The exhaust fan, in the center of the attic, is run at 550 rev. per min., and has a capacity of 22,600 cu. ft. of air per min. It draws the air from each of the rooms below, as well as from the hoods, through a main pipe, 48 in. in diameter. _Steaming and Combustion Tests._--The investigations included under the term, fuel efficiency, relate to the utilization of the various types of fuels found in the coal and oil fields, and deal primarily with the combustion of such fuels in gas producers, in the furnaces of steam boilers, in locomotives, etc., and with the efficiency and utilization of petroleum, kerosene, gasoline, etc., in internal-combustion engines. This work is under the general direction of Mr. R. L. Fernald, and is conducted principally in Buildings Nos. 13 (Plate XVII) and 21. For tests of combustion of fuels purchased by the Government, the equipment consists of two Heine, water-tube boilers, each of 210 h.p., set in Building No. 13. One of these boilers is equipped with a Jones underfeed stoker, and is baffled in the regular way. At four points in the setting, large pipes have been built into the brick wall, to permit making observations on the temperature of the gas, and to take samples of the gas for chemical analysis. The other boiler is set with a plain hand-fired grate. It is baffled to give an extra passage for the gases (Fig. 15). Through the side of this boiler, at the rear end, the gases from the long combustion chamber (Plate XVIII) enter and take the same course as those from the hand-fired grate. Both the hand-fired grate and the long combustion chamber may be operated at the same time, but it is expected that usually only one will be in operation. A forced-draft fan has been installed at one side of the hand-fired boiler, to provide air pressure when coal is being burned at high capacity. This fan is also connected in such a way as to furnish air for the long combustion chamber when desired. A more complete description of the boilers may be found in Professional Paper No. 48, and Bulletin No. 325 of the U.S. Geological Survey, in which the water-measuring apparatus is also described.[13] On account of the distance from Building No. 21 to the main group of buildings, it was considered inadvisable to attempt to furnish steam from Building No. 13 to Building No. 21, either for heating or power purposes. In view, moreover, of the necessity of installing various types and sizes of house-heating boilers, on account of tests to be made thereon in connection with these investigations, it was decided to install these boilers in the lower floor of Building No. 21, where they could be utilized, not only in making the necessary tests, but in furnishing heat and steam for the building and the chemical laboratories therein. [Illustration: Fig. 15. SETTING FOR 210-HORSE POWER, HAND-FIRED BOILERS]