_Answer._ By burning the greatest quantity of fuel possible on the grate in that time. This can be done by keeping the grates free from clinkers and the ash-pan from ashes, and then distributing the coal evenly over the grates in a layer six to twelve inches thick. The thickness of the layer which will give the best results will, however, vary with the quality of the fuel, and must be determined by experience. If the layer is too thick, not enough air will pass through it to burn the coal. If it is too thin, then so much air will pass through that the temperature in the fire will be reduced. The rapidity of combustion will also be promoted by breaking up the coal into lumps the size of a man’s fist or smaller. If fine coal is used it should be wet, otherwise it will be carried into the flues by the blast before it is burned or caked or even reaches the grate. Experience will indicate the amount of air which can advantageously be admitted above the fire in order to secure the maximum production of steam. The best size of the exhaust nozzles and the position of the petticoat pipe must also be determined by experience. It will usually be found, however, that if enough air is admitted above the fire to prevent smoke, it will reduce the maximum amount of steam which can be generated in a given time. The fire should also be fed regularly and with comparatively small quantities of fuel at a time, although if the feeding is too frequent there is more loss from the cooling effect which results from the frequent opening of the furnace door than is gained from the regularity of the firing. In this, too, a fireman must consult experience to guide him. QUESTION 495. _How can a locomotive be fired with the least consumption of coal?_ _Answer._ Two systems of firing are practiced in this country, one known as the “banking system” and the other the “spreading system.” When the banking system is employed, the coal is piled up at the back part of the fire-box, as shown in fig. 218, and slopes down towards the front of the grate, where the layer of coal is comparatively thin and in an active state of incandescence. The heap of coal behind is gradually coked by the heat in the fire-box and the gases are thus expelled. Openings in the furnace door admit air which mingles with the escaping gases, which then pass over the bright fire in front, and are thus supposed to be consumed. When the “bank” of coal behind becomes thoroughly coked, it is pushed forward on the bright fire and fresh coal is again put on behind to be coked. This system of firing is practiced on some roads with good results, but it is doubtful whether it could be used successfully with coal which cakes and clinkers badly.