eat
passes through the chimney, while a further proportion is radiated from
the boiler. Professor John Perry[1] considers that this waste amounts,
under the best conditions at present obtainable, to eleven-twelfths of
the whole. We have to burn a shillingsworth of coal to capture the
energy stored in a pennyworth. Yet the steam-engine of to-day is three
or four times as efficient as the engine of fifty years ago. This is due
to radical improvements in the design of boilers and of the machinery
which converts the heat energy of steam into mechanical motion.

CIRCULATION OF WATER IN A BOILER.

If you place a pot filled with water on an open fire, and watch it when
it boils, you will notice that the water heaves up at the sides and
plunges down at the centre. This is due to the water being heated most
at the sides, and therefore being lightest there. The rising
steam-bubbles also carry it up. On reaching the surface, the bubbles
burst, the steam escapes, and the water loses some of its heat, and
rushes down again to take the place of steam-laden water rising.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

If the fire is very fierce, steam-bubbles may rise from all points at
the bottom, and impede downward currents (Fig. 1). The pot then "boils
over."

Fig. 2 shows a method of preventing this trouble. We lower into our pot
a vessel of somewhat smaller diameter, with a hole in the bottom,
arranged in such a manner as to leave a space between it and the pot
all round. The upward currents are then separated entirely from the
downward, and the fire can be forced to a very much greater extent than
before without the water boiling over. This very simple arrangement is
the basis of many devices for producing free circulation of the water in
steam-boilers.

We can easily follow out the process of development. In Fig. 3 we see a
simple U-tube depending from a vessel of water. Heat is applied to the
left leg, and a steady circulation at once commences. In order to
increase the heating surface we can extend the heated leg into a long
incline (Fig. 4), beneath which three lamps instead of only one are
placed. The direction of the circulation is the same, but its rate is
increased.

[Illustration: FIG. 3.]

A further improvement results from increasing the number of tubes (Fig.
5), keeping them all on the slant, so that the heated water and steam
may rise freely.

THE ENCLOSED FURNACE.

[Illustration: FIG. 4.]