essays-1-第56章
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！
of air which a room of a moderate size contains； and as a large quantity of fresh air always enters the room； and an equal quantity of the warm air of the room is driven out of it every time the door is opened； there is much less danger of the air of a room becoming unwholesome for the want of ventilation than has been generally imagined； particularly in cold weather； when all the different causes which conspire to change the air of warmed rooms act with increased power and effect。
Those who have any doubts respecting the very great change of air or ventilation which takes place each time the door of a warm room is opened in cold weather； need only set the door of such a room wide open for a moment； and hold two lighted candles in the door…way； one near the top of the door； and the other near the bottom of it； the violence with which the flame of that above will be driven outwards； and that below inwards； by the two strong currents of air which； passing in opposite directions； rush in and out of the room at 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
