proper examination the same may be verified for all other vapours. The true peculiarity is the facility with which this form of bodies assumes the liquid state. The moment the pressure of the air is restored, in this experiment, the ethereal vapour collapses into the liquid condition. The same fact may be illustrated in another way. If we take a mattrass, a, Fig. 310, and fill the bulb and tube of it with water, and then introduce a little sulphuric ether into its upper part, the mouth dipping beneath some water contained in a jar, on heating the bulb by a spirit-lamp, the ether presently vaporizes. It may now be remarked-1st, that a vapour occupies a great deal more space than the liquid from which it comes; 2nd, that it has not a misty appearance; 3rd, that, under a reduction of temperature, it collapses into the liquid state-for, on removing the lamp, and suffering the bulb to cool, the vapour disappears. Either by diminution of temperature or increase of pressure, vapours may be condensed into the liquid state, and in this consists the chief distinction between them and gases. Fig. 310. CHAPTER LIII. THE STEAM ENGINE. Elementary Steam Engine Forms of this Machine-Description of the High-Pressure Engine-Principle of the Low-Pressure Engine, Description of the Double-Acting Engine-Estimate of Performance. On the elastic force of steam, and on the rapidity with which it is condensed by application of cold, the construction of the different forms of steam-engine depends. The instrument represented in Fig. 311, gives a clear idea of the elementary parts of a steam-engine. It consists of a eylindrical glass tube, B, terminating in a bulb, A. In the tube a piston moves up and down, air-tight, and a little water having been placed in the bulb, it is brought to the boiling point by the application of a lamp. As the steam forms, it presses the piston upward by reason of its elastic force, and, on dipping the bulb into cold water, the steam condenses and produces a partial vacuum, the piston being driven downward by the pressure of the air. There are a great many modifications of the steam-engine. They may, however, for the most part, be reduced to two kinds: 1st, High-pressure engines; 2nd, Low-pressure engines. B The high-pressure engine, which is the simplest of the two forms, consists essentially of a very strong iron vessel or boiler, in which the steam is generated; a cylinder in which a steam-tight piston moves backward and forward; an arrangement of valves or cocks, so adjusted as alternately to admit the steam above Fig. 311. and below the piston, and also alternately to let it escape into the air; and, lastly, a suitable contrivance by which the oscillations of the piston may be converted into other kinds of motion, suited to the work which the engine has to perform. с The action of the steam in one of these machines may be understood from the annexed diagram, Fig. 312. Let f be the cylinder, in which a solid piston, e, moves, steam-tight, and let us suppose the piston near to the bottom of the cylinder. The steam is now admitted through an aperture, a, and by its elastic force pushes the piston to the top of the cylinder. The movement of the piston-rod rearranges the openings into the cylinder, closing at a particular moment a, through which the steam has already come, and opening b; simultaneously, also, it opens c and closes d. Through c, from the boiler, a fresh supply of steam arrives, while it is shut off from a. This steam cannot escape through d, because that is closed— it therefore takes effect upon the piston and pushes it downward, all the vapour beneath escaping out into the air through b, which Fig. 312. has been opened. This downward movement of the piston-rod rearranges all the valves, reversing the positions they have just had. It therefore opens a, shuts b; opens d, and shuts c. a Steam now comes in from the boiler, through a, but cannot escape through b; it therefore pushes up the piston, driving out the steam, which is on its opposite side, through đ, and in this way a reciprocating motion is produced. The means of opening and shutting the apertures leading into the cylinder at the proper moment differ in different engines-sometimes cocks are used, and sometimes sliding-valves. In this engine, therefore, the piston moves in both ways against the pressure of the air. The steam must be necessarily raised from water at a high boiling point, and hence these machines are much more liable to accident than the low-pressure engine, now to be described. b The rapid condensibility of steam-a principle intimately concerned in the action of the low-pressure engine-may be illustrated in the following manner:-Take a glass flask, a, Fig. 313, and adjust to its mouth a wide bent tube, b, both ends of which are open, having previously placed a quantity of water in the flask. Apply the flame of a spirit-lamp, and bring the water to the boiling point, continuing the ebullition until all the air is driven out of the flask, and nothing but steam remains. Then dip the open end of the tube into the jar, c, containing some cold water, and remove the lamp; the steam in the tube will at once begin to condense, through the influence of the cold water, which soon rises over the bent portion, and precipitates itself into the flask, often with so much violence as to break it to pieces. Fig. 313. Of the low-pressure