All About Engines

Raising Steam 67 may be 250 of them, from if to 2 inches in diameter, but in stationary locomotive boilers they are 3 or 4 inches, and there are fewer of them. In a loco- motive, moreover, the tubes are of brass or, more generally, steel, and the firebox and stays of copper. Copper is more expensive than iron or steel, but stands the action of the fire better. While we shall deal with the locomotive in a separate chapter, it is worth while noting here that the chief difficulty is to raise steam fast enough to draw a heavy train. The size is limited by the height of the bridges and tunnels, by the distance apart of the driving wheels, and by the length of wheel base possible with curves on the existing lines. It is not an uncommon thing for the pressure to drop very seriously when a heavy load is being hauled up a steep incline. About ten years ago the Great Western Railway endeavoured to overcome to some extent the restric- tions on the size of the boiler by making the barrel slightly conical, with the greatest diameter at the firebox end. This, apart from their size, is the most