Your locomotive-builder is apt to be more systematic about these types of engine, and he falls back on what is generally known as Whyte's classification. The basis of this simple system is in the number of wheels of the engine itself. Each type is described by a series of three numbers, the first of these being the number of wheels in front of the drivers, the second the number of drivers, and the third the number of wheels to the rear of these. The eight-wheel American type, the simplest for illustration here, would thus be described as "4-4-0." The trailer, which is described by the third number in this series, is a recent addition to the locomotive family in this country. It came from the constant lengthening of the fire-box, due to the necessity of providing greater steam-power for engines of increasing weight and cylinder capacity. When the fire-box began to overhang too far, the trailer-wheels were introduced, and a device was affixed to the locomotive by which they might receive its weight for hill-climbing purposes. This last device has not proved particularly successful. But the trailer itself has become a fixed device in locomotive construction. When the third figure in Whyte's classification is a cypher it simply means that there are no trailers. Similarly the first figure a cypher, indicates the absence of a forward truck or even wheels, which is common in some forms of switch-engines, where the weight is entirely concentrated on the drivers for better gripping power upon the rail. Such, in brief, is the development of the locomotive. It has been development rather than change, for while some designers have fretted about whether the engine's cab should be in the middle of the boiler or at its end and others have recently developed the Walsheart gears upon the outside of the engine frame, where it is of easier access than the old-style links, the general design of the iron-horse remains practically the same as that given it by our grand-daddies. They planned carefully and they planned for the long years. The essential features of their designs have not been questioned. It has simply been a problem of growth. * * * * * From out of the fiery womb of steel comes the locomotive. If you would better understand the iron horse, find your way to any of the great plants in which he is being built. Begin at the beginning in a factory, which seems, with dozens of shops and great yards, to be almost a miniature city. Begin at the draughting-rooms where each locomotive is given a whole ledger page--sometimes two or three--for specifications. From those specifications, the young draughtsmen take their instructions. They work out their charts and elevations, their detailed plans; and the ink is hardly dry upon their drawings before they are being whisked away to the blueprint rooms. The blueprints are still damp, when in turn they are hurried to the different construction shops of the plant.