Essays; Political, Economical, and Philosophical Part 25

It frequently happens that the iron backs of grates are not vertical, or upright, but inclined backwards.--When these grates are so much too wide as to render it necessary to fill them up behind with fire-stone, the inclination of the back will be of little consequence; for by making the piece of stone with which the width of the grate is to be diminished in the form of a wedge, or thicker above than below, the front of this stone, which in effect will become the back of the grate, may be made perfectly vertical; and the iron back of the grate being hid in the solid work of the back of the Fire-place, will produce no effect whatever; but if the grate be already so narrow as not to admit of any diminution of its width, in that case it will be best to take away the iron back of the grate entirely, and fixing the grate firmly in the brick-work, cause the back of the Fire-place to serve as a back to the grate.--This I have very frequently done, and have always found it to answer perfectly well.

Where it is necessary that the fire in a grate should be very small, it will be best, in reducing the grate with fire-stone, to bring its cavity, destined for containing the fuel, to the form of one half of a hollow hemisphere; the two semicircular openings being one above, to receive the coals, and the other in front, or towards the bars of the grate; for when the coals are burnt in such a confined s.p.a.ce, and surrounded on all sides, except in the front and above, by fire-stone, (a substance peculiarly well adapted for confining heat,) the heat of the fire will be concentrated, and the cold air of the atmosphere being kept at a distance, a much smaller quant.i.ty of coals will burn, than could possibly be made to burn in a grate where they would be more exposed to be cooled by the surrounding air, or to have their heat carried off by being in contact with iron, or with any other substance through which heat pa.s.ses with greater facility than through fire-stone.

Being persuaded that if the improvements in Chimney Fire-places here recommended should be generally adopted, (which I cannot help flattering myself will be the case,) that it will become necessary to reduce, very considerably, the sizes of grates, I was desirous of showing how this may, with the greatest safety and facility, be done.

Where grates, which are designed for rooms of a middling size, are longer than 14 or 15 inches, it will always be best, not merely to diminish their lengths, by filling them up at their two ends with fire-stone, but, forming the back of the Chimney of a proper width, without paying any regard to the length of the grate, to carry the covings through the two ends of the grate in such a manner as to conceal them, or at least to conceal the back corners of them in the walls of the covings.

I cannot help flattering myself that the directions here given in regard to the alterations which it may be necessary to make in Fire-places, in order to introduce the improvements proposed, will be found to be so perfectly plain and intelligible that no one who reads them will be at any loss respecting the manner in which the work is to be performed; -- but as order and arrangement tend much to facilitate all mechanical operations, I shall here give a few short directions respecting the manner of LAYING OUT THE WORK, which may be found useful, and particularly to gentlemen who may undertake to be their own architects, in ordering and directing the alterations to be made for the improvement of their Fire-places.

Directions for laying out the Work.

If there be a grate in the Chimney which is to be altered, it will always be best to take it away; and when this is done, the rubbish must be removed, and the hearth swept perfectly clean.

Suppose the annexed figure No. 1. to represent the ground plan of such a Fire-place; A B being the opening of it in front, A C and B D the two sides or covings, and C D the back.

Figure 2. shows the elevation of this Fire-place.

First draw a strait line, with chalk, or with a lead pencil, upon the hearth, from one jamb to the other,--even with the front of the jambs. The dotted line A B, figure 3, may represent this line.

From the middle C of this line, (A B) another line c d, is to be drawn perpendicular to it, across the hearth, to the middle d, of the back of the Chimney.

A person must now stand upright in the Chimney, with his back to the back of the Chimney, and hold a plumb-line to the middle of the upper part of the breast of the Chimney (d, fig. 5,) or where the ca.n.a.l of the Chimney begins to rise perpendicularly;-- taking care to place the line above in such a manner that the plumb may fall on the line c d, draw on the hearth from the middle of the opening of the Chimney in front to the middle of the back, and an a.s.sistant must mark the precise place e, on that line where the plumb falls.

This being done, and the person in the Chimney having quitted his station, four inches are to be set off the line c d, from e, towards d; and the point f, where these four inches end, (which must be marked with chalk, or with a pencil,) will show how far the new back is to be brought forward.

Through f, draw the line g h, parallel to the line A B, and this line g h will show the direction of the new back, or the ground line upon which it is to be built.

The line c f will show the depth of the new Fire-place; and if it should happen that c f is equal to about ONE-THIRD of the line A B; and if the grate can be accommodated to the Fire-place instead of its being necessary to accommodate the Fire-place to the grate, in that case, half the length of the line c f, is to be set off from f on the line g f h, on one side to k, and on the other to i, and the line i k will show the ground line of the fore part of the back of the Chimney.

