148. _Q._--Is a slow or a rapid combustion the most beneficial?

_A._--A slow combustion is found by experiment to give the best results as
regards economy of fuel, and theory tells us that the largest advantage
will necessarily be obtained where adequate time has been afforded for a
complete combination 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.