d lower
drums are connected by numerous nearly vertical straight tubes. The
whole is enclosed in firebrick casing. The design permits of the
insides of all the tubes being readily inspected, and also of any tube
being taken out and renewed without displacing any other part of the
boiler.

[Illustration: FIG. 12.--Stirling Water-tube Boiler.]

Belleville.

The earliest form of water-tube boiler which came into general use in
the British navy is the Belleville. Two views of this boiler are shown
in fig. 14. It is composed of two parts, the boiler proper and the
"economizer." Each of these consists of several sets of elements
placed side by side; those of the boiler proper are situated
immediately over the fire, and those of the economizer in the uptake
above the boiler, the intervening space being designed to act as a
combustion chamber. Each element is constructed of a number of
straight tubes connected at their ends by means of screwed joints to
junction-boxes which are made of malleable cast iron. These are
arranged vertically over one another, and except in the case of the
upper and lower ones at the front of the boiler, each connects the
upper end of one tube with the lower end of the next tube of the
element. The boxes at the back of the boiler are all close-ended, but
those at the front are provided with a small oval hole, opposite to
each tube end, closed by an internal door with bolt and cross-bar; the
purpose of these openings is to permit the inside of the tubes to be
examined and cleaned. The lower front box of each element of the
boiler proper is connected to a horizontal cross-tube of square
section, called a "feed-collector," which extends the whole width of
the boiler. When the boiler is not in use, any element can be readily
disconnected and a spare one inserted. The lower part of the
steam-chest is connected to the feed-collector by vertical pipes at
each end of the boiler, and prolongations of these pipes below the
level of the feed-collector form closed pockets for the collection of
sediment. The tubes are made of seamless steel. They are generally
about 4-1/2 in. in external diameter: the two lower rows are 3/8 in.
thick, the next two rows 5/16 and the remainder about 1/5 in. The
construction of the economizer is similar to that of the boiler
proper, but the tubes are shorter and smaller, being generally about
2-3/4 in