Essays; Political, Economical, and Philosophical Part 23

You will hear with pleasure that your mode of altering Chimnies, so as to prevent their smoking, to save fuel, and to augment heat, has answered not only with me, but with many of my friends who have tried it; and that the Lord Provest and Magistrates of Edinburgh have voted a sum of money to defray the expences of a bricklayer, who is to be sent there for the purpose of establis.h.i.+ng the same plan in that city. I hope that you will have the goodness to expedite your paper upon the management of Heat, that the knowledge of so useful an art may be as rapidly and as extensively diffused as possible.--With my best wishes for your success in the various important pursuits in which you are now engaged, believe me, with great truth and regard, Your faithful and obedient servant John Sinclair Whitehall, London, 9th February 1796.

CHAPTER. I.

Fire-places for burning coals, or wood, in an open chimney, are capable of great improvement.

Smoking chimnies may in all cases be completely cured.

The immoderate size of the throats of chimnies the princ.i.p.al cause of all their imperfections.

Philosophical investigation of the subject.

Remedies proposed for all the defects that have been discovered in chimnies and their open fire-places.

These remedies applicable to chimnies destined for burning wood, or turf, as well as those constructed for burning coals.

The plague of a smoking Chimney is proverbial; but there are many other very great defects in open Fire-places, as they are now commonly constructed in this country, and indeed throughout Europe, which, being less obvious, are seldom attended to; and there are some of them very fatal in their consequences to health; and, I am persuaded, cost the lives of thousands every year in this island.

Those cold and chilling draughts of air on one side of the body, while the other side is scorched by a Chimney Fire, which every one who reads this must often have felt, cannot but be highly detrimental to health; and in weak and delicate const.i.tutions must often produce the most fatal effects.--I have not a doubt in my own mind that thousands die in this country every year of consumptions occasioned solely by this cause.--By a cause which might be so easily removed!--by a cause whose removal would tend to promote comfort and convenience in so many ways.

Strongly impressed as my mind is with the importance of this subject, it is not possible for me to remain silent.--The subject is too nearly connected with many of the most essential enjoyments of life not to be highly interesting to all those who feel pleasure in promoting, or in contemplating the comfort and happiness of mankind.--And without suffering myself to be deterred, either by the fear of being thought to give the subject a degree of importance to which it is not ent.i.tled, or by the apprehension of being tiresome to my readers by the prolixity of my descriptions,--I shall proceed to investigate the subject in all its parts and details with the utmost care and attention.

--And first with regard to smoking Chimnies:

There are various causes by which Chimnies may be prevented from carrying smoke; but there are none that may not easily be discovered and completely removed.--This will doubtless be considered as a bold a.s.sertion; but I trust I shall be able to make it appear in a manner perfectly satisfactory to my readers that I have not ventured to give this opinion but upon good and sufficient grounds.

Those who will take the trouble to consider the nature and properties of elastic fluids,--of air,--smoke,--and vapour,-- and to examine the laws of their motions, and the necessary consequences of their being rarified by heat, will perceive that it would be as much a miracle if smoke should not rise in a Chimney, (all hindrances to its ascent being removed,) as that water should refuse to run in a syphon, or to descend in a river.

The whole mystery, therefore, of curing smoking Chimnies is comprised in this simple direction. --FIND OUT AND REMOVE THOSE LOCAL HINDRANCES WHICH FORCIBLY PREVENT THE SMOKE FROM FOLLOWING ITS NATURAL TENDENCY TO GO UP THE CHIMNEY; or rather, to speak more accurately, which prevents its being forced up the Chimney by the pressure of the heavier air of the room.

Although the causes, by which the ascent of smoke in a Chimney MAY BE obstructed, are various, yet that cause which will most commonly, and I may say almost universally be found to operate, is one which it is always very easy to discover, and as easy to remove,--the bad construction of the Chimney IN THE NEIGHBOURHOOD OF THE FIRE-PLACE.

