essays-1-第75部分

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ea　of　them　may　be　acquired　from　what　has　already　been　said　upon　that　subject　in　various　parts　of　this　Essay；　and　as　they　may；　in　all　cases；　（a　very　few　only　excepted；）　be　completely　remedied　by　making　the　alterations　in　Fire…places　here　pointed　out；　I　do　not　think　it　necessary　to　enumerate　them　all　in　this　place；　or　to　enter　into　those　long　details　and　investigations　which　would　be　required　to　show　the　precise　manner　in　which　each　of　them　operates；　either　alone；　or　in　conjunction　with　others。

There　is　however　one　cause　of　smoking　Chimnies　which　I　think　it　is　necessary　to　mention　more　particularly。In　modern　built　houses；　where　the　doors　and　windows　are　generally　made　to　close　with　such　accuracy　that　no　crevice　is　left　for　the　passage　of　the　air　from　without；　the　Chimnies　in　rooms　adjoining　to　each　other；　or　connected　by　close　passages；　are　frequently　found　to　affect　each　other；　and　this　is　easy　to　be　accounted　for。When　there　is　a　fire　burning　in　one　of　the　Chimnies；　as　the　air　necessary　to　supply　the　current　up　the　Chimney　where　the　fire　burns　cannot　be　had　in　sufficient　quantities　from　without；　through　the　very　small　crevices　of　the　doors　and　windows；　the　air　in　the　room　becomes　rarefied；　not　by　heat；　but　by　subtraction　of　that　portion　of　air　which　is　employed　in　keeping　up　the　fire；　or　supporting　the　combustion　of　the　fuel；　and　in　consequence　of　this　rarefaction；　its　elasticity　is　diminished；　and　being　at　last　overcome　by　the　pressure　of　the　external　air　of　the　atmosphere；　this　external　air　rushes　into　the　room　by　the　only　passage　left　for　it；　namely；　by　the　open　Chimney　of　the　neighbouring　room：And　the　flow　of　air　into　the　Fire…place；　and　up　the　Chimney　where　the　fire　is　burning　being　constant；　this　expence　of　air　is　supplied　by　a　continued　current　down　the　other　Chimney。

If　an　attempt　be　made　to　light　fires　in　both　Chimnies　at　the　same　time；　it　will　be　found　to　be　very　difficult　to　get　the　fires　to　burn；　and　the　rooms　will　both　be　filled　with　Smoke。

One　of　the　fires；that　which　is　made　in　the　Chimney　where　the　construction　of　the　Fire…place　is　best　adapted　to　facilitate　the　ascent　of　the　Smoke；or　if　both　Fire…places　are　on　the　same　construction；that　which　has　the　wind　most　favourable；　or　in　which　the　fire　happens　to　be　soonest　kindled；will　overcome　the　other；　and　cause　its　Smoke　to　be　beat　back　into　the　room　by　the　cold　air　which　descends　through　the　Chimney。The　most　obvious　remedy　in　this　case　is　to　provide　for　the　supply　of　fresh　air　necessary　for　keeping　up　the　fires　by　opening　a　passage　for　the　external　air　into　the　room　by　a　shorter　road　than　down　one　of　the　Chimnies；　and　when　this　is　done；　both　Chimnies　will　be　found　to　be　effectually　cured。

But　Chimnies　so　circumstanced　may　very　frequently　be　prevented　from　smoking　even　without　opening　any　new　passage　for　the　external　air；　merely　by　diminishing　the　draught；　（as　it　is　called；）　up　the　Chimnies；　which　can　best　be　done　by　altering　both　Fire…places　upon　the　principles　recommended　and　fully　explained　in　the　foregoing　Chapters　of　this　Essay。

Should　the　doors　and　windows　of　a　room　be　closed　with　so　much　nicety　as　to　leave　no　crevices　by　which　a　supply　of　air　can　enter　sufficient　for　maintaining　the　fire；　AFTER　THE　CURRENT　OF　AIR　UP　THE　CHIMNEY　HAS　BEEN　DIMINISHED　AS　MUCH　AS　POSSIBLE　BY　DIMINISHING　THE　THROAT　OF　THE　FIRE…PLACE；　in　that　case　there　would　be　no　other　way　of　preventing　the　Chimney　from　smoking　but　by　opening　a　passage　for　the　admission　of　fresh　air　from　without；but　this；　I　believe；　will　very　seldom　be　found　to　be　the　case。

A　case　more　frequently　to　be　met　with　is　where　currents　of　air　set　down　Chimnies　in　consequence　of　a　diminution　and　rarefaction　of　the　air　in　a　room；　occasioned　by　the　doors　of　the　room　opening　into　passages　or　courts　where　the　air　is　rarefied　by　the　action　of　some　particular　winds。　　In　such　cases　the　evil　may　be　remedied；　either　by　causing　the　doors　in　question　to　close　more　accurately；or；　（which　will　be　still　more　effectual；）　by　giving　a　supply　of　air　to　the　passage　or　court　which　wants　it；　by　some　other　way。

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