in н, the surface of the liquid is pressed so low that the ball falls and opens the valve, and allows it to escape. The air

in H is that which is pumped from the condenser with the liquid, and from which it was disengaged.

Let us suppose the piston at the top of the cylinder: it strikes the tail of the valve T, and raises it, while the stem of the piston-valve R strikes the top of the cylinder, and is pressed into its seat. A free communication is at the same time open between the cylinder, below the piston and the condenser, through the tube D. The pressure of the steam

thus admitted above the piston acting against the vacuum below it, will cause its descent. On arriving at the bottom of the cylinder, the tail of the piston-valve R will strike the bottom, and it will be lifted from its seat, so that a communication will be opened through it with the condenser. At the same moment, a projecting spring K, attached to the piston-rod, strikes the stem of the steam-valve T, and presses it into its seat. Thus while the further admission of steam is cut off, the steam above the piston flows into the condenser, and the piston being relieved from all pressure, is drawn up by the momentum of the fly-wheel, which continues the motion it received from the descending force. On the arrival of the piston again at the top of the cylinder, the valve T is opened and R closed, and the piston descends as before, and so the process is continued.

The mechanism by which motion is communicated from the piston to the fly-wheel is peculiarly elegant. On the axis of the fly-wheel is a small wheel with teeth, which work in the teeth of another larger wheel L. This wheel is turned by a crank, which is worked by a cross-piece attached to the end of the piston-rod. Another equal-toothed wheel м is turned by a crank, which is worked by the other end of the cross-arm attached to the piston-rod.

One of the peculiarities of this engine is, that the liquid which is used for the production of steam in the boiler circulates through the machine without either diminution or admixture with any other fluid, so that the boiler never wants more feeding than what can be supplied from the hot well H. This circumstance forms an important feature in the machine, as it allows of ardent spirits being used in the boiler instead of water, which, since they boil at low heats, promised a saving of fuel. The inventor proposed that the engine should be used as a still, as well as a mechanical power, in which case the whole of the fuel would be saved.

(143.) That part of Cartwright's piston which in the common piston is occupied by the packing of gasket, already explained (141.), was filled by a number of rings, one placed within and above another, and divided into three or four seg

ments. Two rings of brass were made of the full size of the cylinder, and so ground as to fit the cylinder nearly steam-tight. These were cut into several segments A A A (fig. 67.), and Fig. 67. were placed one above the other, so as to fill the space between the top and bottom plates of the piston. The divisions of the segments of the one ring were made to fit between the divisions of the other. Within these another series of rings, B B B, were placed, similarly constructed, SO as to fit

B

within the first series in

the same manner as the first series were made to fit within the cylinder. The joints of the upper series of each set of rings are exhibited in the plan (fig. 67.); the places of the joints of the lower series are position of the rings of each shown in the section (fig. 68.). Fig. 68.

shown by dotted lines; the series one above the other is The joints of the inner series of rings are so placed as to lie between those of the outer series, to prevent the escape of steam which would take place by one continued joint from top to bottom of the packing. The segments into which

the rings are divided are pressed outwards by steel springs in the form of the letter V, the springs which act upon the outer series of segments abutting upon the inner series, and those which act on the inner series abutting upon the solid centre of the piston: these springs are represented in fig. 67.

(144.) An improved form was given to the metallic piston by Barton. Barton's piston consists of a solid cylinder of cast iron, répresented at A in section in fig. 69., and in plan in

fig. 70. In the centre of this is a conical hole, increasing in

ton form circles equal in magnitude to the section of the cylinder, the intermediate part of the body of the piston forms a circle less than the former by the depth of the groove. Let a ring of brass, cast iron, or cast steel, be made to correspond in magnitude and form with this groove, and let it be divided as represented in fig. 70., into four segments cccc, and four corresponding angular pieces D D D D. Let the groove which surrounds the piston be filled by the Fig. 70.

four segments with the four wedge-like angular pieces within them, and let the latter be urged against the former by eight spiral springs, as represented in fig. 69. and fig. 70. These springs will abut against the solid centre by the piston, and will urge the segments c against the cylinder. The spiral springs which urge the wedges are confined in their action by steel pins which pass through their centre, and by being

confined in cylindrical cavities worked into the wedges and into corresponding parts of the solid centre of the piston, as the segments c wear, the springs urge the wedges outwards, and the points of the latter protruding, are gradually worn down so as to fill up the spaces left between the segments, and thus to complete the outer surface of the piston.

Various other forms of metallic pistons have been proposed, but as they do not differ materially in principle from those we have just described, it will not be necessary here to describe them.