essays-1-第54节
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
pening 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 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 b
