(192.)

Since the period at which this railway was opened for the actual purposes of transport, the locomotive engines have been in a state of progressive improvement. Scarcely a month has passed without suggesting some change in the details, by which fuel might be economised, the production of steam rendered more rapid, the wear of the engine rendered slower, the proportionate strength of the different parts improved, or some other desirable end obtained.

Engines constructed in the form of the Rocket, were subject to two principal defects. The cylinders, being placed outside the engine, were exposed to the cold of the atmosphere, which produced a waste of heat more or less considerable by condensation. The points at which the power of the steam to turn the wheels was applied, being at the extremities of the axle and on the exterior of the wheel, a considerable strain was produced, owing to the distance of the point of application of the power from the centre of resistance. If it were possible that the impelling power could act in drawing the train at all times with equal energy on both sides of the engine, then no injurious strain would be produced; but from the relative position of the points on the opposite wheels to which it was necessary to attach the connecting rods, it was inevitable that, at the moment when one of the pistons exerts its full power in driving the wheel, the other piston must be altogether inactive. The impelling power, therefore, at alternate moments of time, acted on opposite wheels, and on each of them at the greatest possible distance from the centre of the axle.

Fig. 94.