. It is one of the principal problems of boiler
engineering to render the amount of heat thus lost as small as possible.

The loss from radiation is in proportion to the amount of surface, its
nature, its temperature, and the time it is exposed. This loss can be
almost entirely eliminated by thick walls and a smooth white or polished
surface, but its amount is ordinarily so small that these extraordinary
precautions do not pay in practice.

It is evident that the temperature of the escaping gases cannot be
brought below that of the absorbing surfaces, while it may be much
greater even to that of the fire. This is supposing that all of the
escaping gases have passed through the fire. In case air is allowed to
leak into the flues, and mingle with the gases after they have left the
heating surfaces, the temperature may be brought down to almost any
point above that of the atmosphere, but without any reduction in the
amount of heat wasted. It is in this way that those low chimney
temperatures are sometimes attained which pass for proof of economy with
the unobserving. All surplus air admitted to the fire, or to the gases
before they leave the heating surfaces, increases the losses.

We are now prepared to see why and how the temperature and the rapidity
of combustion in the boiler furnace affect the economy, and that though
the amount of heat developed may be the same, the heat available for the
generation of steam may be much less with one rate or temperature of
combustion than another.

Assuming that there is no air passing up the chimney other than that
which has passed through the fire, the higher the temperature of the
fire and the lower that of the escaping gases the better the economy,
for the losses by the chimney gases will bear the same proportion to the
heat generated by the combustion as the temperature of those gases bears
to the temperature of the fire. That is to say, if the temperature of
the fire is 2500 degrees and that of the chimney gases 500 degrees above
that of the atmosphere, the loss by the chimney will be 500/2500 = 20
per cent. Therefore, as the escaping gases cannot be brought below the
temperature of the absorbing surface, which is practically a fixed
quantity, the temperature of the fire must be high in order to secure
good economy.

The losses by radiation being practically proportioned to the time
occupied, the more coal burned in a given furnace in a given time, the
less will be the pro