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 ...

Kt34 THE STEAM ENGINE.
the vigour of their constitutions; and again, there will be as
great differences in the performance of the same engine when
in good and bad order, from all the parts being tight and well
oiled, so as to move with little friction, as there is in the
labour of an animal from his being in good or bad health, or
excessively fatigued: but in all these cases there will be a
maximum which cannot be exceeded, and an average which
we ought to expect to obtain.
CHAPTER IX.
DOUBLE-CYLINDER ENGINES.
Hornblower's Engine.-Woolf's Engine.-Cartwright's Engine.
(76.) THE expansive property of steam, of which Watt
availed himself in his single engine by cutting off the supply
of steam before the descent of the piston was completed,
was applied in a peculiar manner by an engineer named
Hornblower, about the year 1.781, and at a later period
by Woolf. Hornblower was the first who conceived the
idea of working an engine with two cylinders of different
sizes, by allowing the steam to flow freely from the boiler
until it fills the smaller cylinder, and then permitting-it to
expand into the greater one, employing it thus to press down
two pistons in the manner which we shall presently describe.
The condensing apparatus of Hornblower, as well as the
other appendages of the engine, do not differ materially from
those of Watt; so that it will be sufficient for our present
purpose to explain the manner in which the steam is made
to act in moving the piston.
Let c, fig. 41, be the centre of the great working beam,
carrying two arch heads, on which the chains of the pistonrods play. The distances of these arch heads from the centre