essays-1-第41章

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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；　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；　and　that　the　conclusions　drawn　from　them　may　be　clear　and　satisfactory；　it　will　be　necessary　to　consider；　first；　what　the　objects　are　which　ought　principally　to　be　had　in　view　in　the　construction　of　a　Fire…place；　and　secondly；　to　see　how　these　objects　can　best　be　attained。

Now　the　design　of　a　Chimney　Fire　being　simply　to　warm　a　room；　it　is　necessary；　first　of　all；　to　contrive　matters　so　that　the　room　shall　be　actually　warmed；　secondly；　that　it　be　warmed　with　the　smallest　expence　of　fuel　possible；　and；　thirdly；　that　in　warming　it；　the　air　of　the　room　be　preserved　perfectly　pure；　and　fit　for　respiration；　and　free　from　smoke　and　all　disagreeable　smells。

In　order　to　take　measures　with　certainty　for　warming　a　room　by　means　of　an　open　Chimney　Fire；　it　will　be　necessary　to　consider　HOW；　or　in　WHAT　MANNER；　such　a　Fire　communicates　heat　to　a　room。　This　question　may　perhaps；　at　the　first　view　of　it；　appear　to　be　superfluous　and　trifling；　but　a　more　careful　examination　of　the　matter　will　show　it　to　be　highly　deserv