In the course all my experience and practice in curing smoking Chimnies,--and I certainly have not had less than five hundred under my hands, and among them many which were thought to be quite incurable,--I have never been obliged, except in one single instance, to have recourse to any other method of cure than merely reducing the Fire-place and the throat of the Chimney, or that part of it which lies immediately above the Fire-place, to a proper form, and just dimensions.

That my principles for constructing Fire-places are equally applicable to those which are designed for burning coal, as to those in which wood is burnt, has lately been abundantly proved by experiments made here in London; for of above an hundred and fifty Fire-places which have been altered in this city, under my direction, within these last two months, there is not one which has not answered perfectly well[1].--And by several experiments which have been made with great care, and with the a.s.sistance of thermometers, it has been demonstrated, that the saving of fuel, arising from these improvements of Fire-places, amounts in all cases to more than HALF, and in many cases to more than TWO THIRDS of the quant.i.ty formerly consumed.--Now as the alterations in Fire-places which are necessary may be made at a very trifling expence, as any kind of grate or stove may be made use of, and as no iron work, but merely a few bricks and some mortar, or a few small pieces of fire-stone, are required; the improvement in question is very important, when considered merely with a view to economy; but it should be remembered, that not only a great saving is made of fuel by the alterations proposed, but that rooms are made much more comfortable, and more salubrious;-- that they may be more equally warmed, and more easily kept at any required temperature;--that all draughts of cold air from the doors and windows towards the Fire-place, which are so fatal to delicate const.i.tutions, will be completely prevented;--that in consequence of the air being equally warm all over the room, or in all parts of it, it may be entirely changed with the greatest facility, and the room completely ventilated, when this air is become unfit for respiration, and this merely by throwing open for a moment a door opening into some pa.s.sage from whence fresh air may be had, and the upper part of a window; or by opening the upper part of on window and the lower part of another, and as the operation of ventilating the room, even when it is done in the most complete manner, will never require the door and window to be open more than one minute; in this short time the walls of the room will not be sensibly cooled, and the fresh air which comes into the room will, in a very few minutes, be so completely warmed by these walls that the temperature of the room, though the air in it be perfectly changed, will be brought to be very nearly the same as it was before the ventilation.

Those who are acquainted with the principles of pneumatics, and know why the warm air in a room rushes out at an opening made for it at the top of a window when colder air from without is permitted to enter by the door, or by any other opening situated lower than the first, will see, that it would be quite impossible to ventilate a room in the complete and expeditious manner here described, where the air in a room is partially warmed, or hardly warmed at all, and where the walls of the room, remote from the fire, are constantly cold; which must always be the case where, in consequence of a strong current up the Chimney, streams of cold air are continually coming in through all the crevices of the doors and windows, and flowing into the Fire-place.

