smoke will be reversed." The fuel is supplied to the hopper z, and is gradually spread over the furnace-bars from the centrifugal force generated by the revolving discs or plates.

fig. 88.

we are indebted to the courtesy of Mr. William Johnson. Fig. 87 is a "longitudinal section" of boiler, fig. 88 a "plan," and fig. 89 an "end elevation." The boiler of which these are drawings, is that adapted to the engine on the double cylinder principle which we have illustrated in figs. 38, 39.

The class of boilers of which this is a good example, are considered by a competent authority to possess numerous features of economy and safety in working; and as affording an example of what a good high-pressure boiler should be, we give it here. The length of boiler is 22 feet by 7 feet 4 inches diameter; it has two internal cylindrical flues, in the entrances to

which are placed the furnaces 6 feet long by 3 feet wide. The flues are each three feet in diameter for a length of 12 feet from the front, after which they taper off to 2 feet 8 inches, as seen in the longitudinal section fig. 87, and by the dotted lines in the plan. The whole of the taper of the flues is given in the inside edges; this admits of a greater space between the two, for the entrance of a man to clean and repair. These flues act advantageously as longitudinal stays to the flat ends of the boiler; but as the points of connection are below the centre of the ends, the upper portions are stayed from the top of the shell by strips of boiler plate a a, 12 inches broad and inch thick, and riveted at each end by angle irons. The arrows in the plan point out the direction of the current of flame, &c. The flame from the internal flues returns back by the side flues bb, thence passing by the side openings c c in the brick division into the separate bottom flues dd, running beneath the whole bottom of boiler, finally joining the main

flue at e. By this arrangement, the most intense heat is confined to the parts where sediment is most likely to be deposited, that is, on the top of the internal flues; and this part being nearest the surface of the water, the steam evolved is carried off in nearly a dry state. To retain the front brickwork above the fire doors, a stout cast-iron beam fis passed across the boiler-end, each extremity of the beam being embedded in the boiler and engine-house walls. The state of the water-level in boiler may be observed either by the glass gauge g, or the index of the stone float h; or it may be tested by the set of gauge-cocks attached to the glass gauge. The index of the stone float is attached to the counterweight i, which works freely inside a cast-iron pillar frame slotted down the front for the passage of the index, and graduated in inches. Immediately before this pillar is placed an alarm whistle, which is rendered self-acting in case of want of water by being connected with the stone float. The steam is taken from the boiler by an open ended T-headed pipe k, which is cottered to a boltstud passing through the receiving dome, The lower end of this pipe has an elbow for bolting to a short elbow pipè l, the end of which is bored to receive the foot of the stop-valve chest m. The flange of the latter rests on the top of the boiler, and bolts pass through it to the flange of the pipe l, the socket of which is thus held up against the conical foot of the stopvalve chest. n is the main safety-valve, fitted on a branch of the cross steam-pipe connecting the valve chest 7 with the main stop-valve elbow o. A second safety-valve p is also fitted to a branch at the stop-valve chest; this valve is loaded to 5 lbs. per square inch more than the main valve, and is intended to act only in case of the latter getting out of order. The bearing faces of this valve p are flat instead of conical, and it has consequently less tendency to stick to its seat. The stop-valve for regulating the steam supply of the engine is worked by a round wheel q, on a shaft passing through the wall of the engine-house. This shaft carries a small bevil wheel, whose box acts as a nut for the screwed spindle r of the stop-valve, which is guided in its vertical movement by a cross-piece working between the side-rods s s. The hand-wheel o, being placed in the enginehouse, directly opposite the starting-gear of the engine, the engine-man has perfect control over it without moving from his post. Should any accident occur to the engine whilst he is in the boiler-house, he can easily shut off the steam, without going round to the engine-house for that purpose. A double valve chest is also fitted at t, for the two purposes of regulating the feed of water and the blow-off. The feed-water is supplied by a pump worked from the side lever of the engine, the water being conveyed by the pipe w to the valve chest t, whence it passes into the boiler; the surplus water, when the valve in this chest is closed, being discharged by a branch pipe v fitted with a lever valve, weighing a little above the steam pressure. The second valve in the chest t is for blowing off the dirty water collected in the bottom of the boiler.

