ess to the inside is obtained by
manholes in the steam-chest and water-chamber ends, but in the smaller
sizes fitted in torpedo boats the water-chambers are too small for
this, and they are each arranged in two parts connected by a bolted
joint, which makes all the tube ends accessible.

The Babcock & Wilcox marine boiler (fig. 17) is much used in the
American and British navies, and it has also been used in several
yachts and merchant steamers. It consists of a horizontal cylindrical
steam-chest placed transversely over a group of elements, beneath
which is the fire, the whole being enclosed in an iron casing lined
with firebrick. Each element consists of a front and back header
connected by numerous water-tubes which have a considerable
inclination to facilitate the circulation. The upper ends of the front
headers are situated immediately under the steam-chest and are
connected to it by short nipples; by a similar means they are
connected at the bottom to a pipe of square section which extends
across the width of the boiler. Additional connexions are made by
nearly vertical tubes between this cross-pipe and the bottom of the
steam-chest. The back headers are each connected at their upper ends
by means of two long horizontal tubes with the steam-chest, the bottom
ends of the headers being closed. The headers are made of wrought
steel, and except the outer pairs, which are flat on the outer
portions, they are sinuous on both sides, the sinuosities fitting into
one another. The tubes are of two sizes, the two lower rows and the
return tubes between the back headers and steam-chest being 3-15/16
in. outside diameter, and the remaining tubes 1-13/16 in. The small
tubes are arranged in groups of two or four to nearly all of the
sinuosities of the headers, the purpose of this arrangement being to
give opportunities for the furnace gases to become well mixed
together, and to ensure their contact with the heating surfaces.
Access for securing the tubes in the headers is provided by a hole
formed on the other side of the header opposite each of the tubes,
where they are grouped in fours, and by one larger hole opposite each
group of two tubes. The larger holes are oval, and are closed by
fittings similar to those used in the land boiler (fig. 18). The
smaller holes are conical, with the larger diameter on the inside,
and are closed by special conical fittin