boiler is capable of supplying sufficient power of steam to the cylinder.
There are two distinct methods of placing the loads upon the engine; one, by placing it on the same carriage with the engine itself; and the other, by causing the carriage which bears the engine to drag after it other carriages containing the load. The latter method has been invariably adopted upon railroads. On common roads, some projectors prefer the one method, some the other. Whichever method be adopted, the pressure necessary to be exerted on the piston, must depend upon the power of the steam to overcome the resistance which the load opposes to its progressive motion upon the road; and this resistance again depends partly on the nature of the road and its inclination to the level, and partly on the weight of the load. Upon level railroads, as has been already observed, the same power is capable of impelling at least twelve times as great a load as upon a good Macadamised turnpike road.
The combination of lightness, power, and speed, which is indispensable to the efficiency of travelling steam-engines, requires that the boilers should be so contrived that a small quantity of water should be exposed to a great heating power. As the furnace must necessarily be small, the fuel must, therefore, be kept in fierce combustion; and for this purpose a powerful draft of air must be maintained through it. The difficulty of accomplishing this, long obstructed the progress of this invention; but a fortunate application of the waste steam which escaped from the cylinder, after having urged the piston, and which had been previously useless, solved this important problem. This steam was carried off by the chim-
