placed under the boiler and near the fire-box; this pump is worked by a lever from one of the engine eccentrics; in some instances the pump is worked from the piston cross-head. The water is drawn from the tank in the tender through a pipe properly coupled, by a ball-and-socket joint, at the part where the engine is attached to the tender; and the water is delivered to the boiler in some instances at the smoke-box end, a little below the water-level; at others near the fire-box, and in others again near the centre part of the boiler.

There are various appliances connected with the boiler. The safetyvalve is generally placed above the fire-box, as in figs. 153-155, and is pressed down by a lever, the pressure being regulated by a spring balance. The nature of the arrangements of this kind of valve is explained at p. 45, fig. 66, Mechanics and Mechanism.

Lock-up safety-valves are falling into disuse, as, from not being easily

got at, they are apt to stick in the seat and become inoperative. In some engines a valve, as shown in fig. 146, is used. This is loaded a little above the usual pressure; this being obtained by the bent springs forcing down the valve into its seat.

The contrivance known as the steam-whistle is placed on the dome or top of the fire-box; this is shown at b, fig. 147. The peculiar noise elicited is caused by the steam rushing up the tube bb connected with the boiler; and the admission to which is regulated by a cock actuated upon by a handle or lever within reach of the engine man. The steam passes through the apertures cc and out at dd, and strikes the thin edge ee of the circular cup f, producing the sound so well known now-a-days in almost every district of the kingdom.

The level of the water in the boiler is ascertained by gauge-cocks, which are placed at different heights to indicate different levels at which the water stands. In addition, a glass water-gauge is attached to the front casing; the arrangement of this apparatus is shown in fig. 148. The tubes cc are in communication with the interior of the boiler; a strong

glass tube bb connects the two tubes cc; the upper tube c communicates with the part of the boiler which should contain steam, the lower with that containing water. On opening the upper and lower handles a a, the water rises in the glass-tube to the same height as in the inside of the boiler. The oscillations of the water which would take place in the tube bb, from the rapid movement of the engine, is in some measure prevented by making the communication between the boiler and tube of a small size. A small cock, as f, is placed at the bottom tube, to clear the tube of its accumulated water. Entrance is obtained to the interior of the boiler by means of a man-hole door of similar construction to that described in another and preceding chapter.

The cylinders of locomotive engines are always of the three-ported species, as in the diagram, fig. 149. a a the piston; b the piston-rod; c the stuffing-box; d the exhaust-port, leading to the exhaust-pipe i; g the valve; h valve-rod; e port to upper side of piston; fport to under side; k cleansing and greasing-cock. Cocks are supplied to cylinders in some instances at top and bottom of cylinder: these are opened when required by levers within reach of the engine-man, to allow the water collected in the cylinders from priming and condensation. The steam is passed from the boiler to the cylinders by a pipe, the entrance to which is at the upper part of a cylindrical vessel o N, fig. 153, or within the dome; above the firebox, as in fig. 155, the entrance to the pipe is placed thus far above the water to prevent priming as much as possible. The supply of steam to the cylinder is regulated by what is termed a regulator." Various contrivances for this purpose are adopted. Fig. 150 explains a form much introduced. Two circular discs work in contact, one of which is fixed, and

66

fig. 150.

the other is made to revolve by a lever connected with it, and actuated on by another lever or handle outside the casing. Apertures are made in both discs to correspond with each other in shape and position; when the apertures in both plates coincide, a free passage is given to the steam; but when the movable disc is turned round, so as to present the solid parts of its face opposite the apertures in the fixed disc, the entrance for the steam is lessened in proportion as the apertures in the fixed disc are closed. The discs are fixed on the entrance to the pipe which passes the steam to the cylinders.

The reciprocating motion of the piston-rod is changed into the circular one of the driving-wheel by means of a connecting-rod, as in fig. 151: a is the piston-rod; b the connectingrod, the brasses of which embrace the cranked axle of the drivingwheels nn, as explained in p. 87, fig. 169, Mechanics and Mechanism; the cranks are placed at right angles to each other, so that the motion is continuous. The slidevalves of the cylinder for admitting the steam to both sides of the piston are worked by eccentrics, as explained in p. 82, Mechanics and Mechanism, the position of which is shown in fig. 151.

fig. 151.

n

2

In order to give the necessary facility for working the engine so as to make it move backwards or forwards as desired, various ingenious arrangements have been introduced: it will suffice for our purpose to describe one of these, and that the most generally adopted. It is known as the "link-motion," and owes its invention to Mr. Stephenson. The movements are effected by four eccentrics, two to give the backward, two the forward motion of the engine; two eccentrics to each cylinder. Let bc, fig. 152, be the two eccentric-rods, b the backing eccentric, and c the forward eccentric : these are connected to the curved link aa, the radius of which is equal to that of a circle described by each eccentric-rod revolving round the centre of its eccentric. The backward and forward eccentric-rods are attached to the extremities of the links. The valve-rod which works the slide-valve of the cylinder is provided with a piece of metal which slides between the grooves made

in the interior faces of the links; the links are kept at a proper distance apart by bolts. The links are capable of being lifted up by means of the lever g, and connecting piece f. When the slide-valve is out

of gear, the eccentric-rods are in the position shown in the diagram, the line of valve-rod bisecting the angle formed by the eccentric-rods. In this case, if the engine is in motion, as just in pulling up at a station, the eccentric-rods merely make the link oscillate or vibrate in the centre, of the valve-spindle, in and out alternately, as shown by the arrows xy.

But if the valve is required to make the engine go ahead, or forward, the link is raised up by means of the handle g, until the forward eccentric-rod b is in a line with the valve-rod; and by this means the throw of the eccentric will be communicated to the valve-rod, and the engine will go forward. To reverse the engine, all that is necessary is to lower the links until the upper eccentric-rod c is placed in a line with the valve-rod, when the engine moves backward.

Having sufficiently, for the elementary purposes of our treatise, given the details of the locomotive, we now proceed to give illustrations of engines, showing the connexion of the various parts. The first we give is the longitudinal section of a "fast passenger-engine," constructed by Mr. Hackworth; and for the drawing of which, and description, we are indebted to Mr. W. Johnson, fig. 153. She has been expressly designed for fast passenger trains, having driving-wheels 6 feet 6 inches diameter, with leading and hind wheels of 4 feet diameter. Her weight, in working order, is 23 tons 15 cwt., and this is distributed in the following manner : on leading wheels, 8 tons 6 cwt.; drivers, 11 tons 4 cwt. ; and hind wheels, 4 tons 5 cwt. The crank axle, A, is carried in bearings in the inner frame, whilst those of the leading and hind wheels, B and C, are in the outer frame; the length from centre to centre of the latter pairs being 13 feet 6 inches; these proportions having been laid down with a strict reference to the stability of the leading wheels, without an undue detraction from the tractive adhesion of the drivers. The barrel, D, of the boiler presents the novelty of welded longitudinal seams: it is composed of five