Illustration: BEAM OF STEAM ENGINE, WOOLWICH ARSENAL.
The great improvements in machinery--whether for looms, locomotive
engines, or steam-ships, for forging anchors, boring cannon,
rolling out and rivetting iron plates together for tubular bridges
and boilers, or any other kind of work--are chiefly owing to the
wonderful ease with which these machines can be driven by the power
of steam. It matters not whether the object to be wrought is the
head of a pin, or the crank of a steam-ship, it is done with both
delicacy of touch and power of arm, a hundredfold beyond what could
be effected by hand in the same time. The motive power of steam
is derived from the property which water has of being expanded
into vapour when heated to a certain degree, and of again resuming
the form of water when cooled; this moreover takes place in the
most easily manageable manner, and either by degrees or suddenly,
according as the heat and pressure balance each other; moreover
water, being easily obtained, and in sufficient quantity for the
purpose, in all places where machinery is required, can always be
applied. Before the use of steam, wind, water, horse, and hand power
were chiefly in use; water-mills were, of course, only erected in
those situations where a good supply of water could be obtained,
and this even often failed in dry weather; windmills also depended
on that uncertain element. Horse and hand powers are limited in
their extent, and are moreover very expensive. The first attempts
at a steam-engine were those in which the steam was only used that
by its condensation a vacuum might be formed in a cylinder under a
piston, so that the weight of the air should cause this to descend
with considerable force--15 lbs. on the square inch. The piston
was balanced by a weight, so that the steam might raise it with
scarcely any pressure; the steam beneath the piston being condensed
by a stream of cold water, the weight of the air again forced down
the piston into the vacuum. This therefore was not a steam but an
air-engine, as all the power exerted was derived from the weight
of the air, and the steam merely used to procure a vacuum. After
this came the low-pressure or condensing engine, and then the
high-pressure or non-condensing engine, both of which are now used,
the former in marine engines and the latter in locomotives.
Illustration: FLY-WHEEL, WOOLWICH ARSENAL.
Illustration: FIG. 1. BOILERS, WOOLWICH ARSENAL.
Illustration: FIG. 2. BOILERS, WOOLWICH ARSENAL.
The steam-engine consists essentially of a boiler or steam-generator,
with a furnace adapted to it, connected by a steam-pipe to a cylinder
having a piston working accurately in it, and valves so contrived
that the steam shall enter alternately above and below the piston. In
the condensing engine, each compartment, above and below the piston,
communicates with the condenser--the vessel in which the steam is
suddenly condensed by cold water--and the valves are so arranged that
when the steam enters above the piston, the space below is opened
to the condenser, and is therefore a partial vacuum; by the time
the piston is driven down by the force of the steam above it, the
space is shut off from the condenser and opened to the steam-pipe,
while the space above is shut off from the steam-pipe and opened
to the condenser. In this way one side of the piston is alternately
pressed by the steam while there is a vacuum on the other side. In
the non-condensing engine the space above and below the piston is
alternately pressed by steam at a great degree of tension; while at
the opposite side of the piston, the space is opened to the air by a
valve. These valves are what are called “sliding valves,” being both
in connection with the same action, which shuts one while it opens
the other; that is, when the piston has nearly descended, it slides
the valve which shuts off the steam from the space above and opens it
to the air, the same action opening the steam-valve below the piston
and shutting it from the air. In this kind of engine the piston is
moved simply by the power of the steam, which first presses it down
and then presses it up again, and as the steam escapes at each stroke
of the piston, and has to be at a great tension or pressure, a large
and rapidly-formed supply of steam is required. In the locomotive and
other high-pressure engines this is effected by having a great number
of tubes passing through the boiler leading from the fire-place to
the flue, so that the fire and heated air shall pass through them
before reaching the flue, and consequently, as these all pass through
the water in the boiler, producing a very rapid generation of steam.
Of the various forms of boilers, the most simple was that in which
the heat was merely applied to the lower part (fig. 1); next may be
named the wagon-head boiler, in which the flue passed all round; some
were made with a cylindrical flue passing though the whole length,
and some with two (fig. 2). Of whatever form the boiler may be, it
should be strong enough to well resist the pressure of the steam,
but to make this sure, a contrivance called a safety-valve is always
used; this consists of a valve held down by a weight, which would
be raised by the steam if it should press so hard as to endanger
the boiler in the least degree; when the safety-valve is forced up,
the steam escapes and the pressure is taken off. Most steam engines
require the up-and-down motion of the piston to be converted into
a circular motion, and this is effected by means of a “crank,”
(see “Cranks”); but this circular motion needs in most cases to be
regulated by a fly-wheel which is so heavy, that upon being set in
motion it continues to revolve for a time by its own weight, so that
the intermitting pulls exerted by the piston-rod on the crank are
blended into one continuous action (see cut); but in steam-ships, and
locomotive engines, fly-wheels cannot be used. In these cases there
are two cylinders and pistons, each fixed to a crank formed in one
axle united to the two wheels, and these cranks are so arranged that
the greatest power is exerted on one when the least is exerted on the
other, and for this purpose they are placed so that when one crank is
upright the other is horizontal. The stroke of the piston-rod is not
always made to act directly on the crank, but has a “beam” interposed
working on bearings in its centre, hence the term beam-engine (see
cut). This beam moves the crank at the opposite end to that which
is moved by the piston and at the same time works the air-pump,
feed-pump, and cold-water-pump, by means of jointed rods.