STEAM-ENGINES.
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.
FIG. 2. BOILERS, WOOLWICH ARSENAL.
FLY-WHEEL, WOOLWICH ARSENAL.
FIG. 1. 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.
FIG. 3.
In those engines which have to perform unequal work, and in which sometimes a great drag is suddenly removed from the engine, some contrivance is necessary to prevent the too rapid motion which would ensue, to the great risk of damaging the engine; this is effected by what is called the “governor;” a contrivance by which a part of the steam is struck off when the action is too rapid, and again let on when it has diminished. This arrangement is shown in [fig. 3]; the two heavy iron balls swing round as the engine works, and the faster they revolve the more they tend to separate, from the natural tendency to fly off called “centrifugal force,” and in separating they bring the other ends of the rods to which they are attached nearer together, and so push up a collar, A, attached to the levers which turn off the steam-tap; and as the action subsides the balls sink down together and the collar also, the steam being thus turned on again. In order that the pressure of the steam in the boiler may be known, a “gauge” is used, which acts on the principle of the barometer, consisting of a column of mercury which is pressed up by the force of the steam, the height to which it rises indicating the pressure. With respect to the details of the steam-engine, they are too various and complicated to be enumerated or described here; but the motion—being regular, continuous, and powerful—can be applied to almost any sort of work by being adapted to the machine suitable for such work, and which receives its motion from the steam-engine, the same as though it were worked by water or by hand.