rly understood without the assistance of drawings； for the more complete the knowledge of any subject is which can be acquired without drawings； the more easy will it be to understand the drawings when it becomes necessary to have recourse to them。
The bringing forward of the Fire into the room； or rather bringing it nearer to the front of the opening of the Fire…place；and the diminishing of the throat of the Chimney； being two objects principally had in view in the alterations in Fire…places here recommended； it is evident that both these may be attained merely by bringing forward the back of the Chimney。 The only question therefore is； how far it should be brought forward？The answer is short； and easy to be understood；bring it forward as far as possible； without diminishing too much the passage which must be left for the smoke。 Now as this passage； which； in its narrowest part； I have called the THROAT OF THE CHIMNEY； ought； for reasons which are fully explained in the foregoing Chapter； to be immediately； or perpendicularly over the Fire； it is evident that the back of the Chimney must always be built perfectly upright。To determine therefore the place for the new back； or how far precisely it ought to be brought forward； nothing more is necessary than to ascertain how wide the throat of the Chimney ought to be left； or what space must be left； between the top of the breast of the Chimney； where the upright canal of the Chimney begins； and the new back of the Fire…place carried up perpendicularly to that height。
In the course of my numerous experiments upon Chimnies； I have taken much pains to determine the width proper to be given to this passage； and I have found； that； when the back of the Fire…place is of a proper width； the best width for the throat of a Chimney； when the Chimney and the Fire…place are at the usual form and size； is FOUR INCHES。Three inches might sometimes answer； especially where the Fire…place is very small； and the Chimney good； and well situated： but as it is always of much importance to prevent those accidental puffs of smoke which are sometimes thrown into rooms by the carelessness of servants in putting on suddenly too many coals at once upon the fire； and as I found these accidents sometimes happened when the throats of Chimneys were made very narrow； I found that； upon the whole； all circumstances being well considered； and advantages and disadvantages compared and balanced； FOUR INCHES is the best width that can be given to the throat of a chimney； and this； whether the Fire…place be destined to burn wood； coals； turf； or any other fuel commonly used for heating rooms by an open fire。
In Fire…places destined for heating very large halls； and where very great fires are kept up； the throat of the Chimney may； if it should be thought necessary； be made four inches and an half； or five inches wide；but I have frequently made Fire…places for halls which have answered perfectly well where the throats of the Chimnies have not been wider than four inches。
It may perhaps appear extraordinary； upon the first view of the matter； that Fire…places of such different sizes should all require the throat of the Chimney to be of the same width； but when it is considered that the CAPACITY of the throat of a Chimney does not depend on its width alone； but on its width and LENGTH taken together； and that in large Fire…places； the width of the back； and consequently the length of the throat of the Chimney； is greater than in those which are smaller； this difficulty vanishes。
And this leads us to consider another important point respecting open Fire…places； and that is； the width which it will； in each case； be proper to give to the back。In Fire…places as they are now commonly constructed； the back is of equal width with the opening of the Fire…place in front；but this construction is faulty on two accounts。First； in a Fire…place； so constructed； the sides of the Fire…place； or COVINGS； as they are called； are parallel to each other； and consequently ill…contrived to throw out into the room the heat they receive from the fire in the form of rays；and secondly； the large open corners which are formed by making the back as wide as the opening of the Fire…place in front occasion eddies of wind； which frequently disturb the fire； and embarrass the smoke in its ascent in such a manner as often to bring it into the room。Both these defects may be entirely remedied by diminishing the width of the back of the Fire…place。 The width which； in most cases； it will be best to give it； is ONE THIRD of the width of the opening of the Fire…place in front。But it is not absolutely necessary to conform rigorously to this decision； nor will it always be possible。It will frequently happen that the back of a Chimney must be made wider than； according to the rule here given； it ought to be。This may be； either to accommodate the Fire…place to a stove； which being already on hand； must； to avoid the expense of purchasing a new one； be employed； or for other reasons；and any small deviation from the general rule will be attended with no considerable inconvenience。It will always be best； however； to conform to it as far as circumstances will allow。
