essays-1-第50节
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t the same time； will be convinced that the change of air which actually takes place must be very considerable indeed； and these currents will be stronger； and consequently the change of air greater； in proportion as the difference is greater between the temperature of the air within the room and of that without。 I have been more particular upon this subject；the ventilation of warmed rooms which are constantly inhabited；as I know that people in general in this country have great apprehensions of the bad consequences to health of living rooms in which there is not a continual influx of cold air from without。 I am as much an advocate for a FREE CIRCULATION of air as any body； and always sleep in a bed without curtains on that account； but I am much inclined to think； that the currents of cold air which never fail to be produced in rooms heated by Fire…places constructed upon the common principle； those partial heats on one side of the body； and the cold blasts on the other； so often felt in houses in this country； are infinitely more detrimental to health than the supposed closeness of the air in a room warmed more equally； and by a smaller fire。
All these advantages； attending the introduction of the improvements in Fire…places here recommended； are certainly important； and I do not know that they are counterbalanced by any one disadvantage whatsoever。 The only complaints that I had ever heard made against them was； that they made the rooms TOO warm； but the remedy to this evil is so perfectly simple and obvious； that I should be almost afraid to mention it； less it might be considered as an insult to the understanding of the persons to whom such information should be given； for nothing surely can be conceived more perfectly ridiculous than the embarrassment of a person on account of the too great heat of his room； when it is in his power to diminish AT PLEASURE the fire by which it is warmed； and yet； strange as it may appear； this has sometimes happened！
Before I proceed to give directions for the construction of Fire…places； it will be proper to examine more carefully the Fire…places now in common use；to point out their faults； and to establish the principles upon which Fire…places ought to be constructed。
The great fault of all the open Fire…places； or Chimnies； for burning wood or coals in an open fire； now in common use； is； that they are much too large； or rather it is THE THROAT OF THE CHIMNEY or the lower part of its open canal； in the neighbourhood of the mantle； and immediately over the fire； which is too large。 This opening has hitherto been left larger than otherwise it probably would have been made； in order to give a passage to the Chimney…sweeper； but I shall show hereafter how a passage for the Chimney…sweeper may be contrived without leaving the throat of the Chimney of such enormous dimensions as to swallow up and devour all the warm air of the room； instead of merely giving a passage to the smoke and heated vapour which rise from the fire； for which last purpose alone it ought to be destined。
Were it my intention to treat my subject in a formal scientific manner； it would be doubtless be proper； and even necessary； to begin by explaining in the fullest manner； and upon the principles founded on the laws of nature； relative to the motions of elastic fluids； as far as they have been discovered and demonstrated； the causes of the ascent of smoke； and also to explain and illustrate upon the same principles； and even to measure； or estimate by calculations； the precise effects of all those mechanical aids which may be proposed for assisting it in its ascent； or rather for removing those obstacles which hinder its motion upwards；but as it is my wish rather to write an useful practical treatise； than a learned dissertation； being more desirous to contribute in diffusing useful knowledge； by which the comforts and enjoyments of mankind may be increased； than to acquire the reputation of a philosopher among learned men； I shall endeavour to write in such a manner as to be easily understood BY THOSE WHO ARE MOST LIKELY TO PROFIT BY THE INFORMATION I HAVE TO COMMUNICATE； and consequently most likely to assist in bringing into general use the improvements I recommend。 This being premised； I shall proceed； without any further preface or introduction； to the investigation of the subject I have undertaken to treat。
As the immoderate size of the throats of Chimnies is the great fault of their construction； it is this fault which ought always to be first attended to in every attempt which is made to improve them； for however perfect the construction of a Fire…place may be in other respects； if the opening left for the passage of the smoke is larger than is necessary for that purpose； nothing can prevent the warm air of the room from escaping through it； and whenever this happens； there is not only an unnecessary loss of heat； but the warm air which leaves the room to go up the Chimney being replaced by cold air from without； the draughts of cold air； so often mentioned； cannot fail to be produced in the room； to the great annoyance of those who inhabit it。 But although both these evils may be effectually remedied by reducing the throat of the Chimney to a proper size； yet in doing this several precautions will be necessary。 And first of all； the throat of the Chimney should be in its proper place； that is to say； in that place in which it ought to be； in order that the ascent of the smoke may be most facilitated； for every means which can be employed for facilitating the ascent of the smoke in the Chimney must naturally tend to prevent the Chimney from smoking： now as the smoke and hot vapour which rise from a fire naturally tend UPWARDS； the proper place for the throat of the Chimney is evidently perpendicularly OVER THE FIRE。
But there is another circumstance to be attended to in determining the proper place for the throat of a Chimney； and that is； to ascertain its distance from the fire； or HOW FAR above the burning fuel it ought to be placed。 In determining this point； there are many things to be considered； and several advantages and disadvantages to be weighed and balanced。
As the smoke and vapour which ascend from burning fuel rise in consequence of their being rarefied by heat； and made lighter than the air of the surrounding atmosphere； and as the degree of their rarefaction； and consequently their tendency to rise； is in proportion to the intensity of their heat； and further； as they are hotter near the fire than at a greater distance from it； it is clear that the nearer the throat of a Chimney is to the fire； the stronger will be； what is commonly called； its DRAUGHT； and the less danger there will be of its smoking。 But on the other hand； when the draught of a Chimney is very strong； and particularly when this strong draught is occasioned by the throat of the Chimney being very near the fire； it may so happen that the draught of air into the fire may become so strong； as to cause the fuel to be consumed too rapidly。 There are likewise several other inconveniences which would attend the placing of the throat of a Chimney VERY NEAR the burning fuel。 In introducing the improvements proposed； in Chimnies already built； there can be no question in regard to the height of the throat of the Chimney； for its place will be determined by the height of the mantle。 It can hardly be made lower than the mantle； and it ought always to be brought down as nearly upon the level with the bottom of it as possible。 If the Chimney is apt to smoke； it will sometimes be necessary either to lower the mantle or to diminish the height of the opening of the Fire…place； by throwing over a flat arch； or putting in a straight piece of stone from one side of it to the other； or； which will be still more simple and easy in practice； building a wall of bricks； supported by a flat bar of iron； immediately under the mantle。
Nothing is so effectual to prevent Chimnies from smoking as diminishing the opening of the Fire…place in the manner here described； and lowering and diminishing the throat of the Chimney； and I have always found； except in the single instance already mentioned； that a perfect cure may be effected by THESE MEANS ALONE； even in the most desperate cases。 It is true； that when the construction of the Chimney is very bad indeed； or its situation very unfavourable to the ascent of the smoke； and especially when both these disadvantages exist at the same time； it may sometimes be necessary to diminish the opening of the Fire…place； and particularly to lower it； and also to lower the throat of the Chimney； more than might be wished： but still I think this can produce no inconveniences to be compared with that greatest of all plagues； a smoking Chimney。
The position of the throat of a Chimney being determined； the next points to be ascertained are its size and form； and the manner in which it ought to be connected with the Fire…place below； and with the open canal of the Chimney above。
But as these investigations are intimately connected with those which relate to the form proper to be given to the Fire…place itself； we must consider them all together。
That these inquiries may be pursued with due method； a
