essays-1-第59节
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Where the top of a Chimney is commanded by high buildings； by clifts； or by high grounds； it will frequently happen； in windy weather； that the eddies formed in the atmosphere by these obstacles will blow down the Chimney； and beat down the smoke into the room。This it is true will be much less likely to happen when the throat of the Chimney is contracted and properly formed than when it is left quite open； and the Fire…place badly constructed； but as it is POSSIBLE that a Chimney may be so much exposed to these eddies in very high winds as to be made to smoke sometimes when the wind blows with violence from a certain quarter； it is necessary to show how the effects of those eddies may be prevented。
Various mechanical contrivances have been imagined for preventing the wind from blowing down Chimnies； and many of them have been found to be useful；there are； however； many of these inventions； which； though they prevent the wind from blowing down the Chimney； are so ill…contrived on other accounts as to obstruct the ascent of the Smoke； and do more harm than good。
Of this description are all those Chimney…pots with flat horizontal plates or roofs placed upon supporters just above the opening of the pot；and most of the caps which turn with the wind are not much better。One of the most simple contrivances that can be made use of； and which in most cases will be found to answer the purpose intended as well or better than more complicated machinery； is to cover the top of the Chimney with a hollow truncated pyramid or cone； the diameter of which above； or opening for the passage of the Smoke； is about 10 or 11 inches。 This pyramid； or cone； （for either will answer；）should be of earthen ware； or of cast iron；its perpendicular height may be equal to the diameter of its opening above； and the diameter of its opening below equal to three times its height。It should be placed upon the top of the Chimney； and it may be contrived so as to make a handsome finish to the brick…work。Where several flews come out near each other； or in the same stack of Chimnies； the form of a pyramid will be better than that of a cone for these covers。
The intention of this contrivance is； that the winds and eddies which strike against the oblique surface of these covers may be reflected upwards instead of blowing down the Chimney。 The invention is by no means new； but it has not hitherto been often put in practice。As often as I have seen it tried it has been found to be of use； I cannot say； however； that I was ever obliged to have recourse to it； or to any similar contrivance； and if I forbear to enlarge upon the subject of these inventions； it is because I am persuaded that when Chimnies are properly constructed IN THE NEIGHBOURHOOD OF THE FIRE…PLACE little more will be necessary to be done at the top of the Chimney than to leave it open。
I cannot conclude this Essay without again recommending； in the strongest manner； a careful attention to the management of fires in open Chimnies； for not only the quantity of heat produced on the combustion of fuel depends much on the manner in which the fire is managed； but even of the heat actually generated a very small part only will be saved； or usefully employed； when the fire is made in a careless and slovenly manner。
In lighting a coal fire more wood should be employed than is commonly used； and fewer coals； and as soon as the fire burns bright； and the coals are well lighted； and NOT BEFORE； more coals should be added to increase the fire to its proper size＇3＇。
The enormous waste of fuel in London may be estimated by the vast dark cloud which continually hangs over this great metropolis； and frequently overshadows the whole country； far and wide； for this dense cloud is certainly composed almost entirely of UNCONSUMED COAL； which having stolen wings from the innumerable fires of this great city has escaped by the Chimnies； and continues to sail about in the air； till having lost the heat which gave it volatility； it falls in a dry shower of extremely fine black dust to the ground； obscuring the atmosphere in its descent； and frequently changing the brightest day into more than Egyptian darkness。
I never view from a distance； as I come into town； this black cloud which hangs over London； without wishing to be able to compute the immense number of chaldrons of coals of which it is composed； for could this be ascertained； I am persuaded so striking a fact would awaken the curiosity； and excite the astonishment of all ranks of the inhabitants； and PERHAPS turn their minds to an object of economy to which they have hitherto paid little attention。
Conclusion。
