. in diameter. The lower boxes of the economizer elements are
connected to a horizontal feed pipe which is kept supplied with water
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.

Another type of large-tube boiler which has been used in the British
and in other navies is the "Niclausse," shown in fig. 15. It is also
in use on land in several electric-light installations. It consists of
a horizontal steam-chest under which is placed a number of elements
arranged side by side over the fire, the whole being enclosed in an
iron casing lined with firebrick where it is exposed to the direct
action of the fire. Each element consists of a header of rectangular
cross-section, fitted with two rows of inclined close-ended tubes,
which slope downwards towards the back of the boiler with an
inclination of 6 deg. to the horizontal. The headers are usually of
malleable cast iron with diap