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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

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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 replentshed 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, yy, 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

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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 Hy-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,
Lifts with strong arm her dark reluctant waves;
Each cavern'd rock, and hidden den explores,
Drags her dark coals, and digs her shining ores :
Next in close cells of ribbed oak confined,
Gale after gale, he crowds the struggling wind;
The imprison'd storms through brazen nostrils roar,
Fan the white flame, and fuse the sparkling ore.
Here, high in air, the rising stream he pours,
To clay-built cisterns, or to lead-lined towers;
Fresh through a thousand pipes the wave distils,
And thirsty cities drink th' exuberant rills:
There the vast millstone, with inebriate whirl,
On trembling floors his forceful fingers twirl,
Whose flinty teeth the golden harvests grind,
Feast without blood, and nourish human kind.
Now his hard hands on Mona's rifted crest,
Bosom'd in rock her azure ores arrest;
With iron lips his rapid rollers seize

The lengthening bars, in thin expansion squeeze;
Descending screws, with ponderous fly-wheels wound
The tawny plates, the new medallions round;
Hard dies of steel the capeous circles cramp,
And with quick fall his massy hammers stamp,
The harp, the lily, and the lion join,
And George and Britain guard the sterling coin.
Soon shall thy arm, unconquered steam! afar
Drag the slow barge, or drive the rapid car;
Or on wide-waving wings, expanded bear,
The flying chariot through the fields of air.

PART II.

POPULAR GEOLOGY.

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