In all cases where the width of the opening of the Fire-place in front (A B) happens to be not greater, or not more than two or three inches greater than THREE TIMES the width of the new back of the Chimney (i k), this opening may be left, and lines drawn from i to A, and from k to B, will show the width and position of the front of the new covings;--but when the opening of the Fire-place in front is still wider, it must be reduced; which is to be done in the following manner:

From c, the middle of the line A B, c a and c b, must be set off equal to the width of the back (i k), added to half its width (f i), and lines drawn from i to a, and from k to b, will show the ground plan of the fronts of the new covings.

When this is done, nothing more will be necessary than to build up the back and covings; and if the Fire-place is designed for burning coals, to fix the grate in its proper place, according to the directions already given.--When the width of the Fire-place is reduced, the edges of the covings a A and b B are to make a finish with the front of the jambs.--And in general it will be best, not only for the sake of the appearance of the Chimney, but for other reasons also, to lower the height of the opening of the Fire-place, whenever its width in front is diminished.

Fig. 4. shows a front view of the Chimney after it has been altered according to the directions here given.--By comparing it with fig. 2. (which shows a front view of the same Chimney before it was altered), the manner in which the opening of the Fire-place in front is diminished may be seen.--In fig. 4. the under part of the door-way by which the Chimney-sweeper gets up the Chimney is represented by white dotted lines. The door-way is represented closed.

I shall finish this chapter with some general observations relative to the subject under consideration; with directions how to proceed where such local circ.u.mstances exist as render modifications of the general plan indispensably necessary.

Whether a Chimney be designed for burning wood upon the hearth, or wood, or coals in a grate, the form of the Fire-place is in my opinion, most perfect when THE WIDTH OF THE BACK is equal to the DEPTH OF THE FIRE-PLACE, and the opening of the Fire-place in front equal to THREE TIMES the width of the back, or, which is the same thing, to THREE TIMES THE DEPTH OF THE FIRE-PLACE.

But if the Chimney be designed for burning wood upon the hearth, upon hand irons, or dogs, as they are called, it will sometimes be necessary to accommodate the width of the back to the length of the wood; and when this is the case, the covings must be accommodated to the width of the back, and the opening of the Chimney in front.

When the wall of the Chimney in front, measured from the upper part of the breast of the Chimney to the front of the mantle, is very thin, it may happen, and especially in Chimnies designed for burning wood upon the hearth, or upon dogs, that the depth of the Chimney, determined according to the directions here given, may be too small.

Thus, for example, supposing the wall of the Chimney in front, from the upper part of the breast of the Chimney to the front of the mantle, to be only four inches, (which is sometimes the case, particularly in rooms situated near the top of a house,) in this case, if we take four inches for the width of the throat, this will give eight inches only for the depth of the Fire-place, which would be too little, even were coals to be burnt instead of wood.--In this case I should increase the depth of the Fire-place at the hearth to 12 or 13 inches, and should build the back perpendicular to the height of the top of the burning fuel, (whether it be wood burnt upon the hearth, or coals in a grate,) and then, sloping the back by a gentle inclination forward, bring it to its proper place, that is to say, PERPENDICULARLY UNDER THE BACK OF THE THROAT OF THE CHIMNEY. This slope, (which will bring the back forward four or five inches, or just as much as the depth of the Fire-place is encreased,) though it ought not to be too abrupt, yet it ought to be quite finished at the height of eight or ten inches above the fire, otherwise it may perhaps cause the Chimney to smoke; but when it is very near the fire, the heat of the fire will enable the current of rising smoke to overcome the obstacle which this slope will oppose to its ascent, which it could not do so easily were the slope situated at a greater distance from the burning fuel[2].

Fig. 7, 8, and 9, show a plan, elevation, and section of a Fire-place constructed or altered upon this princ.i.p.al.--The wall of the Chimney in front at a, fig. 9, being only four inches thick, four inches more added to it for the width of the throat would have left the depth of the Fire-place measured upon the hearth b c only eight inches, which would have been too little;--a niche c and e, was therefore made in the new back of the Fire-place for receiving the grate, which niche was six inches deep in the center of it, below 13 inches wide, (or equal in width to the grate,) and 23 inches high; finis.h.i.+ng above with a semicirular arch, which, in its highest part, rose seven inches above the upper part of the grate.--The door-way for the Chimney-sweeper, which begins just above the top of the niche, may be seen distinctly in both the figures 8 and 9.--The s.p.a.ce marked g, fig. 9, behind this door-way, may either be filled with loose bricks, or may be left void.--The manner in which the piece of stone f, fig. 9, which is put under the mantle of the Chimney to reduce the height of the opening of the Fire-place, is rounded off on the inside in order to give a fair run to the column of smoke in its ascent through the throat of the Chimney, is clearly expressed in this figure.

