The most stubborn of all errors are those which have been acquired
by a sort of inheritance, which have passed dogmatically from father
to son, or, still worse, from mother to daughter. They may become
superstitions without any theological character. The idea that the
weather changes with the moon, that wind “keeps off the rain,” are
physical superstitions in all cases where they are blindly accepted and
promulgated without any examination of evidence.
The idea that our open fireplaces are necessary for ventilation is one
of these physical superstitions, which is producing an incalculable
amount of physical mischief throughout Britain. A little rational
reflection on the natural and necessary movements of our household
atmospheres demonstrates at once that this dogma is not only baseless,
but actually expresses the opposite of the truth. I think I shall be
able to show in what follows, 1st, that they do no useful ventilation;
and, 2d, that they render systematic and really effective ventilation
practically impossible.
Everybody knows that when air is heated it expands largely, becomes
lighter, bulk for bulk, than other air of lower temperature; and
therefore, if two portions of air of unequal temperatures, and free
to move, are in contact with each other, the colder will flow under
the warmer, and push it upwards. The latter postulate must be kept
distinctly in view, for the rising of warm air is too commonly regarded
as due to some direct uprising activity or skyward affinity of its own,
instead of being understood as an indirect result of gravitation. It
is the downfalling of the cooler air that causes the uprising of the
warmer.
Now, let us see what, in accordance with the above-stated simple
laws, must happen in an ordinary English apartment that is fitted,
as usual, with one or more windows more or less leaky, and one or
more doors in like condition, and a hole in the wall in which coal
is burning in an iron cage immediately beneath a shaft that rises to
the top of the house, the fire-hole itself having an extreme height
of only 24 to 30 inches above the floor, all the chimney above this
height being entirely closed. (I find by measurement that 24 inches
is the usual height of the upper edge of the chimney opening of an
ordinary “register” stove. Old farm-house fireplaces are open to the
mantlepiece.)
Now, what happens when a heap of coal is burning in this hole? Some of
the heat—from 10 to 20 per cent, according to the construction of the
grate—is radiated into the room, the rest is conveyed by an ascending
current of air up the chimney. As this ascending current is rendered
visible by the smoke entangled with it, no further demonstration of its
existence is needed.
But how is it pushed up the chimney? Evidently by cooler air, that
flows into the room from somewhere, and which cooler air must get
under it in order to lift it. In ordinary rooms this supply of air is
entirely dependent upon their defective construction—bad joinery;
it enters only by the crevices surrounding the ill-fitting windows
and doors, no specially designed opening being made for it. Usually
the chief inlet is the space under the door, through which pours a
rivulet of cold air, that spreads out as a lake upon the floor. This
may easily be proved by holding a lighted taper in front of the bottom
door-chink when the window and other door—if any—are closed, and the
fire is burning briskly. At the same time more or less of cold air is
poured in at the top and the side spaces of the door and through the
window-chinks. The proportion of air entering by these depends upon the
capacity of the bottom door-chink. If this is large enough it will do
nearly all the work, otherwise every other possible leakage, including
the key-hole, contributes.
But what is the path of the air which enters by these higher level
openings? The answer to this is supplied at once by the fact that
such air being colder than that of the room, it must fall immediately
it enters. The rivulet under the door is thus supplemented by cascades
pouring down from the top and sides of the door and the top and sides
of the windows, all being tributaries to the lake of cold air covering
the floor.
The next question to be considered is, what is the depth of this lake?
In this, as in every other such accumulation of either air or water,
the level of the upper surface of the lake is determined by that of its
outlet. The outlet in this case is the chimney hole, through which all
the overflow pours upwards; and, therefore, the surface of the flowing
stratum of cold air corresponds with the upper part of the chimney
hole, or of the register, where register stoves are used.
Below this level there is abundant ventilation, above it there is none.
The cat that sits on the hearth-rug has an abundant supply of fresh
air, and if we had tracheal breathing apertures all down the sides
of our bodies, as caterpillars have, those on our lower extremities
might enjoy the ventilation. If we squatted on the ground like savages
something might be said for the fire-hole ventilator. But as we are
addicted to sitting on chairs that raise our breathing apparatus
considerably above the level of the top of the register, the maximum
efficiency of the flow of cold air in the lake below is expressed by
the prevalence of chilblains and rheumatism.[31]
The atmosphere in which our heads are immersed is practically stagnant;
the radiations from the fire, plus the animal heat from our bodies,
just warm it sufficiently to enable the cool entering air to push it
upwards above the chimney outlet and the surface of the lower moving
stratum, and to keep it there in a condition of stagnation.
If anybody doubts the correctness of this description, he has only to
sit in an ordinary English room where a good fire is burning—the doors
and windows closed, as usual—and then to blow a cloud by means of
pipe, cigar, or by burning brown paper or otherwise, when the movements
below and the stagnation above, which I have described, will be
rendered visible. If there is nobody moving about to stir the air, and
the experiment is fairly made, the level of the cool lake below will be
distinctly shown by the clearing away of the smoke up to the level of
the top of the register opening, towards which it may be seen to sweep.
Above this the smoke-wreaths will remain merely waving about, with
slight movements due to the small inequalities of temperature caused
by the fraction of heat radiated into the room from the front of the
fire. These movements are chiefly developed near the door and windows,
where the above-mentioned cascades are falling, and against the walls
and furniture, where feeble convection currents are rising, due to the
radiant heat absorbed by their surfaces. The stagnation is the most
complete about the middle of the room, where there is the greatest bulk
of vacant airspace.
When the inlet under the door is of considerable dimensions, there
may be some escape of warmer upper air at the top of the windows, if
their fitting is correspondingly defective. These, however, are mere
accidents; they are not a part of the vaunted chimney-hole ventilation,
but interferences with it.
There is another experiment that illustrates the absence of ventilation
in such rooms where gas is burning. It is that of suspending a canary
in a cage near the roof. But this is cruel; it kills the bird. It would
be a more satisfactory experiment to substitute for the canary-bird any
wingless biped who, after reading the above, still maintains that our
fire-holes are effective ventilators.
Not only are the fire-holes worthless and mischievous ventilators
themselves, but they render efficient ventilation by any other means
practically impossible. The “Arnott’s ventilator” that we sometimes see
applied to the upper part of chimneys is marred in its action by the
greedy “draught” below.
The tall chimney-shaft, with a fire burning immediately below it,
dominates all the atmospheric movement in the house, unless another and
more powerful upcast shaft be somewhere else in communication with the
apartments. But in this case the original or ordinary chimney would be
converted into a downcast shaft pouring air downwards into the room,
instead of carrying it away upwards. I need not describe the sort of
ventilation thus obtainable while the fire is burning and smoking.
Effective sanitary ventilation should supply gentle and
uniformly-diffused currents of air of moderate and equal temperature
throughout the house. We talk a great deal about the climate here
and the climate there; and when we grow old, and can afford it, we
move to Bournemouth, Torquay, Mentone, Nice, Algiers, etc., for
better climates, forgetting all the while that the climate in which
we practically live is not that out-of-doors, but the indoor climate
of our dwellings, the which, in a properly constructed house, may
be regulated to correspond to that of any latitude we may choose.
I maintain that the very first step towards the best attainable
approximation to this in our existing houses is to brick up, cement
up, or otherwise completely stop up, all our existing fire-holes, and
abolish all our existing fires.
But what next? The reply to this will be found in the next chapter.