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68 "AROUND THK LAKES. over the surface of the burning fuel so as to effect complete combustion with the smallest admission of air practicable. By means of the balance of air pressure above and below the fires all tendency for the fire to -blow out at the furnace door, however high the rate of combustion, is entirely removed. By regulating the admission of the air by the valves above and below the fires the highest rate of combustion possible by the air-pressure used can be effected, and in same manner the rate of combustion can be reduced to far below that of natural draft, while complete and economical combustion at all rates is secured. In usual working at sea, in most recent practice, only one ashpit valve is opened and shut unless the very highest power is required. The upper valve for the air admission over the fires is adjusted at the beginning of the voyage to suit the character of the fuel used, and does not require to be afterwards moved during the voyage unless the engines are stopped. If the combustion is required to be suspended, as in the event of the engines being suddenly stopped when working at full power, all that is necessary is to shut the air admission valves, the upper one being very slightly open to maintain a limited circulation of air through the boxes. Blowing oft steam is thus prevented, and the boilers may be kept for hours in this state with the combustion suspended and the steam sustained almost without loss of pressure. This system of draft can be very effectively and economically worked with cold air in consequence of the controlled admissions described rendering the temperature and quantity of the waste gases less than in other modes of working for equal weights of fuel consumed. The most important feature, how- ever, in securing both the highest efficiency and economy is the combination of the heating of the air of combustion by the waste gases with the controlled and regulated admission of air to the furnaces. This arrangement is effected most conveniently by passing the hot fire-gases after they leave the boiler through stacks of vertical tubes enclosed in the up-take, their lower ends being immediately above the smoke-box doors. Tho cold air from the fans enters at the middle of the air heater containing the tubes, and passes horizontally among the vertical tubes to each side, and descends to the reservoir around the furnace fronts. Figs. 3 and 4 are, respectively, a front and side elevation (the latter in section) of the boilers of the American line steamers Indiana, Illinois and Pennsylvania, as fitted with this system of draft. Fig. 5 is an enlarged section of one furnace, with its fittings, represented in operation. These illustrations with the description now given will clearly explain how the system is practically carried out. It should be stated that the air-heating arrangement in more recent practice is much more effective than in the arrangement shown in these figures, and while the air-heating service is only about one-half more in proportion to that in these boilers, the efficiency is greatly increased. The effect of a hot-air supply in promoting rapid and economical combustion is very marked, for though, as is well known, very high rates of combustion can be effected by cold air supply under a high pressure, as in the locomotive boiler, it must be, unless at a heavy sacrifice of economy, under conditions in which the intense heat of the fire is not liable to be quickly