chimneys that I can at present recollect as having fallen under
my observation; communicating the remedies that I have known
successfully used for the different cases, together with the
principles on which both the disease and the remedy depend, and
confessing my ignorance wherever I have been sensible of it. You
will do well, if you publish, as you propose, this letter, to add in
notes, or as you please, such observations as may have occurred to
your attentive mind; and if other philosophers will do the same, this
part of science, though humble, yet of great utility, may in time
be perfected. For many years past, I have rarely met with a case of
a smoky chimney, which has not been solvable on these principles,
and cured by these remedies, where people have been willing to apply
them; which is indeed not always the case; for many have prejudices
in favour of the nostrums of pretending chimney-doctors and fumists,
and some have conceits and fancies of their own, which they rather
chuse to try, than to lengthen a funnel, alter the size of an
opening, or admit air into a room, however necessary; for some are as
much afraid of fresh air as persons in the hydrophobia are of fresh
water. I myself had formerly this prejudice, this _aerophobia_, as I
now account it, and dreading the supposed dangerous effects of cool
air, I considered it as an enemy, and closed with extreme care every
crevice in the rooms I inhabited. Experience has convinced me of
my error. I now look upon fresh air as a friend: I even sleep with
an open window. I am persuaded that no common air from without, is
so unwholesome as the air within a close room that has been often
breathed and not changed. Moist air too, which formerly I thought
pernicious, gives me now no apprehensions: for considering that
no dampness of air applied to the outside of my skin can be equal
to what is applied to and touches it within, my whole body being
full of moisture, and finding that I can lie two hours in a bath
twice a week, covered with water, which certainly is much damper
than any air can be, and this for years together, without catching
cold, or being in any other manner disordered by it, I no longer
dread mere moisture, either in air or in sheets or shirts: and I
find it of importance to the happiness of life, the being freed
from vain terrors, especially of objects that we are every day
exposed inevitably to meet with. You physicians have of late happily
discovered, after a contrary opinion had prevailed some ages, that
fresh and cool air does good to persons in the small pox and other
fevers. It is to be hoped, that in another century or two we may all
find out, that it is not bad even for people in health. And as to
moist air, here I am at this present writing in a ship with above
forty persons, who have had no other but moist air to breathe for six
weeks past; every thing we touch is damp, and nothing dries, yet we
are all as healthy as we should be on the mountains of Switzerland,
whose inhabitants are not more so than those of Bermuda or St.
Helena, islands on whose rocks the waves are dashed into millions of
particles, which fill the air with damp, but produce no diseases,
the moisture being pure, unmixed with the poisonous vapours arising
from putrid marshes and stagnant pools, in which many insects die and
corrupt the water. These places only, in my opinion (which however
I submit to yours) afford unwholesome air; and that it is not the
mere water contained in damp air, but the volatile particles of
corrupted animal matter mixed with that water, which renders such air
pernicious to those who breathe it. And I imagine it a cause of the
same kind that renders the air in close rooms, where the perspirable
matter is breathed over and over again by a number of assembled,
people so hurtful to health. After being in such a situation, many
find themselves affected by that _febricula_, which the English alone
call a _cold_, and, perhaps from the name, imagine that they caught
the malady by _going out_ of the room, when it was in fact by being
in it.
You begin to think that I wander from my subject, and go out of my
depth. So I return again to my chimneys.
We have of late many lecturers in experimental philosophy. I have
wished that some of them would study this branch of that science,
and give experiments in it as a part of their lectures. The addition
to their present apparatus need not be very expensive. A number of
little representations of rooms composed each of five panes of sash
glass, framed in wood at the corners, with proportionable doors,
and moveable glass chimneys, with openings of different sizes, and
different lengths of funnel, and some of the rooms so contrived as
to communicate on occasion with others, so as to form different
combinations, and exemplify different cases; with quantities of green
wax taper cut into pieces of an inch and half, sixteen of which stuck
together in a square, and lit, would make a strong fire for a little
glass chimney, and blown out would continue to burn and give smoke as
long as desired. With such an apparatus all the operations of smoke
and rarified air in rooms and chimneys might be seen through their
transparent sides; and the effect of winds on chimneys, commanded or
otherwise, might be shown, by letting the entering air blow upon them
through an opened window of the lecturer's chamber, where it would
be constant while he kept a good fire in his chimney. By the help of
such lectures our fumists would become better instructed. At present
they have generally but one remedy, which perhaps they have known
effectual in some one case of smoky chimneys, and they apply that
indiscriminately to all the other cases, without success,--but not
without expence to their employers.
