As the cylinders were now placed be tween the wheels, their operation could not be effected in the same manner as in the Rocket. The connecting rods were accordingly made to act on two cranks, constructed upon the axle of the wheels, placed at right angles to each other, so that one may always be at its dead point, while the other was in full action. This double-cranked axle was, from the weak ness consequent upon its form, liable at first to fracture ; but improved methods of forging them subsequently gave them sufficient strength, and now the fracture of a cranked axle rarely occurs.
The two chief improvements in the lo comotive engine, which succeeded those now explained, and which brought that machine to its present state of efficiency, consisted, first, in the substitution of brass for copper tubes ; and, secondly, in the addition of another pair of wheels to support the engine. It was found, by continued experience, that the copper tubes, from some peculiar action of the fire upon them, which has never been explained or understood, were subject to rapid decay ; and in the year 1833, after an experience of about three years of the working of these engines, it occurred to Mr. Dixon, then one'of the superintend ents of the engineering department of the Liverpool and Manchester railway, to try the effect of brass tubes. The ex periment was eminently successful ; they were found to last six or eight times as long as copper tubes of the same dimen sions. Having now brought down the history of the locomotive engine to the present time, we shall give a description of one of these machines in its most im proved form.
of the moat improved Loco motive Engine in operation in 1840.—A lon gitudinal vertical section of a locomotive engine is represented here.
The boiler, as has been explained in the engines already described, is a cylin der placed upon its side ; the fire-box consists of two castings of metal, one within the other, bolted together by riv ets represented at k ; the fire-grate is represented at D. The fire door is rep resented at g, opening upon the platform where the engineer stands. It will be. perceived in the section that the fire-box is on every side surrounded by the water contained between the two casings, the level of the water in the boiler being above the roof of the fire-box. The tubes by which the flame, and the pro ducts of combustion, are drawn from the fire-box into the smoke-box are repre sented at E. The smoke-box, containing the cylinders and the blast pipe, and sup porting the chimney, is represented at F. In the engine from which the drawing was taken, the boiler is a cylinder of 71 feet long and 31 in diameter ; it is clothed with a boarding of wood, represented at a, and bound round by iron hoops screwed together at the bottom. Wood being a slow conductor of heat, this covering has the effect of keeping the boiler warm; and checking the condensation of steam.
As the top of the fire-box would be liable to be destroyed by the action of the fire, if the level of the water in the boiler were suffered to fall below it, so as to leave it uncovered, a leaden plug is inserted in it, which would melt out before the copper would become injuriously heated, and the steam rushing out at the aperture would cause the fire to be extinguished. The tubes E, which serve to conduct the flame through the boiler to the smoke box, are made of the best rolled brass, 1-13th of au inch thick, and 11 of an inch in external diameter ; they are 124 in number, and the distance between tube and tube is three-quarters of an inch. The number of these tubes is at present sel dom less than 90, and varies between that and 150. The tubes act as stays, connect ing the ends of the boiler to strengthen them ; but, besides these, there are rods of wrought-iron, which extend from end to end of the boiler, above the roof of the fire-box. The smoke box F, containing the cylinders, steam pipe, and blast-pipe, is 4 feet wide, and 2 feet long ; it is formed of wrought-iron plates, rivetted in the same manner as those of the fire-box. From the top of the smoke-box, which, like the lire-box, is semi-cylindrical, rises the chimney G, 15 inches diameter, made of 1-inch iron plates, rivetted and bound round by hoops. Near the bottom of the smoke box the working cylinders are placed side by side, in a horizontal position, with. the slide valves upwards.
