nner most
likely to embarrass it in its ascent and drive it back. The current of
air which, passing under the mantle, gets into the chimney, should be
made gradually to bend its course upwards, by which means it will unite
quietly with the ascending current of smoke, and will be less likely to
check and impede its progress. This is to be effected by rounding off
the inside of the breast of the chimney, which may be done by a thick
coating of plaster. When the breast or wall of the chimney in front is
very thin, it may happen, that the depth of the fire-place determined
according to the preceding rules may be too small. Thus supposing the
breast to be only four inches thick, which is sometimes the case,
particularly in rooms situated near the top of a house, taking four
inches for the width of the throat, will give only eight inches for the
depth of the fire-place. In this case, it would be proper to increase
the depth of the fire-place at the hearth to twelve or thirteen inches,
and to build up the back perpendicularly to the height of the top of the
grate, and then sloping the back by a gentle inclination forward, bring
it to its proper place directly under the back part of the throat of
the chimney. This slope, though it ought not to be too abrupt, yet
should be quite finished at the height of eight or ten inches above the
fire, otherwise it may perhaps cause the chimney to smoke; but when it
is very near the fire, its heat will enable the current of rising smoke
to overcome the obstacle which this slope will oppose to its ascent,
which it could not so easily do, were the slope situated at a greater
distance from the burning fuel. There is one important circumstance
respecting chimney fire-places designed for burning coals which remains
to be examined, and that is the grate. Although there are few grates
that may not be used in chimnies, altered or constructed on the
principles recommended by Count Rumford, yet they are not by any means
all equally well adapted for that purpose. Those whose construction is
most simple, and which of course are the cheapest, are beyond comparison
the best on all accounts. Nothing being wanted but merely a grate to
contain the coals, and all additional apparatus being not only useless
but pernicious; all complicated and expensive grates should be laid
aside, and such as are more simple substituted in their room. The proper
width for grates in rooms of a middling size, will be from six to e