d for the purpose. The
reason is, that in a close fire the whole of the draught must pass
through the fire. It thus becomes so heated that, unless the flue is
properly built, it is dangerous throughout its whole course. In one
instance of a heating furnace, the heat in the flue was found to be
300 deg., at a distance of from forty to fifty feet from the fire. In open
fireplaces, the quantity of cold air carried up with the draught keeps
the flue at a moderate heat, from the fire upwards, and, unless the
flue is allowed to become foul, and take fire, this is the safest
possible mode of heating.

Heating by hot air, steam, and hot water are objectionable. First,
because there must be a furnace and furnace flue, and the flue used is
generally that built for an open fire only; and second, the pipes are
carried in every direction, to be as much out of sight as possible. By
this means they are constantly liable to produce spontaneous ignition,
for there appears to be some chemical action between heated iron and
timber, by which fire is generated at a much lower temperature than is
necessary to ignite timber under ordinary circumstances. No
satisfactory explanation of this fact has yet been given, but there is
abundant proof that such is the case. In heating by hot-water pipes,
those hermetically sealed are by far the most dangerous, as the
strength of the pipes to resist the pressure is the only limit of the
heat to which the water, and of course the pipes, may be raised. In
some cases a plug of metal which fuses at 400 deg. is put into the pipes,
but the heat to which the plug is exposed will depend very much on
where it is placed, as, however great may be the heat of the exit
pipe, the return pipe is comparatively cool. But even where the pipes
are left open, the heat of the water at the furnace is not necessarily
212 deg.. It is almost needless to say that 212 deg. is the heat of boiling
water under the pressure of one atmosphere only; but if the pipes are
carried sixty or seventy feet high, the water in the furnace must be
under the pressure of nearer three atmospheres than one, and therefore
the heat will be proportionately increased. Fires from pipes for
heating by hot water have been known to take place within twenty-four
hours after first heating, and some after ten years of apparent
safety.

The New Metropolitan Building Act prescribes rules for the placing
steam, hot-air, and hot-water pipes at a certain distance