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. 253
the powers of steam navigation, and to render the performance of voyages of any length practicable, so far as the efficiency of the machinery is concerned, has naturally stimulated the inventive genius of the country. The preservation
of the boiler by the prevention of deposite and incrustation
is an object of paramount importance; and its attainment
necessarily involves, to a certain degree, another condition
on which the extension of steam voyages must depend, viz.
the economy of fuel. In proportion as the economy of fuel
is increased, in the same proportion will the limit to which
steam navigation may be carried be extended.
(116.) A patent has been obtained by Mr. Thomas
Howard of London for a form of engine possessing much
novelty and ingenuity, and having pretensions to the attainment of a very extraordinary economy of fuiel, in addition
to those advantages which have been already explained as
attending Mr. Hall's engines. In these engines, as in Mr.
Hall's, the steam is constantly reproduced from the same
water, so that pure or distilled water may be used; but Mr.
Howard dispenses with the use of a boiler altogether. The
steam also with which he works is in a state essentially different from the steam used in ordinary engines. In these,
the vapour is raised directly from the water in a boiling
state, and it contains as much water as it is capable of holding at its temperature. Thus, at the temperature of 2120, a
cubic foot of steam used in common engines will contain
about a cubic inch of water; but in the contrivance of Mr.
Howard, a considerable quantity of heat is imparted to the
steam before it passes into the cylinder in addition to what
is necessary to maintain it in the vaporous form.
A quantity of mercury is placed in a shallow wroughtiron vessel over a coke fire, by which it is maintained at the
temperature of from 400~ to 5000. The surface exposed to
the fire is three-fourths of a square foot for each horse power.
The upper surface of the mercury is covered by a very thin
plate of iron, which rests in contact with it, and which is so
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