and engines the _vertical_ boiler is much used.
In Fig. 8 we have three forms of this type--A and B with cross
water-tubes; C with vertical fire-tubes. The furnace in every case is
surrounded by water, and fed through a door at one side.

[Illustration: FIG. 8.--Diagrammatic representation of three types of
vertical boilers.]

The _Lancashire_ boiler is of large size. It has a cylindrical shell,
measuring up to 30 feet in length and 7 feet in diameter, traversed from
end to end by two large flues, in the rear part of which are situated
the furnaces. The boiler is fixed on a seating of fire-bricks, so built
up as to form three flues, A and BB, shown in cross section in Fig. 9.
The furnace gases, after leaving the two furnace flues, are deflected
downwards into the channel A, by which they pass underneath the boiler
to a point almost under the furnace, where they divide right and left
and travel through cross passages into the side channels BB, to be led
along the boiler's flanks to the chimney exit C. By this arrangement the
effective heating surface is greatly increased; and the passages being
large, natural draught generally suffices to maintain proper combustion.
The Lancashire boiler is much used in factories and (in a modified form)
on ships, since it is a steady steamer and is easily kept in order.

[Illustration: FIG. 9.--Cross and longitudinal sections of a Lancashire
boiler.]

In marine boilers of cylindrical shape cross water-tubes and fire-tubes
are often employed to increase the heating surface. Return tubes are
also led through the water to the funnels, situated at the same end as
the furnace.

AIDS TO COMBUSTION.

We may now turn our attention more particularly to the chemical process
called _combustion_, upon which a boiler depends for its heat. Ordinary
steam coal contains about 85 per cent. of carbon, 7 per cent. of oxygen,
and 4 per cent. of hydrogen, besides traces of nitrogen and sulphur and
a small incombustible residue. When the coal burns, the nitrogen is
released and passes away without combining with any of the other
elements. The sulphur unites with hydrogen and forms sulphuretted
hydrogen (also named sulphurous acid), which is injurious to steel
plates, and is largely responsible for the decay of tubes and funnels.
More of the hydrogen unites with the oxygen as steam.

The most important element in coal is the carbon (known chemically by
the symbol C). Its combination with oxygen, cal