All About Engines

86 All^About Engines and when it moves forward this coal is pushed into the furnace. The coal moves forward along the grate from front to back in short jerks, and by the time it reaches the end of the grate only the ashes remain. Another kind of mechanical stoker consists of a chain grate, and is illustrated in connection with the Babcock and Wilcox boiler on Plate 3. In this case the fuel is fed from the hopper on to an end- less chain composed of short, interlocking, cast-iron grate bars. This chain runs on rollers and is driven by a revolving drum at the front end of the stoker, while the shaft which drives the drum may be either overhead or underground. The movement of the chain can be started or stopped, or its speed varied, while the fire is burning. Small repairs, again, such as the renewal of a link, can be carried out without removing the grate, while for thorough overhauling or larger repairs the whole carriage, which runs on rails fixed on each side of the ashpit, can be with- drawn with ease. The necessary air for combustion of the fuel enters through the spaces between the links, which, however, are so close together that the smallest coal can be used. The coal is delivered over the whole width of the grate, and the depth of the fire is regulated by a vertical sliding door beneath which the chain passes. There is a third type, more frequently seen in America than in Great Britain, in which a ram is used, but the coal is delivered in the middle of the furnace from below. As this is forced upwards it falls