When the ring is completed, join the holes with a cold chisel
held obliquely. Then file carefully with a round file, just not cutting the
scratch. As the side of the hole nearest to the bottom corner should run
obliquely to enable the tube to pass, work this out with the file held at
an angle.

As soon as a pair of holes (one in the bottom, the other in the side) have
been made, true up the side hole until a piece of tubing will run through
it at the correct angle. Then bevel off the end to 45 degrees and pass the
tube through again, bringing the bevel up against the bottom hole from the
inside. If it is a trifle difficult to pass, bevel off the edge slightly on
the inside to make a fairly easy driving fit. (Take care not to bulge the
bottom of the kettle.) Mark off the tube beyond the side hole, allowing an
eighth of an inch extra. Cut at the mark, and number tube and hole, so that
they may be paired correctly later on.

When all the tubes are fitted, "tin" the ends with a wash of solder before
returning them to their holes. If there is a gap at any point wide enough
to let the solder run through, either beat out the tube from the inside
into contact, or, if this is impracticable, place a bit of brass wire in
the gap. Use powdered resin by preference as flux for an iron kettle, as it
does not cause the rusting produced by spirit of salt. If the latter is
used, wipe over the solder with a strong ammonia or soda solution, in order
to neutralize the acid.

As the hot gases may tend to escape too quickly through large tubes, it is
well to insert in the upper end of each a small "stop," x--a circle of
tin with an arc cut away on the bottom side. To encourage the gases to pass
up the tubes instead of along the bottom, a ring of metal, y, may be
soldered beyond the bottom holes, if an oil or spirit stove is to be used.
This ring should have notches cut along the kettle edge, so as not to
throttle the flame too much.

[Illustration: FIG. 96--(b), (c), and (d).]

As the tubes for these require bending to shape in each case, the three
types may be grouped together. The tubes of c and d, which require bending
to somewhat sharp curves, may be of 3/8-inch internal diameter. In the last
two cases the direction of the water travel is shown. The up-flow end,
which projects farther through the bottom than the down-flow, is nearer the
centre, where, if a gas stove is used, the heat is more intense than at the
circumference of the bo