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 ...

EXPANSIVE FORCE OF STEAM. 289
the first instance, and to escape at some convenient aperture,
which, opening outwards, will effectually prevent the subsequent readmission of air. The piston-rod and other parts
which pass from the external atmosphere to the interior of
the machine, are likewise so constructed and so supplied
with oil or other lubricating matter, that neither the escape
of steam nor the entrance of air is permitted. We are therefore now to consider the effect of the action of steam against
the piston P, when subjected to a resistance which may be
less in amount, to any extent, than the atmospheric pressure.
In such machines the steam always acts both directly by
its power, and indirectly by its condensation. In calculating
its effects, excluding friction, &c., we have therefore only
to. estimate its total force upon the piston, and to deduct the
force of the uncondensed vapour which will resist the motion
of the piston.
Supposing, then, the total force exerted upon the piston,
after deducting the resistance from the uncondensed vapour,
to be one ton, and the length of the cylinder to be one foot,
each motion of the piston from end to end of the cylinder
will produce a mechanical force equivalent to a ton weight
raised one foot high. If in this case the magnitude of the
piston be equivalent to one square foot, the pressure of the
steam will be equal to'that of the atmosphere, and the quantity of water in the form of steam which the cylinder will
contain will be a cubic inch, while the quantity of steam in
it will be a cubic foot. In proportion as the area of the piston is enlarged, the pressure of the steam will be diminished,
if the moving force is required to remain the same; but
with every diminution of pressure the density of the steam
will be diminished in the same proportion, and the cylinder
will still contain the same quantity of water in the form tof
vapour. In this way steam may be used, as a mechanical
agent, with a pressure to almost any extent less thana that of
the atmosphere, and at temperatures considerably lower than
212~. To obtain the same mechanical force, it is only
2 B 37