In one of the modifications of Barton's piston, on the contrary, the packing is dispensed with, as the flexible springs t t t press upon the wedges c c c, and expand the intermediate plates. A break-joint is readily formed, by making the series of plates double; the second set of plates falling upon the spaces which occur between the first row.
The action of the high pressure engine depends upon the great elastic force acquired by steam, when exposed to the action of heat at very high temperatures.—It may indeed be considered as a return to the principle of Brancas and the Marquis of Worcester, as in this engine no condensing water is necessary; and it acts merely by the elastic or repellant force of steam. In the high pressure engine, the condenser is taken away; and the steam, instead of being converted into water by artificial cold in a close vessel, is allowed to escape into the atmosphere from one side of the piston, while it is acting forcibly on the other.
The advantages of the high pressure engine over that used with a condenser, are cheapness in construction, and a saving of the whole expense attendant on procuring a sufficient supply of condensing water, which in some cases is an object of considerable importance.
In the annexed section, the piston B passes through an air-tight stuffing box, and the steam is entering beneath it, by the four-way cock E. If we now suppose the piston at the top of the cylinder, a new arrangement of the communicating pipe takes place, as the steam which was beneath escapes, while a fresh supply enters above. The four-way cock may be best explained by a section in the opposite direction. Two pipes are seen at the lower extremity of the cock, which communicate with the upper and under sides of the piston. The aperture D opens to the air, while the pipe C serves for the admission of steam from the boiler.
We have now to notice the double cylinder engine constructed by Woolfs, which will be found, by reference to the diagram in the preceding page, to consist of a high pressure cylinder, connected with a condensing apparatus.
A and B represent the two cylinders, in the larger of which the steam is allowed to expand itself, after passing from the high pressure cylinder B. The steam, which in the first instance is of considerable elasticity, is admitted to the cylinder B, by the tube and valve E, and entering the cylinder above its piston, impels it to the bottom. When this is effected, a communication is opened between the upper part of the cylinder B, and the under side of the cylinder A. The communication between the cylinder B and the steam-pipe E, is now reversed, and the steam is made to press on the under side of the piston B, a communication being at the same time formed between the upper part of the cylinder A, and the pipe leading to the condenser which is seen beneath. So that if we suppose the two pistons connected by means of their rods with one end of an ordinary working beam, the upward and downward strokes of each will be performed at the same time. We have hitherto considered the steam as passing direct from the boiler to the cylinder B; this, however, is in reality effected by a more circuitous route, as it is in the first instance admitted to the steam-case of the larger cylinder by the pipe C, and passing round a similar case, encircling the cylinder B, it is then made to enter at E. The pipe at D is merely intended to form a communication for carrying back to the boiler any water that may be produced by condensation in the steam-case, before the engine arrives at a proper temperature for working.
Having thus briefly examined the nature of Mr. Woolf's engine, it may now be advisable to revert to the boiler, by which he proposes to generate steam of sufficient elasticity for the use of the small cylinder, which requires elastic vapour of great expansive force. The boiler, represented by the diagram beneath, consists of a series of tubes, of cast-iron, connected by screw-bolts with the under side of a larger vessel A A, communicating with the engine. The upper boiler is furnished with four, and in some cases, with five apertures; the first of which is intended for the admission of water, to supply the waste which continually arises from evaporation. The safety valves, man-hole, and water-pipe are also shewn.