The locomotive boiler consists of a cylindrical barrel attached to a portion containing the fire-box, which is nearly rectangular both in horizontal and vertical section. The fire-box sides are stayed to the fire-box shell by numerous stays about 1 in. in diameter, usually pitched 4 in. apart both vertically and horizontally. The top of the fire-box in small boilers is stayed by means of girder stays running longitudinally and supported at the ends upon the tube plate and the opposite fire-box plate. In some boilers the girders are partly supported by slings from the crown of the boiler. In larger boilers the crown of the boiler above the fire-box is made flat and the fire-box crown is supported by vertical stays connecting it with the shell crown. Provision is generally made for the expansion of the tube plate, which is of copper, by allowing the two or three cross rows of stays nearest the tube plate to have freedom of motion upwards but not downwards. The ordinary tubes are usually 1-3/4 in. diameter. The fire-bars are generally, though not always, made to slope downwards away from the fire door, and just below the lowest tubes a fire-bridge or baffle is fitted, extending about half-way from the tube plate to the fire-door side of the fire-box. In some cases water-tubes are fitted, extending right across the fire-box. In a boiler for the London & South-Western Railway Co., having a grate area of 31.5 sq. ft. and a total heating surface of 2727 sq. ft., there are 112 water-tubes each 2-3/4 in. diameter. These are arranged in two clusters, each containing 56, one set being placed above the fire-bridge, and the other set nearer the fire-door end of the boiler. The water-tubes are of seamless steel, and are expanded into the fire-box side plates. In way of these tubes the outer shell side plates are supported by stay bars passing right through the water-tubes. The usual pressure of locomotive boilers is about 175 lb. to 200 lb. per sq. in. [Illustration: FIG. 9.] A good example of an express locomotive boiler is shown in fig. 10. In this case the grate area is 30.9 sq. ft. and the heating surface 2500 sq. ft. The barrel is 5 ft. 6 in. diameter, 16 ft. long between tube plates. The fire-box crown is stayed by vertical stays extending to the shell crown, except for the three rows of stays nearest the tube plates. These are supported by cross girders resting upon brackets secured to the outer shell. [Illustration: FIG. 10.--Express Locomotive Boiler, with widened fire-box (Great Northern Railway, England).] Babcock and Wilcox stationary.