But although rooms, furnished with Fire-places constructed upon the principles here recommended, may be easily and most effectually ventilated, (and this is certainly a circ.u.mstance in favour of the proposed improvements,) yet such total ventilations will very seldom, if ever, be necessary.--As long as ANY FIRE is kept up in the room, there is so considerable a current of air up the Chimney, notwithstanding all the reduction that can be made in the size of its throat, that the continual change of air in the room which this current occasions will, generally, be found to be quite sufficient for keeping the air in the room sweet and wholesome; and indeed in rooms in which there is no open Fire-place, and consequently no current of air from the room setting up the Chimney, which is the case in Germany, and all the northern parts of Europe, where rooms are heated by stoves, whose Fire-places opening without are not supplied with the air necessary for the combustion of the fuel from the room;--and although in most of the rooms abroad, which are so heated, the windows and doors are double, and both are closed in the most exact manner possible, by slips of paper pasted over the crevices, or by slips of list or furr; yet when these rooms are tolerably large, and when they are not very much crowded by company, nor filled with a great many burning lamps or candles, the air in them is seldom so much injured as to become oppressive or unwholesome; and those who inhabit them show by their ruddy countenances, as well as by every other sign of perfect health, that they suffer no inconvenience whatever from their closeness.--There is frequently, it is true, an oppressiveness in the air of a room heated by a German stove, of which those who are not much accustomed to living in those rooms seldom fail to complain, and indeed with much reason; but this oppressiveness does not arise from the air of the room being injured by the respiration and perspiration of those who inhabit it;--it arises from a very different cause;-- from a fault in the construction of German stoves in general, but which may be easily and most completely remedied, as I shall show more fully in another place. In the mean time, I would just observe here with regard to these stoves, that as they are often made of iron, and as this metal is a very good conductor of heat, some part of the stove in contact with the air of the room becomes so hot as to calcine or rather to ROAST the dust which lights upon it; which never can fail to produce a very disagreeable effect on the air of the room. And even when the stove is constructed of pantiles or pottery-ware, if any part of it in contact with the air of the room is suffered to become very hot, which seldom fails to be the case in German stoves constructed on the common principles, nearly the same effects will be found to be produced on the air as when the stove is made of iron, as I have very frequently had occasion to observe.

Though a room be closed in the most perfect manner possible, yet, as the quant.i.ty of air injured and rendered unfit for further use by the respiration of two or three persons in a few hours is very small, compared to the immense volume of air which a room of a moderate size contains; and as a large quant.i.ty of fresh air always enters the room, and an equal quant.i.ty of the warm air of the room is driven out of it every time the door is opened, there is much less danger of the air of a room becoming unwholesome for the want of ventilation than has been generally imagined; particularly in cold weather, when all the different causes which conspire to change the air of warmed rooms act with increased power and effect.

Those who have any doubts respecting the very great change of air or ventilation which takes place each time the door of a warm room is opened in cold weather, need only set the door of such a room wide open for a moment, and hold two lighted candles in the door-way, one near the top of the door, and the other near the bottom of it; the violence with which the flame of that above will be driven outwards, and that below inwards, by the two strong currents of air which, pa.s.sing in opposite directions, rush in and out of the room at the same time, will be convinced that the change of air which actually takes place must be very considerable indeed; and these currents will be stronger, and consequently the change of air greater, in proportion as the difference is greater between the temperature of the air within the room and of that without. I have been more particular upon this subject,--the ventilation of warmed rooms which are constantly inhabited,--as I know that people in general in this country have great apprehensions of the bad consequences to health of living rooms in which there is not a continual influx of cold air from without. I am as much an advocate for a FREE CIRCULATION of air as any body, and always sleep in a bed without curtains on that account; but I am much inclined to think, that the currents of cold air which never fail to be produced in rooms heated by Fire-places constructed upon the common principle,-- those partial heats on one side of the body, and the cold blasts on the other, so often felt in houses in this country, are infinitely more detrimental to health than the supposed closeness of the air in a room warmed more equally, and by a smaller fire.

All these advantages, attending the introduction of the improvements in Fire-places here recommended, are certainly important, and I do not know that they are counterbalanced by any one disadvantage whatsoever. The only complaints that I had ever heard made against them was, that they made the rooms TOO warm; but the remedy to this evil is so perfectly simple and obvious, that I should be almost afraid to mention it, less it might be considered as an insult to the understanding of the persons to whom such information should be given; for nothing surely can be conceived more perfectly ridiculous than the embarra.s.sment of a person on account of the too great heat of his room, when it is in his power to diminish AT PLEASURE the fire by which it is warmed; and yet, strange as it may appear, this has sometimes happened!

Before I proceed to give directions for the construction of Fire-places, it will be proper to examine more carefully the Fire-places now in common use;--to point out their faults;-- and to establish the principles upon which Fire-places ought to be constructed.

The great fault of all the open Fire-places, or Chimnies, for burning wood or coals in an open fire, now in common use, is, that they are much too large; or rather it is THE THROAT OF THE CHIMNEY or the lower part of its open ca.n.a.l, in the neighbourhood of the mantle, and immediately over the fire, which is too large.

