LOSS OF HEAT THROUGH EXCESS OF AIR.
In the fire-boxes of American engines, where double dampers are the rule, far more loss of heat is occasioned by excess of air than there is waste of fuel through the gases not receiving their natural supply of oxygen. The blast from the nozzles creates an impetuous draught through the grates; and when to this is added the rapid currents of air impelled into the open ash-pan by the violent motion of the train, the fire-box is found to be the center of a furious wind-storm. The excess of this storm can be regulated by keeping the front damper closed, and letting the engine draw its supply of air through the back damper. When the fire begins to get dirty, and the air-passages between the grates become partly choked, the forward damper can be opened with advantage. So long as an engine steams freely with the front damper closed, it is an indication that there is no necessity for keeping it open. With vicious, heavy firing, all the air that can be injected into the fire-box is needed to effect indifferently complete combustion; and the man who follows this wasteful practice can not get too much air through the fire. Consequently, it is only with moderately light firing that regulation of draught can be practiced. Running with the front damper open all the time is hard on the bottom part of the fire-box, and the ever-varying attrition of cold wind is responsible for many a leaky mud-ring.