cial knowledge of the weight of
carbonic acid, an ignorance of the law of the diffusion of
gases,--upon a realizing sense of the cost of coal, and an
insensibility to the worth of fresh air. Even such unreliable
witnesses as our senses assure us that the air at the top of a high
room--say the upper gallery of an unventilated theatre--is far less
salubrious, though not overheated, than that below. We know, too, how
quickly the sulphurous gas that sometimes escapes from those
warranted furnaces not only ascends through the tin pipes, but rises
in the open stairway if it has a chance. The hurtful carbonaceous
gases doubtless go with it, and are then diffused through the room.
The most forcible objection to allowing the air to escape through the
ceiling is that it is a wanton waste, not only of heat, but of the
fresh air that has just come from the north pole by way of the furnace
and cold-air box, and which, by virtue of its warmth, goes in all its
purity straight to the ceiling. Accordingly the heavy cold air lying
near the floor and laden with poison must be drawn out through the
ventilating flue, till the upper warmth and freshness fall gently on
our heads, like heavenly blessings.

Let me digress here to answer another question. No, don't put your
ventilating flues in the outer walls if you expect the air to rise
through them in cold weather; for it will not, if they reach the
moon, unless it is warmer than that lying at their base. You may as
well expect water to rise from the cistern to the tank in the attic
because the pipe runs there, as that air will rise simply because
there is a passage for it. Sometimes holes are made into the
chimney-flues, but this is robbing the stoves or the fireplaces. It is
better to build an independent flue so close that it shall always feel
the heat from the black warm heart of the chimney, for warmed by some
means it must be. Yet warm air does not choose to rise. It falls like
lead unless lifted by something heavier than itself.

To return to the former point. When you can warm within a ventilating
flue all the air passing through it more economically than you can
warm the same quantity in the room from which it is taken, then you
may admit the air to feed this same flue near the bottom and perhaps
save fuel; but I doubt whether the remaining air will be any purer
than if an equal amount had been allowed to escape near the ceiling.
The answers to your square questions necessarily do