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. [Illustration] 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. [Illustration] 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.