Though the saving of fuel which will result from the improvements in the forms of CHIMNEY FIRE…PLACES here recommended will be very considerable； yet I hope to be able to show in a future Essay； that still greater savings may be made； and more important advantages derived from the introduction of improvements I shall propose in KITCHEN FIRE…PLACES。
I hope likewise to be able to show in an Essay on COTTAGE FIRE…PLACES； which I am now preparing for publication； that THREE QUARTERS； at least； of the fuel which cottagers now consume in cooking their victuals； and in warming their dwellings； may with great ease； and without any expensive apparatus； be saved。
END OF THE FOURTH ESSAY。
EXPLANATION OF THE FIGURES
＇ IMAGE ＇
Fig。 1。 The plan of a Fire…place on the common construction。 A B； the opening of the Fire…place in front。 C D； the back of the Fire…place。 A C and B D； the covings。 See page 341。
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Fig。 2。 This figure shows the elevation； or front view of a Fire…place on the common construction。 See page 341。
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Fig。 3。 This Figure shows how the Fire…place represented by the Fig。 1； is to be altered in order to its being improved。
A B is the opening in front；C D； the back； and A C and B D； the covings of the Fire…place in its original state。
a b； its opening in front；i k； its back；and a i and b k； its covings after it has been altered； e is a point upon the hearth upon which a plum suspended from the middle of the upper part of the breast of the Chimney falls。 The situation for the new back is ascertained by taking the line e f equal to four inches。 The new back and covings are represented as being built of bricks；and the space between these and the old back and covings as being filled up with rubbish。 See page 342。
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Fig。 4。 This Figure represents the elevation or front view of the Fire…place Fig。 3。 after it has been altered。 The lower part of the door…way left for the Chimney…sweeper is shown in this Figure by white dotted lines。 See page 344。
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Fig。 5。 This Figure shows the section of a Chimney Fire…place and of a part of the canal of the Chimney； on the common construction。
a b is the opening in front； b c； the depth of the Fire…place at the hearth； d； the breast of the Chimney。
d e； the throat of the Chimney； and d f； g e； a part of the open canal of the Chimney。
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Fig。 6。 Shows a section of the same Chimney after it has been altered。
k l is the new back of the Fire…place； l i； the tile or stone which closes the door…way for the Chimney…sweeper； d i； the throat of the Chimney； narrow to four inches； a； the mantle； and h； the new wall made under the mantle to diminish the height of the opening of the Fire…place in front。
N。B。 These two Figures are sections of the same Chimney which is represented in each of the four preceding Figures。
＇ IMAGE ＇
Fig。 7。
This Figure represents the ground plan of a Chimney Fire…place in which the grate is placed in a niche； and in which the original width A B of the Fire…place is considerably diminished。
a b is the opening of the Fire…place in front after it has been altered； and d is the back of the niche in which the grate is placed。 See page 347。
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Fig。 8。 Shows a front view of the same Fire…place after it has been altered； where may be seen the grate； and the door…way for the Chimney…sweeper。 See page 347。
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Fig。 9。 Shows a section of the same Fire…place； c d e being a section of the niche； g the door…way for the Chimney…sweeper； closed by a piece of the fire…stone； and f the new wall under the mantle by which the height of the opening of the Fire…place in front is diminished。 See page 347。
＇ IMAGE ＇
Fig。 10。 This Figure shows how the covings are to be placed when the front of the covings （a and b） do not come so far forward as the front of the opening of the Fire…place； or the jambs （A and B）。 See page 348。
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Fig。 11。 This Figure shows how the width and obliquity of the covings are to be accommodated to the width of the back of a Fire…place； in cases where it is necessary to make the back very wide。 See page 349。
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Fig。 12。 This Figure shows how an instrument called a bevel （m n）； useful in laying out the work； in altering Chimney Fire…places； may be constructed。 See page 349。
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Fig。 13。 This shows how； when the breast of a Chimney （d） is too high； it may be brought down by means of a wall （h） placed under the mantle； and a coating of plaster； which in this Figure is represented by the part marked by dots。
