the back； and continued quite to the top of it； which； according to the height to which it will commonly be necessary to carry up the back； will make the opening about twelve or fourteen inches high； which will be quite sufficient to allow the Chimney…sweeper to pass。 When the Fire…place is finished； this door…way is to be closed by a few bricks； by a tile； or a fit piece of stone； placed in it； dry； or without mortar； and confined in its place by means of a rabbet made for that purpose in the brick…work。As often as the Chimney is swept； the Chimney…sweeper takes down this temporary wall； which is very easily done； and when he has finished his work； he puts it again into its place。The annexed drawing （No。 6。） will give a clear idea of this contrivance； and the experience I have had of it has proved that it answers perfectly well the purpose for which it is designed。
I observed above； that the new back； which it will always be found necessary to build in order to bring the fire sufficiently forward； in altering a Chimney constructed on the common principles； need never be thicker than the width of a common brick。I may say the same of the thickness necessary to be given to the new sides； or covings； of the Chimney； or if the new back and covings are constructed of stone； one inch and three quarters； or two inches in thickness will be sufficient。Care should be taken in building up these new walls to unite the back to the covings in a solid manner。
Whether the new back and covings are constructed of stone； or built of bricks； the space between them； and the old back and covings of the Chimney ought to be filled up； to give greater solidity to the structure。This may be done with loose rubbish； or pieces of broken bricks； or stones provided the work be strengthened by a few layers or courses of bricks laid in mortar； but it will be indispensably necessary to finish the work； where these new walls end； that is to say； at the top of the throat of the Chimney； where it ends abruptly in the open canal of the Chimney by a horizontal course of bricks well secured with mortar。 This course of bricks will be upon a level with the top of the door…way left for the Chimney…sweeper。
From these descriptions it is clear that where the throat of the Chimney has an end； that is to say； where it enters into the lower part of the open canal of the Chimney； THERE the three walls which form the two covings and the back of the Fire…place all end abruptly。It is of much importance that they should end in this manner； for were they to be sloped outward and raised in such a manner as to swell out the upper extremity of the throat of the Chimney in the form of a trumpet； and increase it by degrees to the size of the canal of the Chimney； this manner of uniting the lower extremity of the canal of the Chimney with the throat would tend to assist the winds which may attempt to blow down the Chimney； in forcing their way through the throat； and throwing the smoke backward into the room； but when the throat of the Chimney ends abruptly； and the ends of the new walls form a flat horizontal surface； it will be much more difficult for any wind from above； to find； and force its way through the narrow passage of the throat of the Chimney。
As the two walls which form the new covings of the Chimney are not parallel to each other； but inclined； presenting an oblique surface towards the front of the Chimney； and as they are built perfectly upright and quite flat； from the hearth to the top of the throat； where they end； it is evident that an horizontal section of the throat will not be an oblong square； but its deviation from that form is a matter of no consequence； and no attempts should ever be made； by twisting the covings above； where they approach the breast of the Chimney； to bring it to that form。All twists； bends； prominences； excavations； and other irregularities of form； in the covings of a Chimney； never fail to produce eddies in the current of air which is continually passing into； and through an open Fire…place in which a fire is burning；and all such eddies disturb； either the fire； or the ascending currents of smoke； or both； and not unfrequently cause the smoke to be thrown back into the room。Hence it appears； that the covings of Chimneys should never be made circular； or in the form of any other curve； but always quite flat。
For the same reason； that is to say； to prevent eddies； the breast of the Chimney； which forms that side of the throat that is in front； or nearest to the room； should be nearly cleaned off； and its surface made quite regular and smooth。
This may easily be done by covering it with a coat of plaster； which may be made thicker or thinner in different parts as may be necessary in order to bring the breast of the Chimney to be of the proper form。
