essays-1-第59章
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th this hot metal； is heated and rarefied by it； and forcing its way upwards； goes off with the smoke； and as no current of air ever sets from any part of the opening of a Fire…place into the room； it is impossible to conceive how the heat existing in the metal composing any part of the apparatus of the Fire…place； and situated within its cavity； can come； or be brought into the room。
This difficulty may be in part removed； by supposing； what indeed seems to be true in a certain degree； that the heated metal sends off rays； the heat it acquires from the fire； even when it is not heated red hot； but still； as it never can be admitted that the heat； absorbed by the metal and afterwards thrown off by it in rays； is INCREASED by this operation； nothing can be gained by it； and as much must necessary be lost in consequence of the great quantity of heat communicated by the hot metal to the air in contact with it； which； as has already been shown； always makes its way up the Chimney； and flies off into the atmosphere； the loss of heat attending the use of it is too evident to require being farther insisted on。
There is； however； in Chimney Fire…places destined for burning coals； one essential part； the grate； which cannot well be made of any thing else but iron； but there is no necessity whatever for that immense quantity of iron which surrounds grates as they are now commonly constructed and fitted up； and which not only renders them very expensive； but injures very essentially the Fire…place。 If it should be necessary to diminish the opening of a large Chimney in order to prevent its smoking； it is much more simple； economical； and better in all respects； to do this with marble； fire…stone； or even with bricks and mortar； than to make use of iron； which； as has already been shown； is the very worst material that can possibly be employed for that purpose； and as to registers； they not only are quite unnecessary； where the throat of a Chimney is properly constructed； and of proper dimensions； but in that case would do much harm。 If they act at all； it must be by opposing their flat surfaces to the current of rising smoke in a manner which cannot fail to embarrass and impede its motion。 But we have shown that the passage of the smoke through the throat of a Chimney ought to be facilitated as much as possible； in order that it may be enabled to pass by a small aperture。
Register…stoves have often been found to be of use； but it is because the great fault of all Fire…places constructed upon the common principles being the enormous dimensions of the throat of the Chimney； this fault has been in some measure corrected by them； but I will venture to affirm； that there never was a Fire…place so corrected that would not have been much more improved； and with infinitely less expence； by the alterations here recommended； and which will be more particularly explained in the next Chapter。
CHAPTER。 II。
Practical directions designed for the use of workmen； showing how they are to proceed in making the alterations necessary to improve chimney fire…places； and effectually to cure smoking chimnies。
All Chimney Fire…places； without exception； whether they are designed for burning wood or coals； and even those which do not smoke； as well as those which do； may be greatly improved by making the alterations in them here recommended； for it is by no means MERELY to prevent Chimnies from smoking that these improvements are recommended； but it is also to make them better in all other respects as Fire…places； and when the alterations proposed are properly executed； which may be very easily be done with the assistance of the following plain and simple directions； the Chimnies will never fail to answer； I will venture to say； even beyond expectation。 The room will be heated much more equally and more pleasantly with LESS THAN HALF THE FUEL used before； the fire will be more cheerful and more agreeable； and the general appearance of the Fire…place more neat and elegant； and the Chimney WILL NEVER SMOKE。
The advantages which are derived from mechanical inventions and contrivances are； I know； frequently accompanied by disadvantages which it is not always possible to avoid； but in the case in question； I can say with truth； that I know of no disadvantage whatever that attends the Fire…places constructed upon the principles here recommended。 But to proceed in giving directions for the construction of these Fire…places。
That what I have to offer on this subject may be the more easily understood； it will be proper to begin by explaining the precise meaning of all those technical words and expressions which I may find it necessary or convenient to use。
By the THROAT of a Chimney； I mean the lower extremity of its canal； where it unites with the upper part of its open Fire…place。 This throat is commonly found about a foot above the level of the lower part of the mantle； and it is sometimes contracted to a smaller size than the rest of the canal of the Chimney； and sometimes not。
Fig。 5。 shows the section of a Chimney on the common construction； in which d e is the throat。
Fig。 6。 shows the section of the same Chimney altered and improved； in which d i is the reduced throat。
The BREAST of a Chimney； is that part of it which is immediately behind the mantle。It is the wall which forms the entrance from below into the throat of the Chimney in front； or towards the room。It is opposite to the upper extremity of the back of the open Fire…place； and parallel to it； in short it may said to be the back part of the mantle itself。In the figures 5 and 6； it is marked by the letter d。 The WIDTH of the throat of Chimney （d e fig。 5； and d i fig。 6；） is taken from the breast of the Chimney to the back； and its LENGTH is taken at right angles to its width； or in a line parallel to the mantle （a fig。 5。 and 6。）。
Before I proceed to give particular directions respecting the exact forms and dimensions of the different parts of a Fire…place； it may be useful to make such general an practical observations upon the subject as can be clearly 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 carel
