIG. 87.—Longitudinal section of multitubular boiler.]

CONSTRUCTION.

[Illustration: FIG. 88.-Two arrangements for tube holes in multi tubular boiler.]

The centres for the water-tubes, G and H, should be laid out, in accordance with Fig. 88, on the tops of B and C and the lower part of A, along lines scribed in the manner explained on p. 207. Tubes H must be bent to a template to get them all of the same shape and length, and all the tubes be prepared before any are put in place. If the tubes are set 7/8 inch apart, centre to centre, instead of 1-1/4 inches, the heating surface will be greatly increased and the furnace casing better protected.

Assembling.—When all necessary holes have been made and are of the correct size, begin by riveting and silver-soldering in the ends of the drums. Next fix the cross tubes, E, taking care that they and B and C form rectangles. Then slip the F, G, and H tubes half an inch into the main drum, and support A, by means of strips passed between the G and H tubes, in its correct position relatively to B and C. The E tubes can now be pushed into B and C and silver-soldered. The supports may then be removed, and the a and H tubes be got into position and secured. Drum D then demands attention. The connecting tubes, KK, should be silver-soldered in, as the boiler, if properly made, can be worked at pressures up to 100 lbs. per square inch.

The casing is of 1/20-inch sheet iron, and in five parts. The back end must be holed to allow A, B, and C to project 1 inch, and have a furnace-door opening, and an airway at the bottom, 5 inches wide and 1 inch deep, cut in it. The airway may be provided with a flap, to assist in damping down the fire if too much steam is being raised. In the front end make an inspection opening to facilitate cleaning the tubes and removing cinders, etc.

The side plates, m m, are bent as shown in Fig. 86, and bolted to a semicircular top plate, n, bent to a radius of 6 inches. A slot, 1-1/2 inches wide and 11-1/2 inches long, must be cut in the top, n, to allow it to be passed over drum D; and there must also be a 3 or 3-1/2 inch hole for the chimney. A plate, p, covers in D. A little plate, o, is slipped over the slot in n, and asbestos is packed in all round D. The interior of the end, side, and the top plates should be lined with sheet asbestos held on by large tin washers and screw bolts. To protect the asbestos, movable iron sheets may be interposed on the furnace side. These are replaced easily if burnt away. The pieces m m are bent out at the bottom, and screwed down to a base-plate extending the whole length of the boiler.

The fire-bars fill the rectangle formed by the tubes B, El, and E2. A plate extends from the top of E2 to the front plate of the casing, to prevent the furnace draught being "short circuited."

Boiler Fittings.

[Illustration: FIG. 89.-Safety valve.]

Safety Valves.—The best all-round type is that shown in Fig. 89. There is no danger of the setting being accidentally altered, as is very possible with a lever and sliding weight. The valve should be set by the steam gauge. Screw it down, and raise steam to the point at which you wish the safety valve to act, and then slacken off the regulating nuts until steam issues freely. The lock nuts under the cross-bar should then be tightened up. In the case of a boiler with a large heating surface, which makes steam quickly, it is important that the safety-valve should be large enough to master the steam. If the valve is too small, the pressure may rise to a dangerous height, even with the steam coming out as fast as the valve can pass it.

[Illustration: FIG. 90.-Steam gauge and siphon.]

Steam Gauges.—The steam gauge should register pressures considerably higher than that to be used, so that there may be no danger of the boiler being forced unwittingly beyond the limit registered. A siphon piece should be interposed between boiler and gauge (Fig. 90), to protect the latter from the direct action of the steam. Water condenses in the siphon, and does not become very hot.

[Illustration: FIG. 91.-Water gauge.]

Water Gauges should have three taps (Fig. 91), two between glass and boiler, to cut off the water if the glass should burst, and one for blowing off through. Very small gauges are a mistake, as the water jumps about in a small tube. When fitting a gauge, put packings between the bushes and the glass-holders, substitute a piece of metal rod for the glass tube, and pack the rod tightly. If the bushes are now sweated into the boiler end while thus directed, the gauge must be in line for the glass. This method is advisable in all cases, and is necessary if the boiler end is not perfectly flat.

Pumps.—Where a pump is used, the supply should enter the boiler below low-water level through a non-return valve fitted with a tap, so that water can be prevented from blowing back through the pump. As regards the construction of pumps, the reader is referred to p. 164 and to Chapter XXII.

Filling Caps.—The filling cap should be large enough to take the nozzle of a good-sized funnel with some room to spare. Beat the nozzle out of shape, to give room for the escape of the air displaced by the water.

The best form of filling cap has a self-seating ground plug, which, if properly made, is steam-tight without any packing. If needed, asbestos packing can easily be inserted between plug and cap.

Mud-holes.—All but the smallest boilers should have a mud-hole and plug in the bottom at a point not directly exposed to the furnace. In Fig. 82 it is situated at the bottom of the barrel. In Figs. 86 and 87 there should be a mud-hole in one end of each of the three drums, A, B, and C. The plug may be bored at the centre for a blow-off cock, through which the boiler should be emptied after use, while steam is up, and after the fire has been "drawn." Emptying in this way is much quicker than when there is no pressure, and it assists to keep the boiler free from sediment.

[Illustration: FIG. 92.—Steam cock.]

Steam Cocks.-The screw-down type (Fig. 92) is very preferable to the "plug" type, which is apt to leak and stick.

Testing Boilers.—The tightness of the joints of a boiler is best tested in the first instance by means of compressed air. Solder on an all-metal cycle valve, "inflate" the boiler to a considerable pressure, and submerge it in a