With steam raising the safety valve at a pressure of 140 lbs. to the square inch, the engines will whisk a score of carriages out of King’s Cross up the northern height of London at forty miles an hour, and then without a stop rush on to Grantham at near sixty. Standing on the platform at King’s Cross, with a large part of the immense driving-wheel hidden below you as it rests on the rail, you do not realise its tremendous size. Yet, let the engine-driver open the throttle, as it is called—that is, turn on the steam to the cylinders—and that huge wheel will revolve, and with its neighbour on the other side, haul after them that heavy train of carriages, and, gathering speed as they go, they will soon be rushing up the incline at forty miles an hour, and then on at sixty. It is a marvel of mechanism! But then the compound engines that Mr. F. W. Webb, the engineer of the North-Western, builds for that Company can also perform remarkable things. The compound is the great modern improvement (some engineers might doubt whether improvement be the correct word) in the locomotive, effecting, it is said, an economy of from ten to fifteen per cent. in fuel. Now the compounding principle has been developed to such an extent in marine steam engines that it revolutionised steam navigation. But the application of the principle has not been so great in the case of the locomotive. Briefly, the principle is this—the steam is sent out from the boiler at a high pressure, say 160 to 180 lbs. to the square inch, and is used in one or in a pair of high-pressure cylinders, and then used again, by means of its expanding power, in a larger, low-pressure cylinder. Mr. John Nicholson, of the Great Eastern Railway, suggested a compound locomotive before even the compound marine engine had been made, and his design was successful; but in 1881 Mr. Webb, of the North-Western, patented a compound locomotive, with two small high-pressure, and one large low-pressure cylinders, the latter twenty-six inches in diameter. Placed between the front wheels, the bright boss of this cylinder may be seen in shining steel as it flies over the rails. The argument is that the compound burns less fuel and is more powerful than a non-compound of the same weight; but against this is launched the objection that the compound is more expensive to build, to repair, and to maintain. Still further it is argued, that a fast-speeding locomotive has not the time in its hurrying life to expand its steam in the tick of time between each stroke of the piston. [Illustration: THE COMPOUND LOCOMOTIVE “GREATER BRITAIN.” _By kind permission of Mr. F. W. Webb, L. & N. W. Railway._ ]