At the top of the external fire-box, a circular aperture is formed 15 inches in diameter ; and upon this aperture is placed the steam-dome T, 2 feet in height, and secured to the aperture by nuts. The steam-dome is made of brass, nearly half an inch thick. A funnel-shaped tube d, with its wide end upwards, is flanged upon the side of the great steam-pipe S, and is carried upward, so that its mouth is near the top of the steam-dome T. In
order to pass into the steam-pipe S, steam which fills the upper part of the boiler A must ascend the steam-dome and enter the funnel d, as indicated by the bent arrow. This arrangement pre vents, in a great degree, the effect of prim ing, by which word is expressed, techni cally, the spray of water which rises from the water of the boiler, and is mixed with the steam in the upper part of it ; as the steam ascends the steam-dome, this spray, falls back, and nothing but pure steam enters the funnel d. The wider part of the great steam-pipe S is flanged, and screwed at the hinder end to a corres ponding aperture in the back of the fire box, where the engineer stands ; this opening is covered by a circular plate, se cured by screws, having a stuffing-box in its centre, of the same kind as is used for the piston rods of steam cylinders. Through this stuffing-box the spindle or rod a of the regulator passes ; and to its end is attached a winch 4, by which the spindle a is capable of being turned. To the other end of this spindle, at e, is at tached a plate, which moves upon aper tures formed in the cover of the end of the great steam-pipe S ; so that, by turning the winch 71 more or less, this plate e may be removed inure or less from the openings ; and thus the steam may be allowed to in ter the steam-pipe from the steam-dome in greater or less quantity, or may be shut off altogether. The steam-pipe S being in closed within the boiler, is maintained at the same temperature ns the steam in the boiler ; and therefore the steam, in pass imn through it, is not liable to condensa tion. The steam-pipe, passing through the tube plate at the front of the boiler, is turned down at right-angles in the smoke box, where, dividing into two branches, one is conducted to each of the valve boxes of the cylinders. The lower ends of these branches are flanged to the valve boxes at the ends of the cylinders nearest to the boiler ; by these pipes the steam is conducted into the valve-boxes, or steam-chests, from which it is admitted by slide-valves to the cylinders to work the pistons. On the upper sides of the cylinders are the steam-chests U, commu nicating with the passage; m, fig. 5, lead ing to the top of the cylinder, 72 leading to the bottom, and o leading through the side-pipe to the P' blast-pipe. These openings are governed by a slide, so that, when steam is admitted through la, the communication shall be opened between n and o. Thus, when steam is admitted to the top of the cylinder, the steam from the bottom will flow from n through o, into the blast-pipe. When the piston reaches the bottom of the cylinder, then the slide opens a com munication between n. and the steam pipe, and between in and o. Thus steam will be admitted to the bottom of the cylinder, while the steam from the top will escape from m, through o to the blast-pipe. In this way, by the alternate shifting of the slide, steam is admitted alternately to each end of the cylinder, and allowed to escape from the other end, and the alternate motion of the piston and the cylinder is thereby maintained. The pistons used in locomotive engines are of the kind called metallic pistons, and, from their horizontal position, they have a ten dency to wear unequally in the cylinders, their weight pressing them on one side only ; but from their small magnitude, this effect is found to be imperceptible in practice. The cross pipe PI, which com municates with the eduction passage o, in each of the valve-boxes, has an opening in the centre, presented upward. To this opening is flanged the base of the blast pipe p, fig. 4, which rises in a direction slightly curved, and has its mouth presented up ward in the centre of the chimney G. The steam which is discharged at each stroke of the pistons from the cylinders, passes through this pipe, and escapes up the chimney by puffs. When an engine is moving slowly, these puffs are distinctly audible, resembling the coughing of a horse ; hut when at full speed, they suc ceed each other so rapidly that the ear can scarcely distinguish their intervals. It is this stream of waste steam, continu ally rushing up the chimney, that main tains the necessary draught in the fire place ; the upper current thus produced in the funnel causes a corresponding cur rent into the smoke-box F, through the tubes E ; and there is this excellence in the arrangement, that the force of the draught in the chimney being proportional to the quantity of steam produced, it must be therefore proportional to the quantity of fuel necessary to be consumed.