and yet would have their chimney carry up the
smoke, require inconsistencies and expect impossibilities. The obvious
remedy in this case is, to admit more air, and the question will be how
and where this necessary quantity of air from without is to be admitted,
so as to produce the least inconvenience; for if the door or window be
left so much open, it causes a cold draft of air to the fire-place, to
the great discomfort of those who sit there. Various have been the
contrivances to avoid this, such as bringing in fresh air through pipes
in the jambs of the chimney, which, pointing upwards, should blow the
smoke up the funnel; opening passages in the funnel above to let in air
for the same purpose; but these produce an effect contrary to that
intended, for as it is the constant current of air passing from the room
through the opening of the chimney into the flue, which prevents the
smoke coming out into the room, if the funnel is supplied by other means
with the air it wants, and especially if that air be cold, the force of
that current is diminished, and the smoke in its efforts to enter the
room finds less resistance. The wanted air must then indispensably be
admitted into the room to supply what goes off through the opening of
the chimney, and it is advisable to make the aperture for this purpose
as near the ceiling as possible, because the heated air will naturally
ascend and occupy the highest part of the room, thus causing a great
difference of climate at different heights, a defect which will be in
some measure obviated by the admission of cold air near the ceiling,
which descending, will beat down and mingle the air more effectually.
Another cause of smoky chimnies is too short a funnel, as, in this case,
the ascending current will not always have sufficient power to direct
the smoke up the flue. This defect is frequently found in low buildings,
or the upper stories of high ones, and is unavoidable, for if the flue
be raised high above the roof to strengthen its draft, it is then in
danger of being blown down and crushing the roof in its fall. The remedy
in this case is to contract the opening of the chimney so as to oblige
all the entering air to pass through or very near the fire, by which
means it will be considerably heated, and by its great rarefaction,
cause a powerful draft, and compensate for the shortness of its column.
The case of too short a funnel is more general than would be imagined,
and often found