n of the constituent atoms of the combustible, and the
supporter of combustion. In many of the cases, however, which occur in
practice, a slow combustion is not attainable; but the tendencies of slow
combustion are both to save the fuel, and to burn the smoke.

149. _Q._--Is not the combustion in the furnaces of the Cornish boilers
very slow?

A.--Yes, very slow; and there is in consequence very little smoke evolved.
The coal used in Cornwall is Welsh coal, which evolves but little smoke,
and is therefore more favorable for the success of a smokeless furnace; but
in the manufacturing districts, where the coal is more bituminous, it is
found that smoke may be almost wholly prevented by careful firing and by
the use of a large capacity of furnace.

150. _Q._--Do you consider slow combustion to be an advisable thing to
practise in steam vessels?

_A._--No, I do not. When the combustion is slow, the heat in the furnaces
and flues is less intense, and a larger amount of heating surface
consequently becomes necessary to absorb the heat. In locomotives, where
the heat of the furnace is very intense, there will be the same economy of
fuel with an allowance of 5 or 6 square feet of surface to evaporate a
cubic foot of water as in common marine boilers with 10 or 12.

151. _Q._--What is the method of consuming smoke pursued in the
manufacturing districts?

_A._--In Manchester, where some stringent regulations for the prevention of
smoke have for some time been in force, it is found that the readiest way
of burning the smoke is to have a very large proportion of furnace room,
whereby slow combustion may be carried on. In some cases, too, a favourable
result is arrived at by raising a ridge of coal across the furnace lying
against the bridge, and of the same height: this ridge speedily becomes a
mass of incandescent coke, which promotes the combustion of the smoke
passing over it.

152. _Q._--Is the method of admitting a stream of air into the flues to
burn the smoke regarded favorably?

_A._--No; it is found to be productive of injury to the boiler by the
violent alternations of temperature it occasions, as at some times cold air
impinges on the iron of the boiler, and at other times flame,--just as
there happens to be smoke or no smoke emitted by the furnace. Boilers,
therefore, operating upon this principle, speedily become leaky, and are
much worn by oxidation, so that, if the pressure is considerable, they are
liable