by a feed-pumping engine, and the upper boxes are connected to another horizontal pipe from which the heated feed-water is taken into the steam-chest. Both the boiler proper and the economizer are enclosed in a casing which is formed of two thicknesses of thin iron separated by non-conducting material and lined with firebrick at the part between the fire-bar level and the lower rows of tubes. Along the front of the boiler, above the level of the firing-doors, there is a small tube having several nozzles directed across the fire-grate, and supplied with compressed air at a pressure of about 10 lb. per sq. in. In this way not only is additional air supplied, but the gases issuing from the fire are stirred up and mixed, their combustion being thereby facilitated before they pass into the spaces between the tubes. A similar air-tube is provided for the space between the boiler proper and the economizer. Any water suspended in the steam is separated in a special separator fitted in the main steam-pipe, and the steam is further dried by passing through a reducing-valve, which ensures a steady pressure on the engine side of the valve, notwithstanding fluctuations of pressure in the boiler. The boiler pressure is usually maintained at about 50 lb. per sq. in. in excess of that at which the engines are working, the excess forming a reservoir of energy to provide for irregular firing or feeding. [Illustration: FIG. 13.--Woodeson Boiler (Messrs Clarke, Chapman & Co.).] [Illustration: FIG. 14.--Belleville Boiler.] Niclausse.