Mark out and drill the tube holes in the bottom, and then the flue hole, for which a series of small holes must be made close together inside the circumference and united with a fret saw.  Work the hole out carefully till the flue, which should be slightly tapered at the end, can be driven through an eighth of an inch or so.  The flue hole in the top should be made a good fit, full size.

Rivet a collar, x (Fig. 80, a), of strip brass 1/4 inch above the bottom of the flue to form a shoulder.  Another collar, y (Fig. 80, c), is needed for the flue above the top plate.  Put the ends and flue temporarily in place, mark off the position of y, and drill half a dozen 5/32-inch screw holes through y and the flue.  Also drill screw holes to hold the collar to the boiler top.

The steam-pipe is a circle of 5/16-inch copper tube, having one end closed, and a number of small holes bored in the upper side to collect the steam from many points at once.  The other end is carried through the side of the boiler.

[Illustration:  Fig. 81.—­Perspective view of horizontal boiler mounted on wooden base.]

Assembling.—­The order of assembling is:—­Rivet in the bottom; put the steam-pipe in place; rivet in the top; insert the flue, and screw collar y to the top; expand the bottom of the flue by hammering so that it cannot be withdrawn; insert the stays and screw them up tight; silver-solder both ends of the flue, the bottom ends of the stays, and the joint between bottom and barrel.  The water-tubes are then inserted and silver-soldered, and one finishes by soft-soldering the boiler top to the barrel and fixing in the seatings for the water and steam gauges, safety-valve, mud-hole, filler, and pump-if the last is fitted.

The furnace is lined with a strip of stout sheet iron, 7 inches wide and 19-1/4 inches long, bent round the barrel, which it overlaps for an inch and a half.  Several screws hold lining and barrel together.  To promote efficiency, the furnace and boiler is jacketed with asbestos—­or fire-clay round the furnace—­secured by a thin outer cover.  The enclosing is 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.