gs: the conical portion and
bolt are one forging, and the nut is close-ended. In case of the
breakage of the bolt, the fitting would be retained in place by the
steam-pressure. A set of firebrick baffles is placed so as to cover
rather more than half of the spaces between the upper of the two
bottom rows of large tubes, and another set of baffles covers about
two-thirds of the spaces between the upper small tubes. Vertical
baffles are also built between the smaller tubes, as shown in the
longitudinal section. These baffles compel the products of combustion
to circulate among the tubes in the direction shown by the arrows.
Experience has shown that this arrangement gives a better evaporative
efficiency than where the furnace gases are allowed to pass unbaffled
straight up between the tubes. The boilers are usually fitted in pairs
placed back to back, and one side of each is always made accessible.
On this side the casing is provided with numerous small doors, through
any of which a steam jet can be inserted for the purpose of sweeping
the tubes.

[Illustration: FIG 15.--Niclausse Boiler--transverse section.]

Express boilers.

A class of water-tube boilers largely in use in torpedo-boat
destroyers and cruisers, where the maximum of power is required in
proportion to the total weight of the installation, is generally known
as express boilers. In these the tubes are made of smaller diameter
than those used in the boilers already described, and the boilers are
designed to admit of a high rate of combustion of fuel obtained by a
high degree of "forced draught." Of these express boilers the Yarrow
is of similar construction to the large tube Yarrow boiler already
described with the exception that the tubes are smaller in diameter
and much more closely arranged.

Normand.

In the Normand boiler (fig. 19) there are three chambers as in the
Yarrow, connected together by a large number of bent tubes which form
the heating surface, and also connected at each end by large outside
circulating tubes. The two outer rows of heating tubes on each side
are arranged to touch one another to nearly their whole length so as
to form a "water-wall" for the protection of the outer casing. They
enter the steam-chest at about the water-level. The two inner rows of
tubes, which are bent to the form shown in the figure, also form a
water-wall for the larger po