f 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.

_Water-Tube Boilers._--The "Babcock & Wilcox" boiler, as fitted for
land purposes, and illustrated in fig. 11, consists of a horizontal
cylinder forming a steam chest, having dished ends and two specially
constructed cross-boxes riveted to the bottom. Under the cylinder is
placed a sloping nest of tubes, under the upper end of which is the
fire. The sides and back of the boiler are enclosed in brickwork up to
the height of the centre of the horizontal cylinder and the front is
fitted with an iron casing lined with brick at the lower part.
Suitable brickwork baffles are arranged between the tubes themselves,
and between the nests of tubes and the cylinder, to ensure a proper
circulation of the products of combustion, which are made to pass
between the tubes three times. The nest of tubes consists of several
separate elements, each formed by a front and back header made of
wrought steel of sinuous form connected by a number of tubes. The
upper ends of the front headers are connected by short tubes to the
front cross-box of the horizontal cylinder, the lower ends being
closed. The upper ends of the back headers are connected by longer
pipes to the back cross-box, and their lower ends by short pip