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HOWDEN HOT DRAFT SYSTEM. 73 tive boiler. This system obviated several of the objections of the closed ash pit, but it was found to have its own drawbacks. The merits and demerits of this system are concisely stated in the description given of this mode of working furnaces by Mr. Howden in his paper read at the Institution of Naval Architects in 1884, as follows: "A second method resorted to for increasing combustion in boiler furnaces is that of exhausting the air in the chimney or uptake by a fan, thus reducing the pressure in the flues or tubes and furnaces and thereby producing a more rapid current of air through the furnaces both below and above the grate bars. This plan, so far as the supply of air to the furnaces is concerned, is in practice more workable than the plan previously described (the closed ash pit), as the air enters the furnace not only at a greater velocity than is attainable by natural draft and is thereby capable of being more thoroughly intermingled with the fuel all over the furnace, but, being also balanced in pressure above and below the grate bars, the operations of the furnace become much more easily managed. This mode of creating these advantageous conditions in the furnace is, however, objectionable. The passing of the hot gases of combustion through a working fan is, of itself, a mistake practically and theoretically. Even if it were possible for the machine to continue in working order, under this ordeal, for any length of time, the hot gases, if leaving the boiler at no more than 491° above the entering temperature, would be twice the volume of the air which entered the furnace from the stoke hold. A fan to exhaust the air would, therefore, require to be at least double the capacity of one which would have supplied the same quantity direct from the stoke hold to the furnace. This plan, one of the earliest tried, is, therefore, impracticable for large boilers, and has only been occasionally used in boilers of limited size. For particulars and prices apply to the Detroit Dry Dock Company, Detroit, Mich., sole owners for the lakes, and privileged to equip machinery built by this company for use in any part of the United States and Canada.