[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.