o bear on TT near their junction with UU, the "uptake" tubes;
and that every succeeding passage of the pipes brings the gradually
cooling gases nearer to the "downtake" tubes SS.

The pipes TT are easily brushed and scraped after the removal of plugs
from the "headers" into which the tube ends are expanded.

Other well-known water-tube boilers are the Yarrow, Belleville,
Stirling, and Thorneycroft, all used for driving marine engines.

FIRE-TUBE BOILERS.

Fig. 6 shows a locomotive boiler in section. To the right is the
fire-box, surrounded on all sides by a water-jacket in direct
communication with the barrel of the boiler. The inner shell of the
fire-box is often made of copper, which withstands the fierce heat
better than steel; the outer, like the rest of the boiler, is of steel
plates from 1/2 to 3/4 inch thick. The shells of the jacket are braced
together by a large number of rivets, RR; and the top, or crown, is
strengthened by heavy longitudinal girders riveted to it, or is braced
to the top of the boiler by long bolts. A large number of fire-tubes
(only three are shown in the diagram for the sake of simplicity) extend
from the fire-box to the smoke-box. The most powerful "mammoth" American
locomotives have 350 or more tubes, which, with the fire-box, give 4,000
square feet of surface for the furnace heat to act upon. These tubes
are expanded at their ends by a special tool into the tube-plates of the
fire-box and boiler front. George Stephenson and his predecessors
experienced great difficulty in rendering the tube-end joints quite
water-tight, but the invention of the "expander" has removed this
trouble.

The _fire-brick arch_ shown (Fig. 6) in the fire-box is used to deflect
the flames towards the back of the fire-box, so that the hot gases may
be retarded somewhat, and their combustion rendered more perfect. It
also helps to distribute the heat more evenly over the whole of the
inside of the box, and prevents cold air from flying directly from the
firing door to the tubes. In some American and Continental locomotives
the fire-brick arch is replaced by a "water bridge," which serves the
same purpose, while giving additional heating surface.

The water circulation in a locomotive boiler is--upwards at the fire-box
end, where the heat is most intense; forward along the surface;
downwards at the smoke-box end; backwards along the bottom of the
barrel.

OTHER TYPES OF BOILERS.

For small stationary l