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 NAVIG.ATION. 247
only practised occasionally. In the Admiralty steamers, the
engineers are ordered to blow out every two hours. But it
is more usual to do so only once a day.
This method, however, of blowing out furnishes but a
partial remedy for the evils we have alluded to: a loose
deposite will perhaps be removed by such means, but an
incrustation, more or less according to the circumstances and
quality of the water, will be formed; besides which, the
temptation to work the vessel with efficiency for the moment
influences the engine-men to neglect blowing out; and it is
found that this class of persons can rarely be relied upon to
resort to this remedy with that constancy and regularity
which are essential for the due preservation of the boilers.
The class of steam vessels which, at present, are exposed to
the greatest injury from these causes are the sea-going
steamers employed by the Admiralty; and we find, by a
report made by Messrs. Lloyd and Kingston to the Admiralty, in August, 1834, that it is admitted that the method
of blowing out is, even when daily attended to, ineffectual.
"The water in the boiler," these gentlemen observe, "is
kept from exceeding a certain degree of saltness, by periodically blowing a portion of it into the sea; but whatever care
is taken, in long voyages especially, salt will accumulate,
and sometimes in great quantities and of great hardness, so
that it is with difficulty it can be removed. Boilers are
thus often injured as much in a few months as they would
otherwise be in as manv years. The other evil necessarily
resulting from this state of things is, besides the rapid destruction of the boilers, a great waste of fuel, occasioned by
the difficulty with which the heat passes through the incrustation on the inside, by the leaks which are thereby caused,
and by the practice of blowing out periodically, as before
mentioned, a considerable portion of the boiling water."
It would be impracticable to carry on board the vessel a
sufficient quantity of pure fresh water to work the engine
exclusively by its means. To accomplish this, it would be