HISTORY OF THE LOCOMOTIVE.

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gested by Robison in 1759. In 1784 Watt patented a locomotiveengine, which, however, he never executed. About the same time Murdoch, assistant to Watt, made a very efficient working model of a locomotive-engine. In 1802 Trevithick and Vivian patented a locomotive-engine, which was constructed and set to work in 1804 or 1805. It travelled at about 5 miles an hour, with a net load of ten tons. The use of fixed steam-engines to drag trains on railways by ropes, was introduced by Cook in 1808.

After various inventors had long exerted their ingenuity in vain to give the locomotive-engine a firm hold of the track by means of rackwork-rails and toothed driving-wheels, legs and feet, and other contrivances, Blackett and Hedley, in 1813, made the important discovery that no such aids are required, the adhesion between smooth wheels and smooth rails being sufficient. To adapt the locomotiveengine to the great and widely-varied speeds at which it now has to travel, and the varied loads which it now has to draw, two things are essential that the rate of combustion of the fuel, the original source of the power of the engine, shall adjust itself to the work which the engine has to perform, and shall, when required, be capable of being increased to many times the rate at which fuel is burned in the furnace of a stationary engine of the same size; and that the surface through which heat is communicated from the burning fuel to the water shall be very large compared with the bulk of the boiler. The first of these objects is attained by the blast-pipe, invented and used by George Stephenson before 1825; the second by the tubular boiler, invented about 1829, simultaneously by Séguin in France and Booth in England, and by the latter suggested to Stephenson. On the 6th October, 1829, occurred that famous trial of locomotive-engines, when the prize offered by the directors of the Liverpool and Manchester Railway was gained by Stephenson's engine the 'Rocket,' the parent of the swift and powerful locomotives of the present day, in which the blast-pipe and tubular boiler are combined. Since that time the locomotive engine has been varied and improved in various details, and by various engineers. Its weight now ranges from five tons to fifty tons; its load from fifty to five hundred tons; its speed from ten miles to sixty miles an hour."

389. Description of a Locomotive.-A section of a locomotive is represented in Fig. 329. The boiler is cylindrical. Its forward end abuts on a space beneath the chimney, called the smoke-box. At its other end is a larger opening, surrounded above and on the two sides

by the boiler, and called the fire-box. The fuel is heaped up on the bars which form the bottom of the fire-box, and the cinders fall on the line. The fire-box and smoke-box are connected by brass tubes, firmly rivetted to the ends of the boiler; and the products of combus

tion escape by traversing these from end to end. The tubes are very numerous, usually from 150 to 180, thus affording a very large heating surface. The water in the boiler stands high enough to cover all the tubes, as well as the top of the fire-box. Its level is indicated in the same way as in stationary engines; and water is pumped in from the tender as required; its amount being regulated by means of a stop-cock in the pipe e'.

The steam escapes from the boiler by ascending into a dome, which forms its highest part, and thence descending the tube p, this arrangement being adopted in order to free the steam from drops of water. It then passes through a regulator q, which can be opened to a greater or less extent, into the pipe s, which leads to the valvechests and traverses the whole length of the boiler. There are two

APPARATUS FOR REVERSING ENGINES.

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cylinders, one on each side of the engine, each having a valve-chest and slide-valve, by means of which steam is admitted alternately before and behind the pistons. The steam escapes from the cylinder, through the blast-pipe v, up the chimney, and thus increases the draught of the fire. a is one of the pistons, b the piston-rod, cc' the connecting-rod, which is jointed to the crank d on the axle of the driving-wheel m. The cranks of the two driving-wheels, one on each side of the engine, are set at right angles to each other, so that, when one is at a dead point, the other is in the most advantageous position. w is a spring safety-valve, and J the steam-whistle.

390. Apparatus for Reversing: Link-motion.-The method usually employed for reversing engines is known as Stephenson's link

motion, having been first employed in locomotives constructed by Robert Stephenson, son of the maker of the "Rocket." The merit of the invention belongs to one or both of two workmen in his employ-Williams, a draughtsman, who first designed it, and Howe, a pattern-maker, who, being employed by Williams to construct a model of his invention, introduced some important improvements.

The link-motion, which is represented in Fig. 330, serves two purposes; first, to make the engine travel forwards or backwards at pleasure; and, secondly, to regulate the amount of expansion which

shall take place in the cylinder. Two oppositely placed eccentrics, A and A', have their connecting-rods jointed to the two extremities of the link BB', which is a curved bar, having a slit, of uniform width, extending along nearly its whole length. In this slit travels a stud or button C, forming part of a lever, which turns about a fixed point E. The end D of the lever DE is jointed to the connectingrod DN, which moves the rod P of the slide-valve. The link itself is connected with an arrangement of rods LIKH,1 which enables the engine-driver to raise or lower it at pleasure by means of the handle GHF. When the link is lowered to the fullest extent, the end B of the connecting-rod, driven by the eccentric A, is very near the runner C which governs the movement of the slide-valve; this valve, accordingly, which can only move in a straight line, obeys the eccentric A almost exclusively. When the link is raised as much as possible, the slide-valve obeys the other eccentric A', and this change reverses the engine. When the link is exactly midway between the two extreme positions, the slide-valve is influenced by both eccentrics equally, and consequently remains nearly stationary in its middle position, so that no steam is admitted to the cylinder, and the engine stops. By keeping the link near the middle position, steam is admitted during only a small part of the stroke, and consequently undergoes large expansion. By moving it nearer to one of its extreme positions, the travel of the slide-valve is increased, the ports are opened wider and kept open longer, and the engine will accordingly be driven faster, but with less expansion of the steam. As a means of regulating expansion, the link-motion is far from perfect, but its general advantages are such that it has come into very extensive use, not only for locomotives but for all engines which need reversal.

393. Gas-engines. This name includes engines in which work is obtained by the expansion of a mixture of coal-gas and air, on ignition or explosion. In Lenoir's engine a piston is driven alternately in opposite directions by successive ignitions of such a mixture on opposite sides of it, the proportions of gas and air being such as not to yield a true explosion.

In the engine of Otto and Langen (Fig. 331), a true explosive mixture is introduced beneath the piston, and is exploded by means

1I is a fixed centre of motion, and the rods KI, ML are rigidly connected at right angles to each other. M is a heavy piece, serving to counterpoise the link and eccentric rods.