ansverse girder plates. The end plates of the
boiler in the steam space and below the combustion chambers are stayed
by longitudinal stays passing through the whole length of the boiler
and secured by double nuts at each end. The tube plates are
strengthened by stay tubes screwed into them.

Where natural or chimney draught is used the tubes are generally made
3 or 3-1/4 in. outside diameter and are rarely more than 7 ft. long,
but where "forced" draught is employed they are usually made 2-1/2 in.
diameter and 8 to 8-1/2 ft. long. A clear space of 1-1/4 in. between
the tubes is almost always arranged for, irrespective of size of
tubes.

[Illustration: FIG. 8.--Double-ended Marine Boiler.]

Stay tubes are screwed at both ends, the threads of the two ends being
continuous so that they can be screwed into both tube plates;
occasionally nuts are fitted to the front ends. The stay tubes are
expanded into the plates and then beaded over.

Locomotive.

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 o