the
gases to pass between them at the upper part. The rows nearest the
fires are arranged similarly to those in the Thornycroft boiler.
Circulation in the outer sets of tubes is arranged for by outer
circulating pipes of large diameter connecting the steam- and
water-chambers. For the middle water-chamber several nearly vertical
down-comers are provided in the centre of the boiler. Boilers of this
type are extensively used in the British and German navies.

_Material of Boilers._--In ordinary land boilers and in marine boilers
of all types the plates and stays are almost invariably made of mild
steel. For the shell plates and for long stays, a quality having a
tensile strength ranging from 28 to 32 tons per sq. in. is usually
employed, and for furnaces and flues, for plates which have to be
flanged, and for short-screwed stays, a somewhat softer steel with a
strength ranging from 26 to 30 tons per sq. in. is used. The tubes of
ordinary land and marine boilers are usually made of lap-welded wrought
iron. In water-tube boilers for naval purposes seamless steel tubes are
invariably used. In locomotive boilers the shells are generally of mild
steel, the fire-box plates of copper (in America of steel), the fire-box
side stays of copper or special bronze, and other stays of steel. The
tubes are usually of brass with a composition either of two parts by
weight of copper to one of zinc or 70% copper, 30% zinc; sometimes,
however, copper tubes and occasionally steel tubes are used. Where water
tubes are used they are made of seamless steel.

_Boiler Accessories._--All boilers must be provided with certain
mountings and accessories. The water-level in them must be kept above
the highest part of the heating surfaces. In some land boilers, and in
some of the water-tube boilers used on shipboard, the feeding is
automatically regulated by mechanism actuated by a float, but in these
cases means of regulating the feed-supply by hand are also provided. In
most boilers hand regulation only is relied upon. The actual level of
water in the boiler is ascertained by a glass water-gauge, which
consists of a glass tube and three cocks, two communicating directly
with the boiler, one above and one below the desired water-level, and
the third acting as a blow-out for cleaning the gauge and for testing
its working. Three small try-cocks are also fitted, one just at, one
above, and one below the proper water-level. The feeding of t