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 ...

140 THE STEAM ENGINE.
circumstances as a descent, and in this way the process is
continued.
It is evident that the valves may be easily worked by the
mechanism of the engine itself.
In this arrangement the pistons ascend and descend together, and their rods must consequently be attached to the
beam at the same side of the centre. It is sometimes desirable that they should act on different sides of the centre of
the beam, and consequently that one should ascend while
the other descends. It is easy to arrange the valves so as
to effect this. In fig. 45, the lesser piston is at the bottom
of the cylinder, and the greater at the top. On opening the
valves c', E' r:, a vacuum is produced below the greater
piston, and steam flows from the lesser cylinder, through E',
above the greater piston, and presses it down. At the
same time steam being admitted from the boiler through c'
below the lesser piston, forces it up against the diminishing
force of the steam above it, which expands into the greater
cylinder. Thus as the greater piston descends the lesser
ascends. When each has traversed its cylinder, the valves
c', E', a' being closed, and c, E, F opened, the lesser piston
will descend, and the greater ascend, and so on.
(78.) The law according to which the elastic force of steam
diminishes as it expands, of which Mr. Woolf appears to
have been entirely ignorant, is precisely similar to the same
property in air and other elastic fluids. If steam expands
into twice or thrice its volume, it will lose its elastic force
in precisely the same proportion as it enlarges its bulk; and
therefore will have only a half or a third of its former pressure, supposing that as it expands its temperature is kept up.
Although Mr. Woolf's patent contained the erroneous principle which we have noticed, yet, so far as his invention
suggested the idea of employing steam at a very high pressure, and allowing it to expand in a much greater degree
than was contemplated either by Watt or Hornblower, it
became the means of effecting a considerable saving in fuel;