ence
is farther from the wind than the chimney commanded. For instance,
suppose the chimney of a building to be so situated as that its top is
below the level of the ridge of the roof, which, when the wind blows
against it, forms a kind of dam against its progress. In this case, the
wind being obstructed by this dam, will, like water, press and search
for passages through it, and finding the top of the chimney below the
top of the dam, it will force itself down that funnel in order to get
through by some door or window open on the other side of the building,
and if there be a fire in such chimney, its smoke is of course beat down
and fills the room. The only remedy for this inconvenience is, to raise
the funnel higher than the roof, supporting it, if necessary, by iron
bars; for a turn-cap in this case has no effect, the dammed up air
pressing down through it in whatever position the wind may have placed
its opening. Chimnies otherwise drawing well are sometimes made to smoke
by the improper and inconvenient situation of a door. When the door and
chimney are placed on the same side of a room, if the door is made to
open from the chimney, it follows, that when only partly opened, a
current of air is admitted and directed across the opening of the
chimney, which is apt to draw out some of the smoke. Chimnies which
generally draw well, do, nevertheless, sometimes give smoke into the
room, it being driven down by strong winds passing over the tops of
their flues, though not descending from any commanding eminence. To
understand this, it may be considered that the rising light air, to
obtain a free issue from the funnel, must push out of its way, or oblige
the air that is over it to rise. In a time of calm, or of little wind,
this is done visibly; for we see the smoke that is brought up by that
air rise in a column above the chimney. But when a violent current of
wind passes over the top of a chimney, its particles have received so
much force, which keeps them in a horizontal direction, and follow each
other so rapidly, that the rising light air has not strength sufficient
to oblige them to quit that direction, and move upwards to permit its
issue. Add to this, that some of the air may impinge on that part of the
inside of the funnel which is opposed to its progress, and be thence
reflected downwards from side to side, driving the smoke before it into
the room. The simplest and best remedy in this case is the application
of a