The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

STEAM NAVIGATION. 245
the water on every side in thin oblong shells, which traverse
the boiler backward and forward repeatedly, until finally
they terminate in the chimney. By this arrangement the
original expense of the boilers is very considerably increased;
but, on the other hand, their steam-producing power is also
greatly augmented; and from experiments lately made by
Mr. Watt at Birmingham, it appears that they work with
an economy of fuel compared with common land boilers in
the proportion of about two to three. Thus they have the
additional advantage of saving the tonnage as well as the
expense of one-third of the fuel.
One of the most formidable difficulties which has been
encountered in applying the steam engine to the purposes of
navigation has arisen from the necessity of supplying the
boiler with sea water, instead of pure fresh water. This
water (also used for the purpose of condensation) being
injected into the condenser, and mixed with the condensed
steam, is conducted as feeding water into the boiler.
The salt contained in the sea water, not being evaporated,
remains in the boiler. In fact, it is separated from the water
in, the same manner as by the process of distillation. As
the evaporation in the boiler is continued, the proportion of
salt contained in the water is, therefore, constantly increased,
until a greater proportion is accumulated than the water is
capable of holding in solution; a deposition of salt then
commences, and is lodged in the cavities at the bottom of
the boiler. The continuance of this process, it is evident,
would at length fill the boiler with salt.
But besides this, under some circumstances, a deposition
of lime* is made, and a hard incrustation is formed on the
inner surface of the boiler. In some situations, also, sand
and mud are received into the boiler, being suspended in
the water pumped in for feeding it. All these substances,
" Ten thousand grains of pure sea water contain muriate of soda 220 grs.,
sulphate of soda 33 grs., muriate of mag-esia 42 grs., and muriate of lime
8 grs.
x 2