_THE CHIMNEYS._
“Why do chimneys smoke?” asked Paul of his cousin.
“You mean rather to ask me,” replied the latter, “why _some_ chimneys
smoke. Many causes contribute to make chimneys smoke, while there is
only one condition which _must_ be observed if they are not to smoke.
We must therefore do our utmost to fulfil that condition, viz. a
flue proportioned to the fireplace, and the supply of a quantity of
air to the latter proportioned to the combustion. If the flue is too
narrow for the amount of smoke given off by the combustion, this smoke
does not rise easily enough, its advance in ascending is checked by
friction, and the discharge being insufficient for the production,
the smoke comes out into the room. We can stimulate the combustion,
and consequently the ascent of the smoke by a current of external air
directed towards the wood or coal. When the fire is well lighted it
warms the column of air that fills the chimney, and the warmer this
column is the lighter the air is, and the more it tends to rise.
“That is why in some ill-built chimneys a certain time is required
before the smoke will take its proper course—that is to say, the column
of air must be warmed. And until it is so, the smoke passes not into
the flue, but into the room; then we open a window to supply the fire
with air, which brightens it up so as to warm the flue and allow the
smoke to take its proper course. For the same reason all new chimneys
smoke. Flues carried up in masonry are damp and cold, and the air they
contain is heavy; it takes some time to warm and lighten it.
“Instead of opening a window to stimulate the fire (which is a rather
primitive method), we supply each grate with an air draught—that is, we
give it a channel which conducts the external air to the combustible as
soon as the least heat is developed, that, _e.g._ of a piece of paper
lighted. Immediately this exterior air is called in to fill the vacuum
produced by the commencement of combustion, and it stimulates the fire
by bringing it oxygen. The livelier the fire the more rapid is the
draught; and the more rapidly the air comes in, the more brightly does
the wood or coal burn. The air-channel is to a grate what a pair of
bellows are to a forge fire. But the air-channel, as well as the flue,
must bear a due proportion to the fireplace. If the flue is too narrow,
the smoke is obstructed, and comes out into the room; if it is too
wide, it is not uniformly heated, and the external currents of air—the
winds—exert a pressure at its upper extremity which neutralizes the
effect of the draught, and the smoke is beaten down. If the air-channel
is too small for the extent of the grate, it does not bring the
quantity of air necessary for combustion; the fire languishes, it heats
the flue imperfectly, and the lukewarm smoke does not ascend rapidly
enough. If the air-channel is too large, or brings in too considerable
a volume of air, the oxygen of which is not completely taken up, then
a part of the cold air enters the flue and does not stimulate the
draught; or, if there are changes in the temperature, the air-channel
attracts the air from the chimney instead of bringing in air from the
outside. The process is reversed, and the chimney smokes dreadfully.”
It was in the evening, after dinner, and when the family were seated
around the hearth, that Eugène was propounding this theory. “That
appears to me simple enough,” said Madame de Gandelau; “but then why
does the chimney in my room, which I have had altered several times,
smoke on certain days?”
“Because your room, Madame, is situated in the new wing of the house,
the roof of which is lower than that of the older part. They could not
carry the flue high enough to rise above the ridges of the roof of
the old building, for that isolated chimney would not have resisted
the squalls. When the wind comes on your side it finds the obstacle
presented by the loftier building, and rebounds: an eddy is formed,
and whirling about on itself it becomes engulfed in your chimney-flue,
or at least obstructs for a time the passage of the smoke. In such a
case the flues should bifurcate; as the pressure of the wind is never
exerted equally in both orifices, the air rushing into one makes the
smoke issue violently through the other. I know of no other plan: I
have already proposed it to you; but you have thought, not without
reason, that these flues, which seem to raise two despairing arms
towards heaven, would be very ugly; so you have resigned yourself to be
smoked out of your room when a strong gust from the west is blowing.”
“Yet the chimney-doctor put a sheet-iron pipe with a revolving
cowl—what he calls, I think, a _gueule de loup_: he told me it would
work admirably, but it was worse than before.”
“Certainly; when there are eddies and whirlwinds in consequence of
some obstacle, as here. This cowl turns in all directions, and among
its rapid gyrations sometimes presents its mouth to the gust, if only
for an instant. This mouth then performs the office of a funnel, and
the air, rushing into the pipe, sends the smoke in puffs into the very
middle of the room.”
“Exactly so; you think, therefore, that those two ugly flues must be
adopted?”
“Certainly. There are cities near to mountains, all of whose houses,
however lofty, are in the same condition. Geneva, for example, built
between the Salève and the Jura, is commanded, though at a great
distance, by these mountains. The violent winds which sometimes prevail
on the lake are imprisoned between these two chains, form eddies, rush
backwards and forwards, and send violent gusts in every direction;
so that the Genevese are obliged to put these double flues on their
chimneys, which at a distance present the aspect of a forest of
old-fashioned telegraphs.”
“I hope, then, that you will build the chimneys in the new house in
such a way that they will not smoke. You know that Marie would be much
vexed if they should.”
