The Locomotive Of Today

26 The Boiler: Steam Pipe, ExtendedSmokebox, Chimney. The main steam pipe from the boiler to the steam chest passes through the smokebox which assists to an extent as a super-heater for the steam. Vertically within, exactly under the centre of the chimney, stands the biast pipe. Spark arresters, when used, are either arranged horizontally across the box forming a diaphragrn just above the top row of tubes, or are made conical to fit between the top of the biast pipe and base of the chimney, their use being to prevent the small coal or sparks drawn through the tubes from the firebox being dis- charged through the chimney. C shows the complete arrangement of a good form of smokebox with vertical front plate; there are cases, however, where the front plate is inclined, this being done to form a line with the front of cylinders which are set at an angle. All joints must be per- fectly air tight, or the biast of the engine will not be able to create a sufficiently good vacuum in the smokebox to keep up the rapid passage of air through the firebox. necessary for quick combustion, and also because any atmospheric air entering the smokebox direct would cause fresh combustion there by its oxygen combining with the unconsumed carbon which is pulled through the tubes, and the plates would be burnt by the heat so caused. Extended smokeboxes are largely used in America, on the Continent, and to a less extent here. The addition to the cubical capacity of the smokebox causes a steadier and more regular drauglit on the fire, and this is beneficial when inferiør grades of coal are used as fuel. A small smokebox means a sharp effect of the biast on the fire, and a common remedy in the steam shed for a bad steaming engine is to brick up a portion of the smokebox and so diminish its capacity. The smokebox, shown at C, must be emptied of the small ashes it collects by hånd. Where the construction of the smokebox will permit of it, a good plan is to provide a hopper with a trap door opening downwards under the engine for the automatic emptying of the ashes. The chimney may be either tapering or parallel in shape. Its length depends upon the height of the boiler from the rail, as the loading gauge in this country will not pass more than about 13-ft. above rail level. The tapering form, larger at the top than at the base, is claimecl by many to secure a better passage for the gases, and thus help the draught, and it would seem that this is a good claim, as the shape more approximately follows the outline of the escaping cone of steam from the biast pipe. The design of the top also considerably affects the direct discharge of steam and smoke. The main uses of the chimney are first, to form an