This opening has. .h.i.therto been left larger than otherwise it probably would have been made, in order to give a pa.s.sage to the Chimney-sweeper; but I shall show hereafter how a pa.s.sage for the Chimney-sweeper may be contrived without leaving the throat of the Chimney of such enormous dimensions as to swallow up and devour all the warm air of the room, instead of merely giving a pa.s.sage to the smoke and heated vapour which rise from the fire, for which last purpose alone it ought to be destined.

Were it my intention to treat my subject in a formal scientific manner, it would be doubtless be proper, and even necessary, to begin by explaining in the fullest manner, and upon the principles founded on the laws of nature, relative to the motions of elastic fluids, as far as they have been discovered and demonstrated, the causes of the ascent of smoke, and also to explain and ill.u.s.trate upon the same principles, and even to measure, or estimate by calculations, the precise effects of all those mechanical aids which may be proposed for a.s.sisting it in its ascent, or rather for removing those obstacles which hinder its motion upwards;--but as it is my wish rather to write an useful practical treatise, than a learned dissertation, being more desirous to contribute in diffusing useful knowledge, by which the comforts and enjoyments of mankind may be increased, than to acquire the reputation of a philosopher among learned men, I shall endeavour to write in such a manner as to be easily understood BY THOSE WHO ARE MOST LIKELY TO PROFIT BY THE INFORMATION I HAVE TO COMMUNICATE, and consequently most likely to a.s.sist in bringing into general use the improvements I recommend. This being premised, I shall proceed, without any further preface or introduction, to the investigation of the subject I have undertaken to treat.

As the immoderate size of the throats of Chimnies is the great fault of their construction, it is this fault which ought always to be first attended to in every attempt which is made to improve them; for however perfect the construction of a Fire-place may be in other respects, if the opening left for the pa.s.sage of the smoke is larger than is necessary for that purpose, nothing can prevent the warm air of the room from escaping through it; and whenever this happens, there is not only an unnecessary loss of heat, but the warm air which leaves the room to go up the Chimney being replaced by cold air from without, the draughts of cold air, so often mentioned, cannot fail to be produced in the room, to the great annoyance of those who inhabit it. But although both these evils may be effectually remedied by reducing the throat of the Chimney to a proper size, yet in doing this several precautions will be necessary. And first of all, the throat of the Chimney should be in its proper place; that is to say, in that place in which it ought to be, in order that the ascent of the smoke may be most facilitated; for every means which can be employed for facilitating the ascent of the smoke in the Chimney must naturally tend to prevent the Chimney from smoking: now as the smoke and hot vapour which rise from a fire naturally tend UPWARDS, the proper place for the throat of the Chimney is evidently perpendicularly OVER THE FIRE.

But there is another circ.u.mstance to be attended to in determining the proper place for the throat of a Chimney, and that is, to ascertain its distance from the fire, or HOW FAR above the burning fuel it ought to be placed. In determining this point, there are many things to be considered, and several advantages and disadvantages to be weighed and balanced.

As the smoke and vapour which ascend from burning fuel rise in consequence of their being rarefied by heat, and made lighter than the air of the surrounding atmosphere; and as the degree of their rarefaction, and consequently their tendency to rise, is in proportion to the intensity of their heat; and further, as they are hotter near the fire than at a greater distance from it, it is clear that the nearer the throat of a Chimney is to the fire, the stronger will be, what is commonly called, its DRAUGHT, and the less danger there will be of its smoking. But on the other hand, when the draught of a Chimney is very strong, and particularly when this strong draught is occasioned by the throat of the Chimney being very near the fire, it may so happen that the draught of air into the fire may become so strong, as to cause the fuel to be consumed too rapidly. There are likewise several other inconveniences which would attend the placing of the throat of a Chimney VERY NEAR the burning fuel.

