The liedley engines presented two notable ad vances over previous locomotives. The first was the use of a return tubular boiler and the sec ond was the adoption of a smokestack of small diameter in which the exhaust cylinder steam could produce an effective draught. Almost before the practicability of Hedley's engines had been proved on the Wylam Railway, George Stephen son had recommended the adoption of locomotives by the managers of the Killingworth Colliery. It was finally decided to construct a locomotive. and the work was done in the colliery machine-shops after the designs and under the direction of Stephenson. The engine which was finally turned out of the shops was supported on four wheels. and had a wrought-iron horizontal boiler S feet long and 34 inches in diameter, with single internal flue 20 inches in diameter having a grate in one end and a 20-inch stack leading out of the other cud. There were two vertical S X 24 inch cylinders, the motion of whose pistons was conveyed to the four wheels by an ar rangement almost identical with that used by Hedley. The engine was greatly inferior to both Murray's and Hedley's engines previously de scribed. Stephenson's first engine was completed on July 25, 1814, and the following year he com pleted a second. This engine was notable be cause of the fact that the pistons were linked directly to cranks on the driving wheels. and the toothed wheels and chain of gearing previously employed were abandoned. Briefly described. this engine had a horizontal cylindrical boiler with a vertical steam-cylinder mounted on its top at each end. The piston-rod of each cylinder was connected to the centre of a horizontal cross head extending transversely across the boiler and long enough so that connecting-rods attached to its two ends and extending downward to the cranks on the wheels would clear the boiler on each side. The two pairs of driving wheels were connected together by an endless sprocket chain to insure uniformity and coincidence of rotation.
In later engines Stephenson substituted rigid side rods for the sprocket chain and supported a portion of the weight of the engine upon pistons contained in vertical cylinders beneath and coin munieating freely with the interior of the boiler, the pistons being made to press downward upon the bearings of the axles.
The engines built by Stephenson for the Stock ton and Darlington Railway were substantially of the construction just described. They were not very satisfactory machines, and Timothy Ilackworth, the locomotive engineer of this road, undertook to improve upon them. llis engine, the Royal George, was completed in 1827. The boiler was a plain cylinder 13 feet long and 4 feet 4 inches in diameter and had a return flue like the lledley engines used on the lam Rail way. There were six coupled wheels four feet in diameter, and the cylinders, which were placed at the end opposite the fire-door and stack, were so arranged that they overhung the sides of the boiler sufficiently for the downward-projecting piston-rods to clear it and to connect through eonnecting-rods with cranks on the rear pair of wheels. These rear drivers were rigid, but the
others had springs. The Royal George had a cistern into which a portion of the exhaust steam could be turned to heat the feed-water: it also had short-stroke force-pumps worked by eccen trics, adjustable springs instead of weights upon the safety valves, a single lever reversing gear, and a blast-pipe by which the exhaust cylinder steam created a draught in the smokestack.
Nothing further of importance respecting the mechanical development remains to be noted un til the trials instituted by the Liverpool and Manchester Railway in 1829, which are noted in the article on RAILWAYS. Four locomotives were submitted to undergo this trial and three of them were actually tested. The result of the trial was the a and of the prize to the Rocket, submitted by George Stephenson in the name of his son Robert Stephenson. The Rocket (Fig. 3) was a four-wheel engine supported on springs:, and, with a supply of water in the boiler. weighed 4 tons 3 hundredweight. The boiler was a horizontal cy lindrical vessel 6 feet long and 3 feet 4 inehes in diameter. traversed by 25 copper tubes 3 inches in diameter, through which the gaseous prod nets of combustion were conveyed from a 'fire-box' at one end to a tall stack 12 inches in diameter at the other end. The fire-grate was :3 feet wide and 2 feet long in the direction of the boiler, and the fire-box was 3 feet deep and surrounded by a 3-ineh water-spaee which communicated with the boiler proper by means of two external pipes. The cylinders, placed in an inclined position, were fastened to the outside of the boiler near the fire-box, the conneeting-rods working upon crank pins in the driving wheels, which were placed under the front end of the engine. The cylinders were S inches in diameter and had a stroke of inches; the driving wheels were 4 feet inches in diameter. The exhaust steam from each cylinder was carried through a pipe and turned upward into the stack. As the result of the success of the Rocket, whose speed exceeded all expectations, the managers of the Liver pool and Manchester Railway ordered sev eral locomotives from the Stephensons. These engines were, however, larger and heavier than the Rocket, some of them having 10 X 16 inch, and others 11 X 16 inch cylinders, with 5-foot driv ing wheels and weighing six and one-half and seven tons. The arrangement of the cylinders was substantially the same as on the Rocket, but they were less inclined. The tubes in the boiler were increased to from 90 to 92 in num ber and were reduced in diameter to two inches. A smoke-box was also added to the boiler.
Before taking up the next series of steps in the mechanical development of the locomotive a moment's attention should he given to the per formances of the early engines so far mentioned. These are shown in the accompanying table, taken from Zerah Colburn's Locomotire Engineering (London, 1871).