engine we have varieties such as the single-acting and the double-acting engine. In the former, the piston is driven one way by means of steam acting against a vacuum, returning the other way by the counterpoising weight of the machinery. The machine, therefore, in reality, is only in action during half its motion. The double-acting engine has the steam employed to produce both the ascent and descent of the piston into a vacuum on the opposite side.. It therefore works continuously. In expansive engines the supply of steam, instead of being continued during the entire ascent or descent of the piston, is cut off when the movement is one-half or one-third accomplished; the expansion of that steam driving the piston through the rest of the cylinder. The following is a description of the double-acting engine. Fig. 314 represents the boiler and its appurtenances; Fig. 315 the engine. B B, Fig. 314, is the boiler, of a cylindrical shape, the fire, F F, is applied beneath; W W is the Fig. 314. water-level, and S is occupied by steam. At t t there is a bent glass tube, open at both extremities, and so arranged that one end is in the steam space, and the other in the water; it serves to show the level of the water in the boiler. In some cases, two cocks, c and d, are inserted in the boiler, one entering into the steam part and one beneath the water. On opening them, if the water is at its proper level, steam will escape from the upper, and water from the under one. If there is too much water, it escapes from both. The boiler is continually replenished by the feed-pipe, the nature of which has been explained in Chapter XIV. At M there is a barometer-gauge, to show the elastic force of the steam; at e a b a safety-valve, with its weight, w; this opens upward, so that, should the elastic force in the interior of the boiler become too great, the valve opens, and the steam escapes. On the contrary, to prevent the boiler being crushed in by the atmospheric pressure, when the expansive force of the steam happens to decline, there is a second valve at U, with its lever, a c b, and weight, w, which opens inward, that when the external pressure exceeds the weight the air may find access to the inside of the boiler. And, as it is necessary from time to time to clear the boiler from the incrustations or deposits of salt and other impurities, there is an opening, as at L, through which access can be had. This, of course, is, at other times, securely closed. Lastly, from the boiler there passes the steam-pipe, s, which is opened by the valve at N. Fig. 315 represents the engine, properly speaking. At zz it should be imagined as being continuous with z z of Fig. 314; so that in both figures the tubes ii and ss are continuous. In both, s is the tube along which the steam from the boiler is delivered to the cylinder-passing through the four-way cock, a, either down through a or up through b, into the cylinder C, in which the piston, P, moves. Admission for the steam, above or below the piston, is regulated by a system of levers, y y, the necessary motion being communicated by the machine itself. The piston-rod, E, is connected with the beam, B F, working on the fulcrum, A. The connecting-rod is FR. At R it is attached to the crank by a pivot, H H H, being the flywheel, the revolution of which gives uniformity to the motion. The steam, after elevating or depressing the piston, passes through the eduction-pipe, ff, into the condenser, J, which is immersed in a cistern, L, of cold water. In this it is condensed into water by a jet which passes through the injection cock. The resulting warm water is pumped out by the air-pump, 0, into the hot well, W; thence it is carried, by the hot-water pump, b, along the feed-pipe, ii, into the boiler. The cold-water pump, S, supplies the reservoir with cold water. All the pumps are worked by the beam of the engine. The supply of steam is regulated by the governor, G, so as to be kept constant. The performance of steam-engines is commonly estimated by horse-power. The value of the power of one horse is a force sufficient to raise 33,000 pounds one foot high in one minute. We will now explain how motion is communicated to the fly-wheel, and thence to machinery. This we can explain without a diagram. You have, we dare say, often watched a knife-grinder, and perhaps examined his grinding apparatus. You observe that he puts his foot upon a treadle, from which a strap passes to a crooked part of the axis of the fly-wheel. This part is called a crank. Now, there is just such a crank upon the axis of the fly-wheel of a steam-engine. If, then, you imagine a rod, or strap, to proceed from one end of the beam to this crank, you will at once see that it will revolve as the beam goes up and down; in fact, the beam and rod only supply the place of the knife-grinder's treadle and strap. This fly-wheel is of great use; it is, as it were, a reservoir of work, for it soon attains a steady equal motion; and if it should happen that at certain times there is less work to be done, the extra power of the engine is accumulated in, and taken up by, the fly-wheel; and the momentum it acquires also prevents the engine from stopping suddenly, and it is thus calculated to give an uniform motion to the machinery connected with it. The giant power, from earth's remotest caves, The lengthening bars, in thin expansion squeeze; |