In introducing the improvements proposed, in Chimnies already built, there can be no question in regard to the height of the throat of the Chimney, for its place will be determined by the height of the mantle. It can hardly be made lower than the mantle; and it ought always to be brought down as nearly upon the level with the bottom of it as possible. If the Chimney is apt to smoke, it will sometimes be necessary either to lower the mantle or to diminish the height of the opening of the Fire-place, by throwing over a flat arch, or putting in a straight piece of stone from one side of it to the other, or, which will be still more simple and easy in practice, building a wall of bricks, supported by a flat bar of iron, immediately under the mantle.

Nothing is so effectual to prevent Chimnies from smoking as diminis.h.i.+ng the opening of the Fire-place in the manner here described, and lowering and diminis.h.i.+ng the throat of the Chimney; and I have always found, except in the single instance already mentioned, that a perfect cure may be effected by THESE MEANS ALONE, even in the most desperate cases. It is true, that when the construction of the Chimney is very bad indeed, or its situation very unfavourable to the ascent of the smoke, and especially when both these disadvantages exist at the same time, it may sometimes be necessary to diminish the opening of the Fire-place, and particularly to lower it, and also to lower the throat of the Chimney, more than might be wished: but still I think this can produce no inconveniences to be compared with that greatest of all plagues, a smoking Chimney.

The position of the throat of a Chimney being determined, the next points to be ascertained are its size and form, and the manner in which it ought to be connected with the Fire-place below, and with the open ca.n.a.l of the Chimney above.

But as these investigations are intimately connected with those which relate to the form proper to be given to the Fire-place itself, we must consider them all together.

That these inquiries may be pursued with due method, and that the conclusions drawn from them may be clear and satisfactory, it will be necessary to consider, first, what the objects are which ought princ.i.p.ally to be had in view in the construction of a Fire-place; and secondly, to see how these objects can best be attained.

Now the design of a Chimney Fire being simply to warm a room, it is necessary, first of all, to contrive matters so that the room shall be actually warmed; secondly, that it be warmed with the smallest expence of fuel possible; and, thirdly, that in warming it, the air of the room be preserved perfectly pure, and fit for respiration, and free from smoke and all disagreeable smells.

In order to take measures with certainty for warming a room by means of an open Chimney Fire, it will be necessary to consider HOW, or in WHAT MANNER, such a Fire communicates heat to a room.

This question may perhaps, at the first view of it, appear to be superfluous and trifling, but a more careful examination of the matter will show it to be highly deserving of the most attentive investigation.

To determine in what manner a room is heated by an open Chimney Fire, it will be necessary first of all to find out, UNDER WHAT FORM the heat generated in the combustion of the fuel exists, and then to see how it is communicated to those bodies which are heated by it.

In regard to the first of these subjects of inquiry, it is quite certain that the heat which is generated in the combustion of the fuel exists under TWO perfectly distinct and very different forms.

One part of it is COMBINED with the smoke, vapour, and heated air which rise from the burning fuel, and goes off with them into the upper regions of the atmosphere; while the other part, which appears to be UNCOMBINED, or, as some ingenious philosophers have supposed, combined only with light, is sent off from the fire in rays in all possible directions.

With respect to the second subject of inquiry; namely, how this heat, existing under these two different forms, is communicated to other bodies; it is highly probable that the combined heat can only be communicated to other bodies by ACTUAL CONTACT with the body with which it is combined; and with regard to the rays which are sent off by burning fuel, it is certain that THEY communicate or generate heat only WHEN and WHERE they are stopped or absorbed. In pa.s.sing through air, which is transparent, they certainly do not communicate any heat to it; and it seems highly probable that they do not communicate heat to solid bodies by which they are reflected.

