And as heat rarefies all bodies, fluids as well as solids, air as well as water, or mercury,--it follows that two portions of the same fluid, at different temperatures, being brought into contact with each other, that portion which is the hottest being more rarefied or specifically LIGHTER than that which is colder, must be forced upwards by this last.--And this is what always happens in fact.

When hot water and cold water are mixed, the hottest part of the mixture will be found to be at the surface above;--and when cold air is admitted into a warmed room, it will always be found to take its place at the bottom of the room, the warmer air being in part expelled, and in part forced upwards to the top of the room.

Both air and water being transparent and colourless fluids, their internal motions are not easily discovered by the sight, and when these motions are very slow, they make no impression whatever on any of our senses, consequently they cannot be detected by us without the aid of some mechanical contrivance:-- But where we have reason to think that those motions exist, means should be sought, and may often be found, for rendering them perceptible.

If a bottle containing hot water tinged with log-wood, or any other colouring drug, be immersed, with its mouth open, and upwards, into a deep gla.s.s jar filled with cold water, the ascent of the hot water from the bottle through the ma.s.s of cold water will be perfectly visible through the gla.s.s.-- Now nothing can be more evident than that both of these fluids are forced, or PUSHED, and not DRAWN upwards.--Smoke is frequently said to be drawn up the Chimney;--and that a Chimney draws well, or ill;--but these are careless expressions, and lead to very erroneous ideas respecting the cause of the ascent of Smoke; and consequently tend to prevent the progress of improvements in the management of fires.--The experiment just mentioned with the coloured water is very striking and beautiful, and it is well calculated to give a just idea of the cause of the ascent of Smoke. The cold water in the jar, which, in consequence of its superior weight or density, forces the heated and rarefied water in the bottle to give place to it, and to move upwards out of its way, may represent the cold air of the atmosphere, while the rising column of coloured water will represent the column of Smoke which ascends from a fire.

If Smoke required a Chimney to DRAW it upwards, how happens it that Smoke rises from a fire which is made in the open air, where there is no Chimney?

If a tube, open at both ends, and of such a length that its upper end be below the surface of the cold water in the jar, be held vertically over the mouth of the bottle which contains the hot coloured water, the hot water will rise up through it, just a smoke rises in a Chimney.

If the tube be previously heated before it is plunged into the cold water, the ascent of the hot coloured water will be facilitated and accelerated, in like manner as Smoke is known to rise with greater facility in a Chimney which is hot, than in one in which no fire has been made for a long time.--But in neither of these cases can it, with any propriety, be said, that the hot water is DRAWN up the tube.--The hotter the water in the bottle is, and the colder that in the jar, the greater will be the velocity with which the hot water will be forced up through the tube; and the same holds of the ascent of hot Smoke in a Chimney.--When the fire is intense, and the weather very cold, the ascent of the Smoke is very rapid; and under such circ.u.mstances Chimneys seldom smoke.

As the cold water of the jar immediately surrounding the bottle which contains the hot water, will be heated by the bottle while the other parts of the water in the jar will remain cold, this water so heated, becoming specifically lighter than that which surrounds it, will be forced upwards; and if it finds its way into the tube will rise up through it with the coloured hot water.--The warmed air of a room heated by an open Chimney Fire-place has always a tendency to rise, (if I may use that inaccurate expression,) and finding its way into the Chimney frequently goes off with the Smoke.

What has been said, will, I flatter myself, be sufficient to explain and ill.u.s.trate, in a clear and satisfactory manner, the cause of the ascent of Smoke; and just ideas upon that subject are absolutely necessary in order to judge, with certainty, of the merit of any scheme proposed for the improvement of Fire-places; or to take effectual measures, in all cases, for curing smoking Chimnies.--For though the perpetual changes and alterations which are produced by accident, whim, and caprice, do sometimes lead to useful discoveries, yet the progress of improvement under such guidance must be exceedingly slow, fluctuating, and uncertain.

As to the causes of the smoking of Chimnies, they are very numerous, and various; but as a general idea of them may be acquired from what has already been said upon that subject in various parts of this Essay, and as they may, in all cases, (a very few only excepted,) be completely remedied by making the alterations in Fire-places here pointed out; I do not think it necessary to enumerate them all in this place, or to enter into those long details and investigations which would be required to show the precise manner in which each of them operates, either alone, or in conjunction with others.