With all the science, however, that a man shall suppose himself
possessed of in this article, he may sometimes meet with cases
that shall puzzle him. I once lodged in a house at London, which,
in a little room, had a single chimney and funnel. The opening was
very small, yet it did not keep in the smoke, and all attempts to
have a fire in this room were fruitless. I could not imagine the
reason, till at length observing that the chamber over it, which had
no fire-place in it, was always filled with smoke when a fire was
kindled below and that the smoke came through the cracks and crevices
of the wainscot; I had the wainscot taken down, and discovered that
the funnel, which went up behind it, had a crack many feet in length,
and wide enough to admit my arm, a breach very dangerous with regard
to fire, and occasioned probably by an apparent irregular settling of
one side of the house. The air entering this breach freely, destroyed
the drawing force of the funnel. The remedy would have been, filling
up the breach or rather rebuilding the funnel: but the landlord
rather chose to stop up the chimney.
Another puzzling case I met with at a friend's country house near
London. His best room had a chimney in which, he told me, he never
could have a fire, for all the smoke came out into the room. I
flattered myself I could easily find the cause, and prescribe the
cure. I had a fire made there, and found it as he said. I opened
the door, and perceived it was not want of air. I made a temporary
contraction of the opening of the chimney, and found that it was not
its being too large that caused the smoke to issue. I went out and
looked up at the top of the chimney: its funnel was joined in the
same stack with others, some of them shorter, that drew very well,
and I saw nothing to prevent its doing the same. In fine, after
every other examination I could think of, I was obliged to own the
insufficiency of my skill. But my friend, who made no pretension to
such kind of knowledge, afterwards discovered the cause himself. He
got to the top of the funnel by a ladder, and looking down, found
it filled with twigs and straw cemented by earth, and lined with
feathers. It seems the house, after being built, had stood empty some
years before he occupied it; and he concluded that some large birds
had taken the advantage of its retired situation to make their nest
there. The rubbish, considerable in quantity, being removed, and the
funnel cleared, the chimney drew well, and gave satisfaction.
In general smoke is a very tractable thing, easily governed and
directed when one knows the principles, and is well informed of
the circumstances. You know I made it _descend_ in my Pensylvania
stove. I formerly had a more simple construction, in which the same
effect was produced, but visible to the eye (Plate, Figure 7). It
was composed of two plates A B and C D, placed as in the figure.
The lower plate A B rested with its edge in the angle made by the
hearth with the back of the chimney. The upper plate was fixed to
the breast, and lapped over the lower about six inches wide and
the length of the plates (near two feet) between them. Every other
passage of air into the funnel was well stopped. When therefore a
fire was made at E, for the first time with charcoal, till the air
in the funnel was a little heated through the plates, and then wood
laid on, the smoke would rise to A, turn over the edge of that plate,
descend to D, then turn under the edge of the upper plate, and go
up the chimney. It was pretty to see, but of no great use. Placing
therefore the under plate in a higher situation, I removed the
upper plate C D, and placed it perpendicularly (Plate, Figure 8) so
that the upper edge of the lower plate A B came within about three
inches of it, and might be pushed farther from it, or suffered to
come nearer to it, by a moveable wedge between them. The flame then
ascending from the fire at E, was, carried to strike the upper plate,
made it very hot, and its heat rose and spread with the rarefied air
into the room.
I believe you have seen in use with me, the contrivance of a
sliding-plate over the fire, seemingly placed to oppose the rising of
the smoke, leaving but a small passage for it, between the edge of
the plate and the back of the chimney. It is particularly described,
and its uses explained, in my former printed letter, and I mention it
here only as another instance of the tractability of smoke[57].
What is called the Staffordshire chimney, (see the Plate, facing page
238) affords an example of the same kind. The opening of the chimney
is bricked up, even with the fore-edge of its jams, leaving open only
a passage over the grate of the same width, and perhaps eight inches
high. The grate consists of semicircular bars, their upper bar of
the greatest diameter, the others under it smaller and smaller, so
that it has the appearance of half a round basket. It is, with the
coals it contains, wholly without the wall that shuts up the chimney,
yet the smoke bends and enters the passage above it, the draft being
strong, because no air can enter that is not obliged to pass near or
through the fire, so that all that the funnel is filled with is much
heated, and of course much rarefied.
