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 ...

252 THE STEAM ENGINE.
Having mentioned these advantages, which are said to
arise from Mr. Hall's condenser, it is right to state that it is
in fact a reproduction of an early invention of Mr. Watt.
There is in possession of James Watt, Esquire, a drawing of
a condenser laid before parliament in 1776, in which the
same method of condensing without a jet is proposed. Mr.
Watt, however, finding that he could not procure by that
means so sudden or so perfect a vacuum as by injection,
abandoned it. I believe he also found that the tubes of the
condenser became furred with a deposite which impeded the
process of condensation. It would seem, however, that
Mr. Hall has found means to obviate these effects. It is
right to add, that Mr. Hall, in his specification, distinctly
disclaims all claim to the method of condensing by tubes
without jet.
There is another part of Mr. Hall's contrivance which
merits notice. In all engines, a considerable quantity of
steam is allowed to escape from the safety valve. Whenever
the vessel stops, the steam, which would otherwise be taken
from the boiler by the cylinders, passes out through this
valve into the atmosphere. Also, whenever the cylinders
work at under-power, and do not consume the steam as fast
as it is produced by the boiler, the surplus steam escapes
through the valve. Now, according to the principle of Mr.
Hall's method, it is necessary to save the water which thus
escapes in vapour, since otherwise the pure water of the
boiler would be more rapidly wasted. Mr. Hall accordingly
places a safety valve of peculiar construction in communication with a tube which leads to the condenser, so that whenever, either by stopping the engine or diminishing its working power, steam accumulates in the boiler, its increased
pressure opens the safety valve, and it passes through this
pipe to the condenser, where it is reconverted into water,
and pumped off by the air-pump into the cistern from which
the boiler is fed.
The attainment of an object so advantageous as to extend