The plan fig. 7, and elevation fig. 8, show how much the width of the opening of the Fire-place in front is diminished, and how the covings in the new Fire-place are formed.

A perfect idea of the form and dimension of the Fire-place in its original state, as also after its alteration, may be had by careful inspection of these figures.

I have added the drawing fig. 10, merely to show how a fault, which I have found workmen in general whom I have employed in altering Fire-places are very apt to commit, is to be avoided.

--In Chimneys like that represented in this figure, where the jambs A and B project far into the room, and where the front edge of the marble slab, o which forms the coving, does not come so far forward as the front of the jambs, the workmen in constructing the new covings are very apt to place them,--not in the line c A, which they ought to do,--but in the line c o, which is a great fault.--The covings of a Chimney should never range BEHIND the front of the jambs, however those jambs may project into the room;--but it is not absolutely necessary that the covings should MAKE A FINISH with the internal front corners of the jambs, or that they should be continues from the back c, quite to the front of the jambs at A.--They may finish in front at a and b, and small corners A, o, a, may be left for placing the shovels, tongs, etc.

Were the new coving to range with the front edge of the old coving o, the obliquity of the new coving would commonly be too great;--or the angle d c o would exceed 135 degrees, WHICH IT NEVER SHOULD DO,--or at least never by more than a very few degrees.

No inconvenience of any importance will arise from making the obliquity of the covings LESS than what is here recommended; but many cannot fail to be produced by making it much greater;-- and as I know from experience that workmen are very apt to do this, I have thought it necessary to warn them particularly against it.

Fig. 11. shows how the width and obliquity of the covings of a Chimney are to be accommodated to the width of the back, and to the opening in front and depth of the Fire-place, where the width of the opening of the Fire-place is less than three times the width of the new back. As all those who may be employed in altering Chimneys may not, perhaps, known how to set off an angle of any certain numbers of degrees,--or may not have at hand the instruments necessary for doing it,--I shall here show how an instrument may be made which will be found to be very useful in laying out the work for the bricklayers.

Upon a board about 18 inches wide and four feet long, or upon the floor or a table, draw three equal squares A, B, C, fig. 12. of about 12 or 14 inches each side, placed in a strait line, and touching each other.--From the back corner c of the center square B, draw a diagonal line across the square A, to its outward front corner f, and the adjoining angle formed by the lines d c and c f will be equal to 135 degrees,--the angle which the plane of the back of a Chimney Fire-place ought to make with the plane of its covings.--And a bevel m, n, being made to this angle with thin slips of hard wood, this little instrument will be found to be very useful in marking out on the hearth, with chalk, the plans of the walls which are to form the covings of Fire-places.

As Chimneys which are apt to smoke will require the covings to be placed less obliquely in respect to the back than others which have not that defect, it would be convenient to be provided with several bevels;--three or four, for instance, forming different angles.--That already described, which may be called No. 1. will measure the obliquity of the covings when the Fire-place can be made of the most perfect form:--another No. 2. may be made to a smaller angle, d c e,--and another No. 3. for Chimnies which are very apt to smoke at the still smaller angle d c i.--Or a bevel may be so contrived, by means of a joint, and an arch, properly graduated, as to serve for all the different degrees of obliquity which it may ever be necessary to give to the covings of Fire-places.

Another point of much importance, and particularly in Chimneys which are apt to smoke, is to form the throat of the Chimney properly, by carrying up the back and covings to a proper height. This, workmen are apt to neglect to do, probably on account of the difficulty they find in working where the opening of the ca.n.a.l of the Chimney is so much reduced.--But it is absolutely necessary that these walls should be carried up five or six inches at least above the upper part of the breast of the Chimney, or to that point where the wall which forms the front of the throat begins to rise perpendicularly. --If the workman has intelligence enough to avail himself of the opening which is formed in the back of the Fire-place to give a pa.s.sage to the Chimney-sweeper, he will find little difficulty in finis.h.i.+ng his work in a proper manner.