There is however one cause of smoking Chimnies which I think it is necessary to mention more particularly.--In modern built houses, where the doors and windows are generally made to close with such accuracy that no crevice is left for the pa.s.sage of the air from without, the Chimnies in rooms adjoining to each other, or connected by close pa.s.sages, are frequently found to affect each other, and this is easy to be accounted for.--When there is a fire burning in one of the Chimnies, as the air necessary to supply the current up the Chimney where the fire burns cannot be had in sufficient quant.i.ties from without, through the very small crevices of the doors and windows, the air in the room becomes rarefied, not by heat, but by subtraction of that portion of air which is employed in keeping up the fire, or supporting the combustion of the fuel, and in consequence of this rarefaction, its elasticity is diminished, and being at last overcome by the pressure of the external air of the atmosphere, this external air rushes into the room by the only pa.s.sage left for it, namely, by the open Chimney of the neighbouring room:--And the flow of air into the Fire-place, and up the Chimney where the fire is burning being constant, this expence of air is supplied by a continued current down the other Chimney.

If an attempt be made to light fires in both Chimnies at the same time, it will be found to be very difficult to get the fires to burn, and the rooms will both be filled with Smoke.

One of the fires,--that which is made in the Chimney where the construction of the Fire-place is best adapted to facilitate the ascent of the Smoke,--or if both Fire-places are on the same construction,--that which has the wind most favourable, or in which the fire happens to be soonest kindled,--will overcome the other, and cause its Smoke to be beat back into the room by the cold air which descends through the Chimney.--The most obvious remedy in this case is to provide for the supply of fresh air necessary for keeping up the fires by opening a pa.s.sage for the external air into the room by a shorter road than down one of the Chimnies; and when this is done, both Chimnies will be found to be effectually cured.

But Chimnies so circ.u.mstanced may very frequently be prevented from smoking even without opening any new pa.s.sage for the external air, merely by diminishing the draught, (as it is called,) up the Chimnies; which can best be done by altering both Fire-places upon the principles recommended and fully explained in the foregoing Chapters of this Essay.

Should the doors and windows of a room be closed with so much nicety as to leave no crevices by which a supply of air can enter sufficient for maintaining the fire, AFTER THE CURRENT OF AIR UP THE CHIMNEY HAS BEEN DIMINISHED AS MUCH AS POSSIBLE BY DIMINISHING THE THROAT OF THE FIRE-PLACE; in that case there would be no other way of preventing the Chimney from smoking but by opening a pa.s.sage for the admission of fresh air from without;--but this, I believe, will very seldom be found to be the case.

A case more frequently to be met with is where currents of air set down Chimnies in consequence of a diminution and rarefaction of the air in a room, occasioned by the doors of the room opening into pa.s.sages or courts where the air is rarefied by the action of some particular winds. In such cases the evil may be remedied, either by causing the doors in question to close more accurately,--or, (which will be still more effectual,) by giving a supply of air to the pa.s.sage or court which wants it, by some other way.

Where the top of a Chimney is commanded by high buildings, by clifts, or by high grounds, it will frequently happen, in windy weather, that the eddies formed in the atmosphere by these obstacles will blow down the Chimney, and beat down the smoke into the room.--This it is true will be much less likely to happen when the throat of the Chimney is contracted and properly formed than when it is left quite open, and the Fire-place badly constructed; but as it is POSSIBLE that a Chimney may be so much exposed to these eddies in very high winds as to be made to smoke sometimes when the wind blows with violence from a certain quarter, it is necessary to show how the effects of those eddies may be prevented.

Various mechanical contrivances have been imagined for preventing the wind from blowing down Chimnies, and many of them have been found to be useful;--there are, however, many of these inventions, which, though they prevent the wind from blowing down the Chimney, are so ill-contrived on other accounts as to obstruct the ascent of the Smoke, and do more harm than good.

Of this description are all those Chimney-pots with flat horizontal plates or roofs placed upon supporters just above the opening of the pot;--and most of the caps which turn with the wind are not much better.--One of the most simple contrivances that can be made use of, and which in most cases will be found to answer the purpose intended as well or better than more complicated machinery, is to cover the top of the Chimney with a hollow truncated pyramid or cone, the diameter of which above, or opening for the pa.s.sage of the Smoke, is about 10 or 11 inches.

