es to a
horizontal mud drum to which a blow-off cock and pipe are attached.
The headers are furnished with holes on two opposite sides; those on
one side form the means of connexion between the headers and tubes,
and the others allow access for fixing the tubes in position and
cleaning. The outer holes are oval, and closed by special fittings
shown in fig. 18, the watertightness of the joints being secured by
the outer cover plates. The holes being oval, the inside fitting can
be placed in position from outside, and it is so made as to cover the
opening and prevent any great outrush of steam or water should the
bolt break. Any desired working pressure can be provided for in these
boilers; in some special cases it rises as high as 500 lb. per sq.
in., but a more usual pressure is 180 lb. Like all water-tube
boilers, they require to be frequently cleaned if impure feed-water is
used, but the straightness of their tubes enables their condition to
be ascertained at any time when the boiler is out of use, and any
accumulation of scale to be removed. The superheaters, which are
frequently fitted, consist of two cross-boxes or headers placed
transversely under the cylindrical drum and connected by numerous
C-shaped tubes. They are situated between the tubes and the
steam-chest, and are exposed to the heat of the furnace gases after
their first passage across the tubes. The steam is taken by an
internal pipe passing through the bottom of the drum into the upper
cross-box, then through the C tubes into the lower box, and thence to
the steam pipe. When steam is being raised, the superheater is flooded
with water, which is drained out through a blow-off pipe before
communication is opened with the steam-pipe. In large boilers of this
type, two steam-chests are placed side by side connected together by
two cross steam pipes and by the mud drum. Each, however, has its own
separate feed supply. The largest boiler made has two steam chests
4-1/2 ft. diameter by 25-1/2 ft. long, a grate surface of 85 sq. ft.,
and a total heating surface of 6182 sq. ft.

Stirling.

Another type of water-tube boiler in use for stationary purposes is
the "Stirling" (fig. 12). This boiler consists of four or five
horizontal drums, of which the three upper form the steam-space, and
the one or two lower contain water. The lower drums, where two are
fitted, are connected to each o