work, which must be done, which requires mere physical force, while he will engage in the more delicate operation of guiding and regulating the power with which it arms him—and it will lessen his hours of labour, and thereby give him some leisure for the rational recreation, intellectual cultivation, and more refined pursuits, which ought to characterise a being endowed with the superior faculties which have been given to him.

SECTION VI.

HIGH-PRESSURE ENGINES.

317. In the engines which have been described in the last sections, steam, about or near the ordinary pressure of the atmosphere, is used. Such steam is raised at a moderate temperature, and is attended with little risk of an explosion. But it can have no motive power, if resisted by the atmospheric pressure: to give such steam an impelling force, the space into which it acts (pushes the piston) must be a vacuum, or nearly so. Hence the necessity for a condensing apparatus, embracing condenser, air-pump, cold-water-pump, &c. These are termed CONDENSING or Low-PRESSURE Engines. They are both costly and cumbrous; and if it is desired to have an engine as cheap, simple, and light as possible, this is effected by dispensing with the condensing apparatus. The steam, in such a case, will be resisted by the atmospheric pressure; and, therefore, to do any work, must have a pressure much higher than steam acting against a vacuum. Simply to balance the

air's pressure, it must have an equal force; and it will exert no impelling power until it exceeds it considerably. Such engines are termed HIGH-PRESSURE, or NONCONDENSING.

318. Savery's engine was (in the second part of its action) a high-pressure engine. One of the simplest was that invented about 1720, by Leopold, a German, author of the Theatrum Machinarum.

Two cylinders with pistons are placed above a boiler, each having an aperture at its lower part, communicating with the boiler, or with the open air, according to the position of a valve, called a four-way cock, interposed between the boiler and cylinders. The cylinders are open above, and the piston rods are attached to levers or beams, which they work. In Fig. 22, steam is entering the cylinder s, and pushing up the piston, the force of the steam being sufficient to overcome the pressure of the air, the weight and friction of the piston, and the resistance at the other end of the beam. In the cylinder r the piston is descending by its own weight, (the steam, which, by the position of the four-way cock, has free access to the air, rushing out,) and thus pulling down the extremity of the beam to which it is attached. When the piston in s has reached the top of its cylinder, and that in is at the bottom, the four-way cock is turned so as to admit steam from the boiler into r, and the steam in s to the air. Thus, steam is again admitted to r to push up its piston, and the steam in s has access to the air-it rushes out, its elasticity is greatly weakened, and the piston in s descends. The four-way cock was contrived by Papin.

FIG. 22.

This engine was proposed by Leopold for raising water, pump-rods being attached to the extremities of the beams. This was the first high-pressure engine in which the motion was transmitted by a cylinder, piston, and beam; and the steam, after having performed its work, was made to escape into the air the method now employed in most high-pressure engines.

319. The next high-pressure engine, the first that came into use, was that of Messrs. Trevithick and Vivian, a very simple and beautiful contrivance, and which, while it is applicable to the usual purposes for which condensing engines were used, was the first engine applied to locomotion, in drawing waggons on railways. Their engine was constructed about 1802, and, two years after, was in use upon a railway at Merthyr Tydvil in South Wales.

320. In the engine of Trevithick and Vivian, the boiler was of a peculiar construction. It was of a cylindrical form, with flat ends. The flue carrying off the heated air from the grate, made a bend like a U through the boiler, from one end of it to the other, where it terminated in the chimney. The flame and heated air thus winding through the boiler, communicate a considerable quantity of heat to the water. The cylinder was in a great part immersed in the boiler, by which its temperature and the elasticity of the steam were maintained; and it was closed above, and the steam was made to produce the downwards as well as the upwards stroke. The four-way cock was used. It was placed near the top of the cylinder, and communicated with both ends of the cylinders by tubes. By its action, steam was alternately admitted from the boiler to above the piston, and from below the piston to the air; and

next, from the boiler to below the piston, and from above the piston to the air. Thus the engine acted with a force proportioned to the excess of the steam pressure above the air's pressure. The steam, upon leaving the cylinder, passed into a tube, which led through a vessel of water, which, being thus heated, was supplied to the boiler by a force-pump. It then entered the chimney, assisted in creating a draught, and escaped into the air. The end of the piston-rod was preserved vertical by slides, as will be described in the next section-on Direct Action Engines. By a flywheel the motion was equalised; and, if necessary, it could be regulated by a governor, as already described.

321. As the steam in this engine had a high elastic force, about 70 lbs. to the square inch, and was, therefore, more liable to burst the boiler, besides the usual safety-valve, another was provided, not under the control of the engineer; and in case of the water falling too low, and the boiler thereby becoming too hot, and forming steam too rapidly, or corroding, a small part of the side of the boiler, just below the lowest level at which the water should be, was formed of a fusible metal, as lead, or some metallic composition which melted at that temperature when danger might ensue, and gave exit to the steam. The steam-gauge would also act as a valve, the mercury being expelled by the force of the steam when too high in the limb exposed to the air, and of course too low (or altogether out) of the other.

322. High-pressure engines, being cheap in the first cost, occupying little room, and being easily moved, are now coming much into use. The steam is used of a pressure from about 25 to 80 lbs. on the square inch; sometimes, as in many American engines, from 100 to

150 lbs. pressure on the inch; but so high a power of steam is not used in this country. It seems not improbable that high pressure steam, applied expansively, and used along with condensation, as in the Cornwall engines, will prove the most economical mode of working

steam.

SECTION VII.

DIRECT-ACTION ENGINES.

323. LATELY, a class of engines have come much into use, in which the heavy and cumbrous beam is dispensed with, and the piston-rod is applied more or less directly to the crank itself, by which there is a considerable saving in expense, in room, and in weight, of material; all of which are so important in marine engines. Such engines are called DIRECT ACTION ENGINES.

324. Of all Direct Action Cylinder and Piston Engines, the most direct, and, perhaps, one of the most successful, is the VIBRATING, or OSCILLATING ENGINE. In this very elegant contrivance, the piston-rod is attached directly to the crank itself, and plays the part of connecting rod. To adapt the rectilineal motion of the piston-rod to the circular motion required in the shaft or axis of the engine, the cylinder, instead of being fixed, is suspended on an axis at its middle part, on which it has motion backwards and forwards, assuming alternately the positions of the two strokes of an X, and vibrating about as much as a beam on its axis. Thus, the pistonrod has a double motion-up and down—and lateral, the latter resembling the motion of the earth's axis from