In placing the plumb-line against the breast of the Chimney, in order to ascertain how far the new back is to be brought forward, great care must be taken to place it at the very top of the breast, where the ca.n.a.l of the Chimney BEGINS TO RISE PERPENDICULARLY; otherwise, when the plumb-line is placed too low, or against the slope of the breast, when the new back comes to be raised to its proper height, the throat of the Chimney will found to be too narrow.

Sometimes, and indeed very often the top of the breast of a Chimney lies very high, or far above the fire (see the figures 13 and 14, where d shows the top of the breast of the Chimney); when this is the case it must be brought lower, otherwise the Chimney will be very apt to smoke.--So much has been said in the First Chapter of this Essay of the advantages to be derived from bringing the throat of a Chimney near to the burning fuel, that I do not think it necessary to enlarge on them in this place,-- taking it for granted that the utility and necessity of that arrangement have already been made sufficiently evident;-- but a few directions for workmen, to show them how the breast (and consequently the throat) of a Chimney can most readily be lowered, may not be superfluous.

Where the too great height of the breast of a Chimney is owing to the great height of the mantle, (see fig. 13,) or, which is the same thing, of the opening of the Fire-place in front, which will commonly be found to be the case; the only remedy for the evil will be to bring down the mantle lower;--or rather, to make the opening of the Fire-place in front lower, by throwing across the top of this opening, from one jamb to the other, and immediately under the mantle, a very flat arch;--a wall of bricks and mortar, supported on straight bars of iron;--or a piece of stone (h, fig. 13).--When this is done, the slope of the old throat of the Chimney, or of the back side of the mantle, is to be filled up with plaster, so as to form one continued flat, vertical, or upright plane surface with the lower part of the wall of the ca.n.a.l of the Chimney, and a new breast is to be formed lower down, care being taken to round it off properly, and make it finish at the lower surface of the new wall built under the mantle;--which wall forms in fact a new mantle.

The annexed drawing fig. 13, which represents the section of a Chimney in which the breast has been lowered according to the method here described, will show these various alterations in a clear and satisfactory manner. In this figure, as well as in most of the others in this Essay, the old walls are distinguished from the new ones by the manner in which they are shaded;-- the old walls being shaded by diagonal lines, and the new ones by vertical lines. The additions, which are formed of plaster, are shaded by dots instead of lines.

Where the too great height of the breast of a Chimney is occasioned, not by the height of the mantle, but by the too great width of the breast, in that case, (which however will seldom be found to occur,) this defect may be remedied by covering the lower part of the breast with a thick coating of plaster, supported, if necessary, by nails or studs, driven into the wall which forms the breast, and properly rounded off at the lower part of the mantle.--See fig. 14.

CHAPTER. III.

Of the cause of the ascent of smoke.

Ill.u.s.tration of the subject by familiar comparisons and experiments.

Of chimnies which affect and cause each other to smoke.

Of chimnies which smoke from want of air.

Of the eddies of wind which sometimes blow down chimnies, and cause them to smoke.

Explanation of the figures.

Though it was my wish to avoid all abstruse philosophical investigations in this Essay, yet I feel that it is necessary to say a few words upon a subject generally considered as difficult to be explained, which is too intimately connected with the matter under consideration to be pa.s.sed over in silence.-- A knowledge of the cause of the ascent of Smoke being indispensably necessary to those who engage in the improvement of Fire-places, or who are desirous of forming just ideas relative to the operations of fire, and the management of heat, I shall devote a few pages to the investigation of that curious and interesting subject.--And as many of those who may derive advantage from these inquiries are not much accustomed to philosophical disquisitions, and would not readily comprehend either the language or the diagrams commonly used by scientific writers to explain the phaenomena in question, I shall take pains to express myself in the most familiar manner, and to use such comparisons for ill.u.s.tration as may easily be understood.

If small leaden bullets, or large goose shot, be mixed with peas, and the whole well shaken in a bushel, the shot will separate from the peas, and will take its place at the bottom of the bushel; forcing by its greater weight the peas which are lighter, to move upwards, contrary to their natural tendency, and take their places above.

If water and linseed oil, which is lighter than water, be mixed in a vessel by shaking them together, upon suffering this mixture to remain quite, the water will descend and occupy the bottom of the vessel, and the oil, being forced out of its place by the greater pressure downwards of the heavier liquid, will be obliged to rise and swim on the surface of the water.

If a bottle containing linseed oil be plunged in water with its mouth upwards, and open, the oil will ascent out of the bottle, and pa.s.sing upwards through the ma.s.s of water, in a continued stream, will spread itself over its surface.

In like manner when two fluids of any kind, of different densities, come into contact, or are mixed with each other, that which is the lightest will be forced upwards by that which is the heaviest.