--This pyramid, or cone, (for either will answer,)--should be of earthen ware, or of cast iron;--its perpendicular height may be equal to the diameter of its opening above, and the diameter of its opening below equal to three times its height.--It should be placed upon the top of the Chimney, and it may be contrived so as to make a handsome finish to the brick-work.--Where several flews come out near each other, or in the same stack of Chimnies, the form of a pyramid will be better than that of a cone for these covers.

The intention of this contrivance is, that the winds and eddies which strike against the oblique surface of these covers may be reflected upwards instead of blowing down the Chimney.-- The invention is by no means new, but it has not hitherto been often put in practice.--As often as I have seen it tried it has been found to be of use; I cannot say, however, that I was ever obliged to have recourse to it, or to any similar contrivance; and if I forbear to enlarge upon the subject of these inventions, it is because I am persuaded that when Chimnies are properly constructed IN THE NEIGHBOURHOOD OF THE FIRE-PLACE little more will be necessary to be done at the top of the Chimney than to leave it open.

I cannot conclude this Essay without again recommending, in the strongest manner, a careful attention to the management of fires in open Chimnies; for not only the quant.i.ty of heat produced on the combustion of fuel depends much on the manner in which the fire is managed, but even of the heat actually generated a very small part only will be saved, or usefully employed, when the fire is made in a careless and slovenly manner.

In lighting a coal fire more wood should be employed than is commonly used, and fewer coals; and as soon as the fire burns bright, and the coals are well lighted, and NOT BEFORE, more coals should be added to increase the fire to its proper size[3].

The enormous waste of fuel in London may be estimated by the vast dark cloud which continually hangs over this great metropolis, and frequently overshadows the whole country, far and wide; for this dense cloud is certainly composed almost entirely of UNCONSUMED COAL, which having stolen wings from the innumerable fires of this great city has escaped by the Chimnies, and continues to sail about in the air, till having lost the heat which gave it volatility, it falls in a dry shower of extremely fine black dust to the ground, obscuring the atmosphere in its descent, and frequently changing the brightest day into more than Egyptian darkness.

I never view from a distance, as I come into town, this black cloud which hangs over London, without wishing to be able to compute the immense number of chaldrons of coals of which it is composed; for could this be ascertained, I am persuaded so striking a fact would awaken the curiosity, and excite the astonishment of all ranks of the inhabitants; and PERHAPS turn their minds to an object of economy to which they have hitherto paid little attention.

Conclusion.

Though the saving of fuel which will result from the improvements in the forms of CHIMNEY FIRE-PLACES here recommended will be very considerable, yet I hope to be able to show in a future Essay, that still greater savings may be made, and more important advantages derived from the introduction of improvements I shall propose in KITCHEN FIRE-PLACES.

I hope likewise to be able to show in an Essay on COTTAGE FIRE-PLACES, which I am now preparing for publication, that THREE QUARTERS, at least, of the fuel which cottagers now consume in cooking their victuals, and in warming their dwellings, may with great ease, and without any expensive apparatus, be saved.

END OF THE FOURTH ESSAY.

EXPLANATION OF THE FIGURES

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Fig. 1.

The plan of a Fire-place on the common construction.

A B, the opening of the Fire-place in front.

C D, the back of the Fire-place.

A C and B D, the covings.

See page 341.

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Fig. 2.

This figure shows the elevation, or front view of a Fire-place on the common construction. See page 341.

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Fig. 3.

This Figure shows how the Fire-place represented by the Fig. 1, is to be altered in order to its being improved.

A B is the opening in front,--C D, the back, and A C and B D, the covings of the Fire-place in its original state.

a b, its opening in front,--i k, its back,--and a i and b k, its covings after it has been altered, e is a point upon the hearth upon which a plum suspended from the middle of the upper part of the breast of the Chimney falls. The situation for the new back is ascertained by taking the line e f equal to four inches.

The new back and covings are represented as being built of bricks;--and the s.p.a.ce between these and the old back and covings as being filled up with rubbish. See page 342.

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Fig. 4.

This Figure represents the elevation or front view of the Fire-place Fig. 3. after it has been altered. The lower part of the door-way left for the Chimney-sweeper is shown in this Figure by white dotted lines. See page 344.

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Fig. 5.

This Figure shows the section of a Chimney Fire-place and of a part of the ca.n.a.l of the Chimney, on the common construction.

a b is the opening in front; b c, the depth of the Fire-place at the hearth; d, the breast of the Chimney.

d e, the throat of the Chimney, and d f, g e, a part of the open ca.n.a.l of the Chimney.

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