With regard to the form of the breast of a Chimney； this is a matter of very great importance； and which ought always to be particularly attended to。The worst form it can have is that of a vertical plane； or upright flat；and next to this the worst form is an inclined plane。Both these forms cause the current of warm air from the room； which will； in spite of every precaution； sometimes find its way into the Chimney； to cross upon the current of smoke； which rises from the fire； in a manner most likely to embarrass it in its ascent； and drive it back。 The inclined plane which is formed by a flat register placed in the throat of a Chimney produces the same effects； and this is one reason； among many others； which have induced me to disapprove of register stoves。
The current of air； which； passing under the mantle； gets into the Chimney； should be made GRADUALLY TO BEND ITS COURSE UPWARDS； by which means it will be QUIETLY with the ascending current of smoke； and will be less likely to check it； or force it back into the room。Now this may be effected with the greatest ease and certainty； merely by ROUNDING OFF the breast of the Chimney or back part of the mantle； instead of leaving it flat； or full of holes and corners； and this of course ought always to be done。
I have hitherto given no precise directions in regard to the height to which the new back and covings ought to be carried： This will depend not only on the height of the mantle； but also； and more especially； on the height of the breast of the Chimney； or of that part of the Chimney where the breast ends and the upright canal begins。The back and covings must rise a few inches； five or six for instance； higher than this part； otherwise the throat of the Chimney will not be properly formed：but I know of no advantages that would be gained by carrying them up still higher。
I mentioned above； that the space between the walls which form the new back and covings； and the old back and sides of the Fire…place； should be filled up：but this must not be understood to apply to the space between the wall of dry bricks； or the tile which closes the passage for the Chimney…sweeper； and the old back of the Chimney； for that space must be left void； otherwise； though this tile （which at most will not be more than two inches in thickness；） were taken away； there would not be any room sufficient for him to pass。
In forming this door…way； the best method of proceeding is to place the tile or flat piece of stone destined for closing it； in its proper place； and to build round it； or rather by the sides of it； taking care not to bring any mortar near it； in order that it may be easily removed when the door…way is finished。With regard to the rabbet which should be made in the door…way to receive it and fix it more firmly in its place； this may either be formed at the same time when the door…way is built； or it may be made after it is finished； by attaching to its bottom and sides； with strong mortar； pieces of thin roof tiles。 Such as are about half an inch in thickness will be best for this use； if they are thicker； they will diminish too much the opening of the door…way； and will likewise be more liable to be torn away by the Chimney…sweeper in passing up and down the Chimney。
It will hardly be necessary for me to add； that the tile； or flat stone； or wall of dry bricks； which is used for closing up the door…way； must be of sufficient height to reach quite up to a level with the top of the walls which form the new back and covings of the Chimnies。
I ought； perhaps； to apologize for having been so very particular in these description and explanations； but it must be remembered that this chapter is written principally for the information of those who； having had few opportunities of employing their attention in abstruse philosophical researches； are not sufficiently practised in these intricate investigations； to seize； with facility； new ideas；and consequently； that I have frequently been obliged TO LABOUR to make myself understood。
I have only to express my wishes that my reader may not be more FATIGUED with this labour than I have been；for we shall them most certainly be satisfied with each other。But to return once more to the charge。
There is one important circumstance respecting Chimney Fire…places； destined for burning coals； which still remains to be farther examined；and that is the Grate。
Although there are few grates that may not be used in Chimneys constructed or altered upon the principles here recommended； yet they are not； by any means； all equally well adapted for that purpose。Those whose construction is the most simple； and which of course are the cheapest； are beyond comparison the best； ON ALL ACCOUNTS。Nothing being wanted in these Chimnies but merely a grate for containing the coals； and in which they will burn with a clear fire；and all additional apparatus being； not only useless； but very pernicious； all complicated and expensive grates should be laid aside； and such as more simple substituted in the room of them。And in the choice of a grate； as in every thing else； BEAUTY and ELEGANCE may easily be united with the MOST PERFECT SIMPLICITY。Indeed they are incompatible with every thing else。
In placing the grate； the thing principally to be attended to is； to make the back of it coincide with the back of the Fire…place； but as many of the grates now in common use will be found to be too large； when the Fire…pl