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 ...

DOUBLE-ACTING STEAM ENGINE. 103
Again, suppose the crank and connector to be in the position H I K, (fig. 16,) the piston being consequently at the
bottom of the cylinder. If steam now press the piston
upward, the pivot H and the connector H I will be pressed
downward, and this pressure will urge the crank I Ic in the
direction I K. It is evident that such a force cannot turn
the crank round the centre K, and can be attended with no
other effect than a pressure on the axle or pivots of the
wheel.
Hence, in these two positions, the engine can have no
effect whatever in turning the crank. What, then, it may
be asked, extricates the machine from this mechanical
dilernma, in which it is placed twice in every revolution, on
arriving at those positions in which the crank escapes the
influence of the power? There is a tendency in bodies,
when once put in motion, to continue that motion until
stopped by some opposing force, and this tendency carries
the crank out of those two critical situations. The velocity
which is given to it, while it is under the influence of the
impelling force of the beam, is retained in a sufficient degree
to carry it through that situation in which it is deserted by
this impelling force. Although the rotatory motion intended
to be produced by the crank is, therefore, not absolutely
destroyed by this circumstance, yet it is rendered extremely
irregular, since, in passing through the two positions already
described, where the machine loses its power over the crank,
the motion will be very slow, and, in the positions of the
crank most remote from these, where the power of the beam
upon it is greatest, the motion will be very quick. As the
crank revolves from each of those positions where the power
of the machine over it is greatest, to where that power is
altogether lost, it is continually diminished,, so that, in fact,
the crank is driven by a varying power, and therefore produces a varying motion. This will be easily understood by
considering the successive positions of the crank and con
nector represented in fig. 16.