In these respects they seem to bear a great resemblance to the solar rays. But in order not to distract the attention of my reader, or carry him too far away from the subject more immediately under consideration, I must not enter too deeply into these inquiries respecting the nature and properties of what has been called RADIANT HEAT. It is certainly a most curious subject of philosophical investigation, but more time would be required to do it justice than we now have to spare. We must therefore content ourselves with such a partial examination of it as will be sufficient for our present purpose.

A question which naturally presents itself here is.

What proportion does the radiant heat bear to the combined heat?--Though that point has not yet been determined with any considerable degree of precision, it is, however, quite certain, that the quant.i.ty of heat which goes off combined with the smoke, vapour, and heated air is much more considerable, perhaps three of four times greater at least, than that which is sent off from the fire in rays.--And yet, small as the quant.i.ty is of this radiant heat, it is the only part of the heat generated in the combustion of fuel burnt in an open Fire-place which is ever employed, or which can ever be employed, in heating a room.

The whole of the combined heat escapes by the Chimney, and is totally lost; and, indeed, no part of it could ever be brought into a room from an open Fire-place, without bringing along with it the smoke with which it is combined; which, of course, would render it impossible for the room to be inhabited. There is, however, one method by which combining heat, and even that which arises from an open Fire-place, may be made to a.s.sist in warming a room; and that is by making it pa.s.s through something a.n.a.logous to a German stove, placed in the Chimney above the fire.--But of this contrivance I shall take occasion to treat more fully hereafter; in the mean time I shall continue to investigate the properties of open Chimney Fire-places, constructed upon the most simple principles, such as are now in common use; and shall endeavour to point out and explain all those improvements of which THEY appear to me to be capable. When fuel is burnt in Fire-places upon this simple construction, where the smoke escapes immediately by the open ca.n.a.l of the Chimney, it is quite evident that all the combined heat must of necessity be lost; and as it is the radiant heat alone which can be employed in heating a room, it becomes an object of much importance to determine how the greatest quant.i.ty of it may be generated in the combustion of the fuel, and how the greatest proportion possible of that generated may be brought into the room.

Now the quant.i.ty of radiant heat generated in the combustion of a given quant.i.ty of any kind of fuel depends very much upon the management of the fire, or upon the manner in which the fuel is consumed. When the fire burns bright, much radiant heat will be sent off from it; but when it is SMOTHERED UP, very little will be generated; and indeed very little combined heat, that can be employed to any useful purpose: most of the heat produced will be immediately EXPENDED in giving elasticity to a thick dense vapour or smoke which will be seen rising from the fire; -- and the combustion being very incomplete, a great part of the inflammable matter of the fuel being merely rarefied and driven up the Chimney without being inflamed, the fuel will be wasted to little purpose. And hence it appears of how much importance it is, whether it be considered with a view to economy, or to cleanliness, comfort, and elegance, to pay due attention to the management of a Chimney Fire.

Nothing can be more perfectly void of common sense, and wasteful and slovenly at the same time, than the manner in which Chimney Fires, and particularly where coals are burned, are commonly managed by servants. They throw on a load of coals at once, through which the flame is hours in making its way; and frequently it is not without much trouble that the fire is prevented from going quite out. During this time no heat is communicated to the room; and what is still worse, the throat of the Chimney being occupied merely by a heavy dense vapour, not possessed of any considerable degree of heat, and consequently not having much elasticity, the warm air of the room finds less difficulty in forcing its way up the Chimney and escaping, than when the fire burns bright;--and it happens not unfrequently, especially in Chimneys and Fire-places ill constructed, that this current of warm air from the room which presses into the Chimney, crossing upon the current of heavy smoke which rises slowly from the fire, obstructs it in its ascent, and beats it back into the room; hence it is that Chimnies so often smoke when too large a quant.i.ty of fresh coals is put upon the fire. So many coals should never be put on the fire at once as to prevent the free pa.s.sage of the flame between them. In short, a fire should never be smothered; and when proper attention is paid to the quant.i.ty of coals put on, there will be very little use for the poker; and this circ.u.mstance will contribute very much to cleanliness, and to the preservation of furniture.

