ssels of the British navy, in some respects resembles the
"Niclausse." The separate headers of the latter, however, are replaced
by one large water-chamber formed of steel plates with welded joints,
and instead of the tubes being secured by "lanternes" to two plates
they are secured to the inner plate only by conical joints, the holes
in the outer plate being closed by small round doors fitted from the
inside. In fixing the tubes each is separately forced into its
position by means of a small portable hydraulic jack. The lower ends
of the caps are closed by cap-nuts made of a special heat-resisting
alloy of copper and manganese. Circulation is provided for by a
diaphragm in the water-chamber and by inner tubes as in the Niclausse
boiler. Baffle plates are fitted amongst the tubes to ensure a
circulation of the furnace gases amongst them. Above the main set of
tubes is a smaller set arranged horizontally, and connected directly
to the steam receiver. These are fitted with internal tubes, and an
internal diaphragm is provided so that steam from the chest circulates
through these tubes on its way to the stop-valves. This supplementary
set of tubes is intended to serve as a superheater, but the amount of
surface is not sufficient to obtain more than a very small amount of
superheat.

Yarrow.

The Yarrow boiler (fig. 16) is largely in use in the British and also
in several other navies. It consists of a large cylindrical steam
chest and two lower water-chambers, connected by numerous straight
tubes. In the boilers for large vessels all the tubes are of 1-3/4 in.
external diameter, but in the large express boilers the two rows
nearest to the fire on each side are of 1-1/4 in. and the remainder of
1 in. diameter. They are arranged with their centres forming
equilateral triangles, and are spaced so that they can be cleaned
externally both from the front of the boiler and also cross-ways in
two directions. In some boilers the lower part of the steam-chest is
connected with the water-chambers by large pipes outside the casings
with the view of improving the circulation.

The largest size of single-ended large tube boiler in use has a steam
drum 4 ft. 2 in. diameter, a grate area of 73.5 sq. ft. and 3750 sq.
ft. of heating surface, but much larger double-ended boilers have been
made, these being fired from both ends.

In most of the boilers made, acc