Much more of the prosperity of a winter country depends on the plenty
and cheapness of fuel, than is generally imagined. In travelling I
have observed, that in those parts where the inhabitants can have
neither wood nor coal nor turf but at excessive prices, the working
people live in miserable hovels, are ragged, and have nothing
comfortable about them. But when fuel is cheap (or where they have
the art of managing it to advantage) they are well furnished with
necessaries, and have decent habitations. The obvious reason is,
that the working hours of such people are the profitable hours, and
they who cannot afford sufficient fuel have fewer such hours in the
twenty four, than those who have it cheap and plenty: for much of the
domestic work of poor women, such as spinning, sewing, knitting; and
of the men in those manufactures that require little bodily exercise,
cannot well be performed where the fingers are numbed with cold,
those people, therefore, in cold weather, are induced to go to bed
sooner, and lie longer in a morning, than they would do if they could
have good fires or warm stoves to sit by; and their hours of work are
not sufficient to produce the means of comfortable subsistence. Those
public works, therefore, such as roads, canals, &c. by which fuel
may be brought cheap into such countries from distant places, are of
great utility; and those who promote them may be reckoned among the
benefactors of mankind.
I have great pleasure in having thus complied with your request, and
in the reflection, that the friendship you honour me with, and in
which I have ever been so happy, has continued so many years without
the smallest interruption. Our distance from each other is now
augmented, and nature must soon put an end to the possibility of my
continuing our correspondence: but if consciousness and memory remain
in a future state, my esteem and respect for you, my dear friend,
will be everlasting.
B. FRANKLIN.
_Notes for the Letter upon Chimneys._
No. I.
The latest work on architecture that I have seen, is that entitled
_Nutshells_, which appears to be written by a very ingenious man, and
contains a table of the proportions of the openings of chimneys; but
they relate solely to the proportions he gives his rooms, without
the smallest regard to the funnels. And he remarks, respecting those
proportions, that they are similar to the harmonic divisions of a
monochord.[58] He does not indeed lay much stress on this; but it
shows that we like the appearance of principles; and where we have
not true ones, we have some satisfaction in producing such as are
imaginary.
No. II.
The description of the sliding plates here promised, and which hath
been since brought into use under various names, with some immaterial
changes, is contained in a former letter to J. B. Esq. as follows:
TO J. B.[59] ESQ. AT BOSTON, IN NEW-ENGLAND.
_London, Dec. 2, 1758,_
DEAR SIR,
I have executed here an easy simple contrivance, that I have long
since had in speculation, for keeping rooms warmer in cold weather
than they generally are, and with less fire. It is this. The opening
of the chimney is contracted, by brick-work faced with marble slabs,
to about two feet between the jams, and the breast brought down to
within about three feet of the hearth. An iron frame is placed just
under the breast, and extending quite to the back of the chimney,
so that a plate of the same metal may slide horizontally backwards
and forwards in the grooves on each side of the frame. This plate
is just so large as to fill the whole space, and shut the chimney
entirely when thrust quite in, which is convenient when there is no
fire. Drawing it out, so as to leave a space between its further edge
and the back, of about two inches; this space is sufficient for the
smoke to pass; and so large a part of the funnel being stopt by the
rest of the plate, the passage of warm air out of the room, up the
chimney, is obstructed and retarded, and by that means much cold air
is prevented from coming in through crevices, to supply its place.
This effect is made manifest three ways. First, when the fire burns
briskly in cold weather, the howling or whistling noise made by the
wind, as it enters the room through the crevices, when the chimney
is open as usual, ceases as soon as the plate is slid in to its
proper distance. Secondly, opening the door of the room about half
an inch, and holding your hand against the opening, near the top of
the door, you feel the cold air coming in against your hand, but
weakly, if the plate be in. Let another person suddenly draw it out,
so as to let the air of the room go up the chimney, with its usual
freedom where chimneys are open, and you immediately feel the cold
air rushing in strongly. Thirdly, if something be set against the
door, just sufficient, when the plate is in, to keep the door nearly
shut, by resisting the pressure of the air that would force it open:
then, when the plate is drawn out, the door will be forced open by
the increased pressure of the outward cold air endeavouring to get in
to supply the place of the warm air, that now passes out of the room
to go up the chimney. In our common open chimneys, half the fuel is
wasted, and its effect lost; the air it has warmed being immediately
drawn off. Several of my acquaintance, having seen this simple
machine in my room, have imitated it at their own houses, and it
seems likely to become pretty common. I describe it thus particularly
to you, because I think it would be useful in Boston, where firing is
often dear.