Cylinder- Valves and Pistons.-The steam is admitted to the upper and under sides of the piston by means of slide-valves generally, although in some cases lifting or poppet-valves are used. Slide-valves are of three varieties—the "three-port valve," the "long D valve," and the "short D valve." Figs. 90 and 91 show the arrangement of the "three-port valve.” In the valve-facing of the cylinder, as d d fig. 90, three apertures are cut, as a b c; that at b ends by a channel shown in fig. 91 to the upper side of

the cylinder, that at c to the lower side, and that at a to the atmosphere. A valve-casing, as represented by e e e, is secured to the face of the cylinder; and into this the steam is admitted by the pipe c c proceeding from the boiler; a valve h h slides on the face of the cylinder, and is moved up and down by the rod g actuated by the engine; the valve h h is of suf

ficient size to cover two of the ports, one of which is always the exhaust port a. In the diagram the valve is so placed as to be admitting the steam from below the piston-this being at the top of cylinder-to pass up by the port c in the direction of the arrows, and out to the atmosphere by the exhaust port a a. As the steam which fills the casing cannot pass either by a or c, as the valve h h covers both of the apertures, it passes up the channel b by the port b, and presses on the top of the piston. The valve hh is therefore always so arranged, that when the piston is at the top of its stroke, the valve will cover the lower steam-port c and the exhaust a; while it is at the bottom of its stroke, the valve will then cover the upper steam port bb and the exhaust. The valve-rod g is worked by an eccentric, as already explained in the work on Mechanics and Mechanism. The valve next to be described, and known as the "long D valve," is illustrated by the diagram in fig. 92. The ports for admitting steam to the upper and under sides of the piston are situated, one at top, as a, and the other at bottom, as b: the valve is semicircular, as at m n, and hollow at the back of the face m, which slides on the cylinder-facing. The valve is of length sufficient to cover the bottom and top ports a d; steam is admitted through the pipe c to the hollow space pp, situated between top and bottom faces of slide-valve. Supposing top and bottom faces, as m, to cover both ports ad, the steam from the boiler entering through the pipe c will not gain admittance either through a or d; but will remain only in the hollow part pp.

But suppose the valve to be moved by the rod ss, so that the lower

ας

O

m

port is opened, and a communication made between it and the condenser through the pipe v, at the same time the upper port is opened and a communication made between it and the hollow space pp; the steam then enters above the pis, depressing it. By means of the eccentric the slide-valve is moved in the contrary direction; the upper port a is opened, and a commut nication made between the upper side of the piston and the condenser, the steam passing down the hollow at the back of the valve as oo; the steam from the pipe c entering the cylinder by the port d, and the piston is turned up. By this arrangement it will be seen that an immediate communication is made between the condenser and under side of piston through the port d; but the steam from the upper side of piston passing through the port a, has to come down the channel along the whole length of valve before getting to the condenser by the pipe v. To prevent the steam from passing up or down as the ports are uncovered, the space between the back of valve, as t, and the inside of casing b b, is packed.

fig. 92.

The "short D slide," which remains to be described, consists of two slide-pieces, a b, fig. 93, connected by two rods cd. In some cases only one rod is used, in others three; the section of this valve is similar to that of

m

n

d

fig. 92, or the long D slide. In place of having one exhaust-passage, the short D valve "has two separate exhaust or eduction-passages fitted, to allow of the steam to pass to the condenser, one at the top of the slidecase, and the other at the bottom." Lifting or spindle-valves are similar in construction to those used by Watt in his engines. In working large engines, from the pressure of the steam forcing the valve against the working surfaces, considerable difficulty arises. in starting the engine; in some instances a small engine is employed to work the slide on first starting, in others the steam is admitted to both sides of the valve. The "balance-valve" is sometimes used in marine engines. These are much easier to work than the slide-valve. The "equilibrium-valves," used in pumping engines, consist "substantially of a cylinder open at both ends, and capable of being moved on a fixed piston with an upright stem. The cylinder stands over the hole in the steam

fig. 93.