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 ...

WATT'S SINGLE-ACTING STEAM ENGINE. 87
manner in which this is effected will be more particularly
described hereafter. A part of the heat which would otherwise be lost is thus restored to the boiler, to assist in the
production of fresh steam. We may consider a portion of;
the heat to be in this manner circulating continually through
the machine. It proceeds from the boiler in steam, works
the piston, passes into the condenser, and is reconverted
into hot water; thence it is passed to the hot well, from
whence it is pumped back into the boiler, and is again
converted into steam, and so proceeds in constant circulation.
From what has been described, it appears that there are
four pistons attached to the great beam, and worked by the
piston of the steam cylinder. On the same side of the
centre with the cylinder is the piston-rod of the air-pump,
and on the opposite side are the piston-rods of the hotwater pump and the cold-water pump; and lastly, at the
extremity of the beam opposite to that at which the steam
piston works, is the piston of the pump to be wrought by
the engine.
(53.) The position of these piston-rods with respect to
the centre of the beam depends on the play necessary to be
given to the piston. If the play of the piston be short, its
rod will be attached to the beam near the centre; and if
longer, more remote from the centre. The cylinder of the
air-pump is commonly half the length of the steam cylinder,
and its piston-rod is attached to the beam at the point exactly
in the middle between the end of the beam and the centre.
The hot-water pump not being required to raise a considerable quantity of water, its piston requires but little play,
and is therefore placed near the centre of the beam, the
piston-rod of the cold-water pump being farther from the
centre.
(54.) It appears to have been about the year 1763, that
Watt made these improvements in the steam engine, and
constructed a model which fully realized his expectations.