Mentioning chimneys puts me in mind of a property I formerly had
occasion to observe in them, which I have not found taken notice of
by others; it is, that in the summer time, when no fire is made in
the chimneys, there is, nevertheless, a regular draft of air through
them, continually passing upwards, from about five or six o'clock
in the afternoon, till eight or nine o'clock the next morning, when
the current begins to slacken and hesitate a little, for about half
an hour, and then sets as strongly down again, which it continues to
do till towards five in the afternoon, then slackens and hesitates
as before, going sometimes a little up, then a little down, till, in
about a half an hour, it gets into a steady upward current for the
night, which continues till eight or nine the next day; the hours
varying a little as the days lengthen and shorten, and sometimes
varying from sudden changes in the weather; as if, after being long
warm, it should begin to grow cool about noon, while the air was
coming down the chimney, the current will then change earlier than
the usual hour, &c.
This property in chimneys I imagine we might turn to some account,
and render improper, for the future, the old saying, _as useless
as a chimney in summer_. If the opening of the chimney, from the
breast down to the hearth, be closed by a slight moveable frame or
two, in the manner of doors, covered with canvas, that will let the
air through, but keep out the flies; and another little frame set
within upon the hearth, with hooks on which to hang joints of meat,
fowls, &c. wrapt well in wet linen cloths, three or four fold, I
am confident, that if the linen is kept wet, by sprinkling it once
a day, the meat would be so cooled by the evaporation, carried on
continually by means of the passing air, that it would keep a week
or more in the hottest weather. Butter and milk might likewise be
kept cool, in vessels or bottles covered with wet cloths. A shallow
tray, or keeler, should be under the frame to receive any water that
might drip from the wetted cloths. I think, too, that this property
of chimneys might, by means of smoke-jack vanes, be applied to some
mechanical purposes, where a small but pretty constant power only is
wanted.
If you would have my opinion of the cause of this changing current of
air in chimneys, it is, in short, as follows. In summer time there
is generally a great difference in the warmth of the air at mid-day
and mid-night, and, of course, a difference of specific gravity
in the air, as the more it is warmed the more it is rarefied. The
funnel of a chimney, being for the most part surrounded by the house,
is protected, in a great measure, from the direct action of the
sun's rays, and also from the coldness of the night air. It thence
preserves a middle temperature between the heat of the day, and the
coldness of the night. This middle temperature it communicates to
the air contained in it. If the state of the outward air be cooler
than that in the funnel of the chimney, it will, by being heavier,
force it to rise, and go out at the top. What supplies its place from
below, being warmed, in its turn, by the warmer funnel, is likewise
forced up by the colder and weightier air below, and so the current
is continued till the next day, when the sun gradually changes the
state of the outward air, makes it first as warm as the funnel of
the chimney can make it (when the current begins to hesitate) and
afterwards warmer. Then the funnel, being cooler than the air that
comes into it, cools that air, makes it heavier than the outward air,
of course it descends; and what succeeds it from above being cooled
in its turn, the descending current continues till towards evening,
when it again hesitates and changes its course, from the change of
warmth in the outward air, and the nearly remaining same middle
temperature in the funnel.
Upon this principle, if a house were built behind Beacon-hill, an
adit carried from one of the doors into the hill horizontally, till
it meet with a perpendicular shaft sunk from its top, it seems
probable to me, that those who lived in the house would constantly,
in the heat even of the calmest day, have as much cool air passing
through the house, as they should chuse; and the same, though
reversed in its current, during the stillest night.
I think, too, this property might be made of use to miners; as,
where several shafts or pits are sunk perpendicularly into the
earth, communicating at bottom by horizontal passages, which is a
common case, if a chimney of thirty or forty feet high were built
over one of the shafts, or so near the shaft, that the chimney might
communicate with the top of the shaft, all air being excluded but
what should pass up or down by the shaft, a constant change of air
would, by this means, be produced in the passages below, tending to
secure the workmen from those damps, which so frequently incommode
them. For the fresh air would be almost always going down the open
shaft, to go up the chimney, or down the chimney, to go up the shaft.
Let me add one observation more, which is, that if that part of the
funnel of a chimney, which appears above the roof of a house, be
pretty long, and have three of its sides exposed to the heat of the
sun successively, viz. when he is in the east, in the south, and in
the west, while the north side is sheltered by the building from the
cool northerly winds; such a chimney will often be so heated by the
sun, as to continue the draft strongly upwards, through the whole
twenty-four hours, and often for many days together. If the outside
of such a chimney be painted black, the effect will be still greater,
and the current stronger.
No. III.