either by passing under or round the furnace, or in one case through hollow
fire bars. The regulation of the air supply is effected by hand, and it is
clear that some of these arrangements are liable to admit an unnecessary
supply of air, while others scarcely admit enough, especially when fresh
coal is put on. This is the difficulty with all these arrangements when
used with ordinary hand--i.e., intermittent--stoking. Two plans are open to
us to overcome the difficulty. Either the stoking and the air supply must
both be regular and continuous, or the air supply be made intermittent to
suit the stoking. The first method is carried out in any of the many forms
of mechanical stoker, of which this of Sinclair's is an admirable specimen.
Fresh fuel is perpetually being pushed on in front, and by alternate
movement of the fire bars the fire is kept in perpetual motion till the
ashes drop out at the back. To such an arrangement as this a steady air
supply can be adjusted, and if the boiler demand is constant there is no
need for smoke, and an inferior fuel may be used. The other plan is to vary
the air supply to suit the stoking. This is effected by Prideaux automatic
furnace doors, which have louvers to remain open for a certain time after
the doors are shut, and so to admit extra air immediately after coal has
been put on, the supply gradually decreasing as distillation ceases. The
worst of air admitted through chinks in the doors, or through partly open
doors, is that it is admitted cold, and scarcely gets thoroughly warm
before it is among the stuff it has to burn. Still this is not a fatal
objection, though a hot blast would be better. Nothing can be worse than
shoveling on a quantity of coal and shutting it up completely. Every
condition of combustion is thus violated, and the intended furnace is a
mere gas retort.

_Gas Producers_.--Suppose the conditions of combustion are purposely
violated; we at once have a gas producer. That is all gas producers are,
extra bad stoves or furnaces, not always much worse than things which