We have supposed that our cylinder, when full of steam, contained just 1 pound weight at GO lbs. pressure. Let us now find out how much useful work this 1)01111(1 of steam las done for us, and we will then show how the same weight may be made to do a great deal more, by utilizing more of its great store of heat. Let 11s suppose that the area of the cylinder is 2 sq. ft., while its length (the stroke of the piston) is 81 feet. It will thus have a capacity of 7 cubic ft., as before assumed. In the first case described, we should have a pressure of 45 lbs. per square inch exerted on an area•of 288 sq. in., through a distance of 37 feet. This is equal to 45,360 foot-pounds of work. In the second case, we have a pressure of 59 lbs. per square inch on the same area, and through the !Same distance. This is equal to 50,472 foot-pounds of work, or about of the total heat supplied by the fuel.* We may now proceed to examine the way in which the same weight of steam, generated by the consumption of an identical weight of fuel. may be made to perform many times more work by "working expansively." One of the properties possessed by steam, in common with all other gases, is a ten dency to expand indefinitely. In article STEAM we mentioned and illustrated the fact that its pressure varied nearly inversely as its volume. For simplicity's sake we shall here assume that steam is a perfect gas, and follows Boyle's law, the pressure varying ezact/y inversely as the volume. We shall now describe the way in which this expansi bility of steam is taken advantage of by the engineer. If we have a cylinder of the same area as before, but of twice the length, but only intend to admit 1 lb. of steam into it at a time, it will he necessary, when the piston has traveled 31 ft. of its stroke, to shut the entrance valve, so as to prevent more steam entering;• this is called," cutting off" the steam. The piston: however, still continues its motion in thd same direction as before, propelled by the internal separative energy among the particles of steam. But as it is pressed forward, the space occupied by the steam is always increasing, and its pressure always decreasing in proportion, until at length. when the piston has reached the end of its stroke, the steam occupies exactly double its original volume—viz., 14 cubid ft., and is reduced in pressure to half its original pressure—viz., to 30 lbs. per square inch. We have thus, during the first half of the stroke, a constant pressure on the piston of GO lbs. per square inch, and during the second half a pressure gradually decreasing from 60 to 30 pounds. The mean pressure during this second half of the stroke will be found on calculation to be almost exactly 40 pounds. Let us now, in the same way as before, see what work we have been able to get out of our pound of steam by expanding it in this way. In the first half of the stroke we have 59.472 foot-pounds of twork exactly as before, and then we have in addition a mean pressure of 40-1, or 39 lhs. per square inch, exerted over 288 sq. in. for a distance of 31 feet. This equals 39.312 foot-pounds, making a total of 98,784 foot-pounds of work obtained from the steam which only gave us 59,472 before. The economy of working expansively, how ever, goes much further than this. If the cylinder had been four times its original length, and the steam had been cut off at the same point as before (which would then bo quarter instead of half stroke), we should have obtained from the 1 lb. of steam 144,34'S foot-pounds of If we had gone still further, and expanded the pound of steam into eight times its original volume, we should have obtained no less than 179,98-1 foot pounds of work, which is more than three times as much as at first.* All modern engines are worked more or less on this principle of expansion, and the general ten dency seems to be every year to adopt higher initial pressures, and larger ratios of 1 expansion.
Having thus briefly sketched the history of the steam-engine, and the theory of Its action, we may now proceed to some consideration of its mechanism. Fig. 3 represents Watt's "double-acting" condensing engine, which we have already mentioned. By "double-acting engine" we mean an engine such as was sketched in fig. 2, in which the steam acts on both sides of the piston instead of only on one, as in Newcomen's engine.
Watt's engine, though not of the form now generally used, contains all the parts now considered essential; and we may therefore it before saying anything about these parts in detail. The steam from the boiler passes direct to the valve-chest v, which is simply a long box attached to the cylinder a. In this chest are placed valves, which are so regulated as to open communication between the boiler, cylinder, and condenser, in such a way that when the top of the cylinder is open to the boiler, the bottom com municates with the condenser, and vice versa. When the steam has done its work, it passes out through the bent pipe into the condenser f, where it is met by a jet of water (not shown in the engraving), and condensed, as before explained. g is a pump called the air-pump, which continually draws away the contents of the condenser, and dis charges them into a cistern It, called the hot-well. A small force-pump, j, draws part of the water from this cistern, and sends it back again to the boiler, there to be reconverted into steam, while the rest of the water is allowed to run to waste. A suction-pump k, supplies water to the large tank round the condenser, and also for the condepsing jet. Inside the cylinder are the piston and the rod (called the piston-rod) connecting it with the beam Lb. In Newcomen's engine, the rod had only to pull the beam down and not to push it up; it could, therefore, be connected to it by a chain, as shown in fig. 1. In the double-acting engine, the piston-rod is required both to pull and to push the beam, so that the chain is no longer admissible. It is obvious that as the head of the rod must move in a straight line, while every point in the beam describes an arc of a circle, the two cannot be rigidly connected. Watt invented the arrangement of rods shown in fig. 1, by which the piston-rod head is always guided in a straight line, while the end of the beam is left free to pursue its own course. This is called a "parallel motion." 'The end of the oeam furthest from the‘cylinder is connected by a rod ce, called a con necting-rod, to the crank 1, which is firmly fixed on the shaft; and by this means the reciprocating motion of the beam is converted into the rotary motion of the "crank shaft" r. The governor in, and•the fly-wheel ee, will be explained further on.
The cylinder and its piston are both made of cast-iron. The former is very accurately bored in a lathe, and ought always to be covered outside with non-conducting material to prevent radiation of heat. It is frequently inclosed in another cylinder, and the annular space, or " jacket" between them filled with steam from the boiler, principally with the object of preventing liquefaction in the cylinder, which is fatal to economical working. The openings for the entrance or discharge of the steam are called ports.
The valve or valves which regulate the admission of steam to the cylinder vary much in construction and design. In ordinary engines one valve, called a slide-raire, does the whole work for each cylinder in a way which we shall explain by the aid of fig. 2. This figure shows the valve in two positions—namely, those corresponding to the times when the piston is at the middle of its stroke, going in the two different directions—c and d are the ports; b is the " exhaust port.," or opening through which the steam passes to the condenser; and a is the slide-valve working inside the steam-chest (not shown). The sketch to the left shows the position of the vale when the piston is moving upward. The steam enters the cylinder through d, as shown by the arrows, while the steam in the other cud is free to rush out by c, under the valve, and through b into the condenser. By the time the piston has reached the same position, going in the opposite direction, .the valve is in the position shown in the right-hand sketch, and the motion of the steam is exactly reversed. When it is de sired to "cut off" the steam earlier than half-stroke, a separate valve, called an expansion valve (of which there are innumerable varieties) is generally used. The rod to which the piston is attached is called the piston-rod, and the rod which actually drives the crank the connecting-rod. In Watt's engine, and similar machines, these are connected to opposite ends of a beam, but in the common type of engine the two rods are directly attached.