Those who have feeling enough to be made miserable by any thing careless, slovenly, and wasteful which happens under their eyes,--who know what comfort is, and consequence are worthy of the enjoyments of a CLEAN HEARTH and a CHEERFUL FIRE, should really either take the trouble themselves to manage their fires, (which, indeed, would rather be an amus.e.m.e.nt to them than a trouble,) or they should instruct their servants to manage them better.

But to return to the subject more immediately under consideration.

As we have seen what is necessary to the production or generation of radiant heat, it remains to determine how the greatest proportion of that generated and sent off from the fire in all directions may be made to enter the room, and a.s.sist in warming it. How as the rays which are thrown off from burning fuel have this property in common with light, that they generate heat only WHEN and WHERE they are stopped or absorbed, and also in being capable of being reflected WITHOUT GENERATING at the surfaces of various bodies, the knowledge of these properties will enable us to take measures, with the utmost certainty, for producing the effect required,--that is to say, for bringing as much radiant heat as possible into the room.

This must be done, first, by causing as many as possible of the rays, as they are sent off from the fire in straight lines, to come DIRECTLY into the room; which can only be effected by bringing the fire as far forward as possible, and leaving the opening of the Fire-place as wide and as high as can be done without inconveniences; and secondly, by making the sides and back of the Fire-place of such form, and constructing them of such materials, as to cause the direct rays from the fire, which strike against them, to be sent into the room BY REFLECTION in the greatest abundance.

Now it will be found, upon examination, that the best form for the vertical sides of a Fire-place, or the COVINGS, (as they are called,) is that of an upright plane, making an angle with the plane of the back of the Fire-place, of about 135 degrees.-- According to the present construction of Chimnies this angle is 90 degrees, or forms a right angle; but as in this case the two sides or covings of the Fire-place (AC, BD, Fig. 1.) are parallel to each other, it is evident that they are very ill contrived for throwing into the room by reflection the rays from the fire which fall on them.

To have a clear and perfect idea of the alterations I propose in the forms of Fire-places, the reader need only observe, that, whereas the backs of Fire-places, as they are now commonly constructed, are as wide as the opening of the Fire-place in front, and the sides of it are of course perpendicular to it, and parallel to each other,--in the Fire-places I recommend, the back (i k, Fig. 3) is only about one-third of the width of the opening of the Fire-place in front (a,b), and consequently that the two sides of covings of the Fire-place (a i and b k), instead of being perpendicular to the back, are inclined to it at an angle of about 135 degrees; and in consequence of this position, instead of being parallel to each other, each of them presents an oblique front towards the opening of the Chimney, by means of which the rays which they reflect are thrown into the room.

A bare inspection of the annexed drawings (Fig. 1. and Fig. 3.) will render this matter perfectly clear and intelligible.

In regard to the materials which it will be most advantageous to employ in the construction of Fire-places, so much light has, I flatter myself, already been thrown on the subject we are investigating, and the principles adopted have been established on such clear and obvious facts, that no great difficulty will attend the determination of that point.--As the object in view is to bring radiant heat into the room, it is clear that that material is best for the construction of a Fire-place which reflects the most, or which ABSORBS THE LEAST of it; for that heat which is ABSORBED cannot be REFLECTED--Now as bodies which absorb radiant heat are necessarily heated in consequence of that absorption, to discover which of the various materials that can be employed for constructing Fire-places are best adapted for that purpose, we have only to find out by an experiment, very easy to be made, what bodies acquire LEAST HEAT when exposed to the direct rays of a clear fire;--for those which are least heated, evidently absorb the least, and consequently reflect the most radiant heat. And hence it appears that iron, and, in general, metals of all kinds, which are well known to GROW VERY HOT when exposed to the rays projected by burning fuel, are to be reckoned among the VERY WORST materials that it is possible to employ in the construction of Fire-places.