ther at about the middle of their
height by horizontal tubes, and to the upper drums by numerous nearly
vertical tubes which form the major portion of the heating surfaces.
The central upper drum is at a slightly higher level than the others,
and communicates with that nearest the back of the boiler by a set of
curved tubes entirely above the water-level, and with the front drum
by two sets--the upper one being above and the lower below the
water-level. The whole boiler is enclosed in brickwork, into which the
supporting columns and girders are built. Brickwork baffles compel the
furnace gases to take specified courses among the tubes. It will be
seen that the space between the boiler front and the tubes form a
large combustion chamber into which all the furnace gases must pass
before they enter the spaces between the tubes; in this chamber a
baffle-bridge is sometimes built. Another chamber is formed between
the first and second sets of tubes. The feed-water enters the back
upper drum, and must pass down the third set of tubes into the lower
drum before it reaches the other parts of the boiler. Thus the coldest
water is always where the temperature of the furnace gases is lowest;
and as the current through the lower drum is slight, the solid matters
separated from the feed-water while its temperature is being raised
have an opportunity of settling to the bottom of this drum, where the
heating is not great and where therefore their presence will not be
injurious. When superheaters are required, they are made of two drums
connected by numerous small tubes, and are somewhat similar in
construction to the boiler proper. The superheater is placed between
the first and second sets of tubes, where it is exposed to the furnace
gases before too much heat has been taken from them. Arrangements are
provided for flooding the superheater while steam is being raised, and
for draining it before the steam is passed through it.

[Illustration: FIG. 11.--Babcock & Wilcox Water-tube Boiler fitted
with Superheaters.]

Woodeson.

A somewhat similar boiler is made by Messrs. Clarke, Chapman & Co.,
and is known as the "Woodeson" boiler (fig. 13). It consists of three
upper drums placed side by side connected together by numerous short
tubes, some above and some below the water-level, and of three smaller
lower drums also connected by short cross tubes. The upper an