(b) are shown two forms of fire-tube kettles (a and
b) and two of water-tube (c and d). For use over a spirit or Swedish
petroleum stove the first two types are most convenient; the third will
work well on a stove or an open fire; and the last proves very efficient on
an open fire. One may take it that, as a general rule, areas of heating
surface being equal, the water-tube kettle will boil more quickly than the
fire-tube.

Fire-tube Kettles.

The tubing of Figs. 96 (a) and 96 (b) presents a little difficulty in each
case. The straight tube is the more difficult to insert, owing to the
elliptical shape of the ends; whereas the bent tube requires only circular
holes, but must be shaped on a template.

The tubing used for (a) should have at least 5/8-inch internal diameter,
for (b) 1/2 inch, and be of thin copper. Hot gases will not pass willingly
through tubes much smaller than this, in the absence of induced or forced
draught.

For convenience in fitting, the tubes should run at an angle of 45 degrees
to the bottom and side of the kettle, as this gives the same bevel at each
end. Find the centre of the bottom, and through it scratch plainly four
diameters 45 degrees apart. From their ends draw perpendiculars up the side
of the kettle.

[Illustration: FIG. 96 (a).]

Now draw on a piece of paper a section of the kettle, and from what is
selected as a convenient water-level run a line obliquely, at an angle of
45 degrees, from the side to the bottom. Measuring off from this diagram,
you can establish the points in the side and bottom at which the upper and
longer side of the tubes should emerge. Mark these off.

Next bevel off a piece of tubing to an angle of 45 degrees, cutting off
roughly in the first instance and finishing up carefully with a file till
the angle is exact. Solder to the end a piece of tin, and cut and file this
to the precise shape of the elliptical end. Detach by heating, scribe a
line along its longest axis, and attach it by a small countersunk screw to
the end of a convenient handle.

Place this template in turn on each of the eight radii, its long axis in
line with it, being careful that the plate is brought up to the marks
mentioned above, and is on the bottom corner side of it. Scratch round
plainly with a fine steel point.

To remove the metal for a tube hole, it is necessary to drill a succession
of almost contiguous holes as near the scratch as possible without actually
cutting it.