Where a Chimney is designed for warming a room of a middling size； and where the thickness of the wall of the Chimney in front； measured from the front of the mantle to the breast of the Chimney； is nine inches； I should set off four inches more for the width of the throat of the Chimney； which； supposing the back of the Chimney to be built upright； as it always ought to be； will give thirteen inches for the depth of the Fire…place； measured upon the hearth； from the opening of the Fire…place in front； to the back。In this case thirteen inches would be a good size for the width of the back； and three times thirteen inches； or thirty…nine inches； for the width of the opening of the Fire…place in front； and the angle made by the back of the Fire…place and the sides of it； or covings； would be just 135 degrees； which is the best position they can have for throwing heat into the room。
But I will suppose that in altering such a Chimney it is found necessary； in order to accommodate the Fire…place to a grate or stove already on hand； to make the Fire…place sixteen inches wide。 In that case； I should merely increase the width of the back； to the dimensions required； without altering the depth of the Chimney； or increasing the width of the opening of the Chimney in front。 The covings； it is true； would be somewhat reduced in their width； by this alteration； and their position with respect to the plane of the back of the Chimney would be a little changed； but these alterations would produce no bad effects of any considerable consequence； and would be much less likely to injure the Fire…place； than an attempt to bring the proportions of its parts nearer to the standard； by increasing the depth of the Chimney； and the width of its opening in front；or than an attempt to preserve that particular obliquity of the covings which is recommended as the best； （135 degrees；） by increasing the width of the opening of the Fire…place； without increasing its depth。
In order to illustrate this subject more fully； we will suppose one case more。We will suppose that in the Chimney which is to be altered； the width of the Fire…place in front is either wider or narrower than it ought to be； in order that the different parts of the Fire…place； after it is altered； may be of the proper dimensions。 In this case； I should determine the depth of the Fire…place； and the width of the back of it； without any regard to the width of the opening of the Fire…place in front； and when this is done； if the opening of Fire…place should be only two or three inches too wide； that is to say； only two or three inches wider than is necessary in order that the covings may be brought into their proper position with respect to the back； I should not alter the width of this opening； but should accommodate the covings to this width； by increasing their breadth； and increasing the angle they make with the back of the Fire…place； but if the opening of the Fire…place should be more than three inches too wide； I should reduce it to the proper width by slips of stone； or by bricks and mortar。
When the width of the opening of the Fire…place； in front； is very great； compared with the depth of the Fire…place； and with the width of the back； the covings in that case being very wide； and consequently very oblique； and the Fire…place very shallow； any sudden motion of the air in front of the Fire…place； （that motion； for instance； which would be occasioned by the clothes of a woman passing hastily before the fire； and very near it；） would be apt to cause eddies in the air； WITHIN THE OPENING OF THE FIRE…PLACE； by which puffs of smoke might easily be brought into the room。 Should the opening of the Chimney be too narrow； which however will very seldom be found to be the case； it will； in general； be advisable to let it remain as it is； and to accommodate the covings to it； rather to attempt to increase its width； which would be attended with a good deal of trouble； and probably a considerable expence。
From all that has been said it is evident； that the points of the greatest importance； and which ought most particularly to be attended to； in altering Fire…places upon the principles here recommended； are； the bringing forward the back to its proper place； and making it of a proper width。But it is time that I should mention another matter upon which it is probable that my reader is already impatient to receive information。Provision must be made for the passage of the Chimney…sweeper up the Chimney。This may easily be done in the following manner： In building up the new back of the Fire…place； when this wall； （which need never be more than the width of a single brick in thickness；） is brought up so high that there remains no more than about ten or eleven inches between what is then the top of it； and the inside of the mantle； or lower extremity of the breast of the Chimney； an opening； or door…way； eleven or twelve inches wide； must be begun in the middle of the back； and continued quite to the top of it； which； according to the height to which it will comm