f water with the steam. This trouble, very likely to occur if the
boiler has to run an engine too large for it, means a great loss of
efficiency, but it may be partly cured by making the steam pass through
coils exposed to the furnace gases on its way to the engine. This
"superheating" evaporates the globules and dries the steam, besides raising
its temperature. The small water-tube is preferable to the small fire-tube
connecting furnace and chimney, as its surface is exposed more directly to
the flames; also it increases, instead of decreasing, the total volume of
water in the boiler.

A Vertical Boiler.

[Illustration: FIG. 80.--Details of vertical boiler.]

The vertical boiler illustrated by Fig. 80 is easily made. The absence of a
water jacket to the furnace is partly compensated by fitting six
water-tubes in the bottom. As shown, the barrel is 8 inches long and 6
inches in outside diameter, and the central flue 1-1/2 inches across
outside solid-drawn 1/16-inch tubing, flanged ends, and four 1/4-inch
stays--disposed as indicated in Fig. 80 (a) and (b)--are used. The 5/16 or
3/8 inch water-tubes must be annealed and filled with lead or resin before
being bent round wooden templates. After bending, run the resin or lead out
by heating. The outflow end of each pipe should project half an inch or so
further through the boiler bottom than the inflow end.

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 mounte