As the putting in of the tubes entails some trouble, it is worth while to select a good kettle for treatment.  Get one that is made of thick tinned sheet iron (cast-iron articles are unsuitable), or even of copper, if you are intent on making a handsome gift which will last indefinitely.  The broad shallow kettle is best suited for tubing, as it naturally has a fair heating surface, and its bottom area gives room for inserting plenty of tubes.  Also, the tubes can be of good length.  Let us, therefore, assume that the kettle will be of at least 8 inches diameter.

In Figs. 96 (a) and 96 (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.