“_We will do our best_; and the local conditions are favourable to
begin with; we are not commanded by other elevations, we have not any
eddies of wind to fear; along the plateau on which we are building the
breezes are regular. Moreover, we have only straight high roofs, and
all the flues rise above the ridges. We shall build these flues with
brick, and make them of ample size. Nothing obliges us to give them
very oblique directions; they rise vertically, or nearly so. Lastly, we
shall have a system of air-flues, arranged from the very foundations,
in a cool aspect; for that must be attended to, since if the air-flues
are open to the south—for example—the air they receive from without,
even during the winter, is warmer than that of the room where the fire
is lighted; and then the air-flue draws the smoke which comes down
into the room. At least the fire cannot be lighted; the wood is only
charred, and does not burn.
“At Paris they often have a single flue for several fireplaces
(arranged one over the other), and parallel with it a ventilation-flue
sending a branch to each of these fireplaces. That is a good plan,
especially in houses where there are as many as five fireplaces,
one over the other; because the weakening of the walls by the
juxtaposition of a number of flues is thus avoided. The fires exercise
a reciprocal attraction, and this system prevents smoke in the rooms.
These flues must have a section sufficient for all the fireplaces—that
is, for five ordinary chimneys, one above another, a section of 1 foot
9 inches superficial, or 16 inches square. But here, where we have only
three stories and sufficient room, I prefer having separate flues for
each chimney; especially as with the system of single flues all the
fires must be lighted: which is always the case in a large town. If
they are not, it will happen in rapid changes of temperature that the
smoke will pass into one of the higher or lower fireplaces instead of
following the vertical column. This inconvenience, which by the by is
only accidental, is remedied by well-arranged dampers.”
“But,” said Paul, “does not this cold air of the air-channel chill the
apartments?”
“This cold air comes into the fireplace itself, not into the apartment;
it is evident that if a fire is not made, the air-flue introduces cold
air, which contributes to lowering the temperature of the apartment. It
may be shut off by a damper. But keep this in mind: to make a fire burn
wood, or coal, or anything else, oxygen is needed. You have learned
that in your studies in chemistry and physics. Air, therefore, is
required; without air no fire. Formerly they did not take the trouble
to provide air-flues for fireplaces, because the air came into the
rooms under the doors and through the ill-closed windows, and also
because the apartments being very large contained air enough to keep up
a fire for some time. And our grandfathers’ chimneys, be it observed,
smoked pretty considerably. We of the present day are less hardy,
and like to have smaller rooms, well shut in, and we are afraid of
draughts; that is all very well, but the chimney must have a draught,
since without it the fuel will not burn and therefore not warm you. It
is evident that this column of cold air, which you call in to stimulate
the combustion, takes with it, in ascending the flue, a considerable
amount of heat. Many plans have therefore been devised for preventing
the heated air from passing rapidly away. It is caused to turn in the
flues, and obliged to remain as long as possible, or at least to leave
in the walls of the numerous passages which it traverses a part of the
heat it has absorbed. These passages in their turn warm a surrounding
cavity or room, which is also supplied with air. This air, dilated by
heat, tends to escape. Issues are made for it, which are called hot-air
escapes. This is the principle of the hot-air apparatus.”
“_A propos_ of heating apparatuses,” said Madame de Gandelau, “are you
intending to construct one in the new house?”
“Certainly; its place is marked on the plan of the cellars below the
entrance hall, and its flue goes up in the interior angle of the great
staircase. A heating apparatus is indispensable in a country house,
especially if it is not lived in throughout the winter. It is the means
of preventing a good deal of injury to the house. Heating once or
twice a week during the cold and damp season is sufficient to keep the
apartments fairly dry.”
“Do you not think the heat of the hot-air apparatus injurious to
health?”
“The warm air issuing from the heating apparatus is unwholesome,
because in becoming warm it has lost a part of its oxygen, and because
oxygen is as necessary to us in supporting life as it is to fuel in
supporting combustion. We can avoid some of the injurious results to
the animal economy arising from the deoxidized air by making it pass
over basins filled with water on leaving the heat receiver; but this
means is only palliative, and we thus lose part of the warmth.
“I consider hot-air apparatuses desirable only for warming apartments
that are not lived in, such as entrance-halls, staircases, and
passages; but if hot-air escapes are provided in drawing-rooms,
dining-rooms, and bedrooms, care must be taken not to allow them to be
open while the apartments are occupied. Open them only to dry the rooms
when you are absent; and when this is done, open the windows, and close
the hot-air escapes when you shut the windows.”
“And the baths—how will you heat them?”
“By means of a boiler arranged near the heating apparatus, with
circulating pipes reaching to the bath-rooms on the first floor, which
are over the heating chamber, or nearly so.”
“Have you arranged for baths for the servants also?”
“Yes, under the bakehouse and wash-house, below the ground floor.”
“You have provided for everything, I see. This conversation about the
chimneys has been one of which you will do well to give a summary in
your notes, Paul!”
“I will do so, mother.”