safely leave this to the unassisted action of
nature as we do in culinary operations? We may, if we do not care for
the three most important aims in steam-boiler construction, namely,
efficiency, durability, and safety, each of which is more or less
dependent upon a proper circulation of the water. As for efficiency, we
have seen one proof in our kettle. When we provided means to preserve
the circulation, we found that we could carry a hotter fire and boil
away the water much more rapidly than before. It is the same in a steam
boiler. And we also noticed that when there was nothing but the
unassisted circulation, the rising steam carried away so much water in
the form of foam that the kettle boiled over, but when the currents were
separated and an unimpeded circuit was established, this ceased, and a
much larger supply of steam was delivered in a comparatively dry state.
Thus, circulation increases the efficiency in two ways: it adds to the
ability to take up the heat, and decreases the liability to waste that
heat by what is technically known as priming. There is yet another way
in which, incidentally, circulation increases efficiency of surface, and
that is by preventing in a greater or less degree the formation of
deposits thereon. Most waters contain some impurity which, when the
water is evaporated, remains to incrust the surface of the vessel. This
incrustation becomes very serious sometimes, so much so as to almost
entirely prevent the transmission of heat from the metal to the water.
It is said that an incrustation of only one-eighth inch will cause a
loss of 25 per cent in efficiency, and this is probably within the truth
in many cases. Circulation of water will not prevent incrustation
altogether, but it lessens the amount in all waters, and almost entirely
so in some, thus adding greatly to the efficiency of the surface.

[Illustration: Fig. 3]

A second advantage to be obtained through circulation is durability of
the boiler. This it secures mainly by keeping all parts at a nearly
uniform temperature. The way to secure the greatest freedom from unequal
strains in a boiler is to provide for such a circulation of the water as
will insure the same temperature in all parts.

3rd. Safety follows in the wake of durability, because a boiler which is
not subject to unequal strains of expansion and contraction is not only
less liable to ordinary repairs, but also to rupture and disastrous
explosion. By far the mos