the chimney, the immoderate size of which is a most essential
fault in their construction; for, however good the formation of a
fire-place may be in other respects, if the opening left for the passage
of the smoke is larger than is necessary for that purpose, nothing can
prevent the warm air of the room from escaping through it; and whenever
this happens, there is not only an unnecessary loss of heat, but the
warm air, which leaves the room to go up the chimney, being replaced by
cold air from without, produces those drafts of air so often complained
of. But though these evils may be remedied, by reducing the throat of
the chimney to a proper size, yet, in doing this, several considerations
will be necessary to determine its proper situation. As the smoke and
hot vapour which rise from a fire naturally tend upwards, it is evident
that it will be proper to place the throat of the chimney
perpendicularly over the fire; but to ascertain its most advantageous
distance, or how far above the burning fuel it ought to be placed, is
not so easy, and requires several advantages and disadvantages to be
balanced. As the smoke and vapour rise in consequence of their being
rarefied by heat, and made lighter than the air of the surrounding
atmosphere, and as the degree of their rarefraction is in proportion to
the intensity of their heat, and as this heat is greater near the fire
than at a distance from it, it is clear, that the nearer the throat of a
chimney is to the fire, the stronger will be what is commonly called its
draught, and the less danger there will be of its smoking, or of dust
coming into the room when the fire is stirred. But, on the other hand,
when a very strong draught is occasioned by the throat of the chimney
being very near the fire, it may happen that the influx of air into the
fire may become so strong as to cause the fuel to be consumed too
rapidly. This however will very seldom be found to be the case, for the
throats of chimnies are in general too high. In regard to the materials
which it will be most advantageous to employ in the construction of
fire-places, little difficulty will attend the determination of that
point. As the object in view is to bring radiant heat into the room, it
is clear that that material is best for the construction of a fire-place
which reflects the most, or which absorbs the least of it, for that heat
which is absorbed cannot be reflected. Now, as bodies which absorb
radiant heat ar