(193.)

The next step in the improvement of the machine was made with a view to remove these two defects. The cylinders were transferred from the exterior of the engine to the [Pg355] interior of the casing called the smoke-box, B, fig. 94., which supports the chimney, and which receives the heated air issuing from the tubes which traverse the boiler. Thus placed, the cylinders are always maintained as hot as the air which issues from the flues, and all condensation of steam by their exposure is prevented. The piston-rods are likewise brought closer together, and nearer the centre of the engine: the connecting rods, no longer attached to the wheels, are made to act upon two cranks constructed upon the axle of the wheels, and placed at right angles to each other. From the position of these cranks, one would always be at its dead point when the other is in full action. The action of the steam upon them would, therefore, be generally unequal; but this would not produce the same strain as when the connecting rods are attached to points upon the exterior of the wheels, owing to the cranks being constructed on the axle at points so much nearer its centre. By this means it was found that the working of the machine was more even, and productive of much less strain, than in the arrangement adopted in the Rocket, and the earlier engines. On the other hand, a serious disadvantage was incurred by a double-cranked axle. The weakness necessarily arising from such a form of axle could only be removed by great thickness [Pg356] and weight of metal; and even this precaution, at first, did not prevent their occasional fracture. The forging of them was, however, subsequently much improved: the cranks, instead of being formed by bending the metal when softened by heat, were made by cutting the square of the crank out of the solid metal; and now it rarely happens that one of these axles fails.

The adoption of smaller tubes, and a greater number of them, with a view more perfectly to extract the heat from the air in passing to the chimney, rendered a more forcible draft necessary. This was accomplished by the adoption of a more contracted blast-pipe leading from the eduction-pipes of the cylinders and presented up the chimney. A representation of such a blast-pipe, with the two tubes leading from the cylinders and uniting together near the point, which is presented up the chimney, is given at p p in fig. 104. The engine thus improved is represented in fig. 94.

A represents the cylindrical boiler, the lower half of which is traversed by tubes, as described in the Rocket. They are usually from eighty to one hundred in number, and about an inch and a half in diameter; the boiler is about seven feet in length; the fire-chamber is attached to one end of it, at F, as in the Rocket, and similar in construction: the cylinders are inserted in a chamber at the other end, immediately under the chimney. The piston-rods are supported in the horizontal position by guides; and connecting rods extend from them, under the engine, to the two cranks placed on the axle of the large wheels. The effects of an inequality in the road are counteracted by springs, on which the engine rests; the springs being below the axle of the great wheels, and above that of the less. The steam is supplied to the cylinders, and withdrawn, by means of the common sliding valves, which are worked by an eccentric wheel placed on the axle of the large wheels of the carriage. The motion is communicated from this eccentric wheel to the valve by sliding rods. The stand is placed for the attendant at the end of the engine, next the fire-place F; and two levers L project from the end which communicate with the valves by means of rods, by which the engine is governed so as to reverse the motion. [Pg357]

The wheels of these engines have been commonly constructed of wood with strong iron ties, furnished with flanges adapted to the rails. But Mr. Stephenson afterward substituted, in some instances, wheels of iron with hollow spokes. The engine draws after it a tender carriage containing the fuel and water; and, when carrying a light load, is capable of performing the whole journey from Liverpool to Manchester without a fresh supply of water. When a heavy load of merchandise is drawn, it is usual to take in water at the middle of the trip.