boilers were old and worn out, and not fit for service. Boilers must get corroded and weakened, by constant exposure to heat. We trust that there are few engines with such boilers plying on the shores or rivers of Great Britain; but we think that the owners of steam-vessels owe it to the public at least to adopt the precautions recommended by the Parliamentary Committee, and one or two other regulations and simple contrivances, which would render explosions next to impossible.

SECTION VII.

HIGH-PRESSURE ENGINES.

332. 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

R

it considerably. Such engines are termed HIGH-PRESSURE, or NON-CONDENSING.

333. Savery's engine was (in the second part of its action) a high-pressure engine; so were those of Worcester and De Caus. One of the simplest was that devised by Leopold, a German, author of a work of some value, entitled, Theatrum Machinarum. This engine, invented about 1720, is represented in the fol lowing figure:

Fig. 35.

Two cylinders with pistons were 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. 35, steam is entering the cylinder r, 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 h. In the cylinder s 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 g, to which it is attached. When the piston in r has reached the top of its cylinder, and that in s is at the bottom, the four-way cock is turned to the position shewn in Fig. 36. Thus, steam is again admitted to s, to push up its piston, and the steam in has access to the air-it rushes out, its elasticity is greatly weakened, and the piston in r descends. The four-way cock was contrived by Papin. This engine was proposed by Leopold for raising water, pump-rods being attached to the extremities of the beams g and h. pressure engine in which the 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 all high-pressure engines.

This was the first highmotion was transmitted

334. 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.

335. In the engine of Trevithick and Vivian, the boiler

was of 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 was thus heated and 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 attached to a cross-bar, whose motion was preserved vertical by slides, as bf in Fig. 32, p. 167. At its extremities were two side-rods, as in Fig. 28, P. 165, which worked the cranks in the usual manner. By a fly-wheel the motion was equalised; and, if necessary, could be regulated by a governor, as already described.

336. 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.

SECTION VIII.

LOCOMOTIVE ENGINES ON RAILROADS,

337. In a movable engine, it is evident that lightness and compactness are two principal objects. Hence the condensing engine of Watt, with its bulky and cumbrous beam, condenser, air-pump, &c., however well adapted for a stationary engine, is totally inapplicable for the purposes of locomotion. The object of the condensing apparatus is to produce a vacuum, that steam, of little or the ordinary elastic force, may act with considerable power. In locomotive engines, all this apparatus is dispensed with; the steam acting on one side of a piston is resisted by air pressing on the other side; the steam must have an equal force simply to balance the air's pressure; it exerts no motive power until it exceeds that, and therefore, to do any work, must have a pressure very much higher than steam acting against a vacuum. Hence, for locomotive engines HIGH-PRESSURE STEAM is always employed, the steam, after performing its office, being allowed to escape into the air.

338. The first locomotive engine on a railway, was