essays-1-第48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！


y　into　the　tube　will　rise　up　through　it　with　the　coloured　hot　water。The　warmed　air　of　a　room　heated　by　an　open　Chimney　Fire…place　has　always　a　tendency　to　rise；　（if　I　may　use　that　inaccurate　expression；）　and　finding　its　way　into　the　Chimney　frequently　goes　off　with　the　Smoke。

What　has　been　said；　will；　I　flatter　myself；　be　sufficient　to　explain　and　illustrate；　in　a　clear　and　satisfactory　manner；　the　cause　of　the　ascent　of　Smoke；　and　just　ideas　upon　that　subject　are　absolutely　necessary　in　order　to　judge；　with　certainty；　of　the　merit　of　any　scheme　proposed　for　the　improvement　of　Fire…places；　or　to　take　effectual　measures；　in　all　cases；　for　curing　smoking　Chimnies。For　though　the　perpetual　changes　and　alterations　which　are　produced　by　accident；　whim；　and　caprice；　do　sometimes　lead　to　useful　discoveries；　yet　the　progress　of　improvement　under　such　guidance　must　be　exceedingly　slow；　fluctuating；　and　uncertain。

As　to　the　causes　of　the　smoking　of　Chimnies；　they　are　very　numerous；　and　various；　but　as　a　general　idea　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　a　very　small　part　only　will　be　saved；　or　usefully　employed；　when　the　fire　is　made　in　a　careless　and　slovenly　manner。

In　lighting　a　coal　fire　more　wood　should　be　employed　than　is　commonly　used；　and　fewer　coals；　and　as　soon　as　the　fire　burns　bright；　and　the　coals　are　well　lighted；　and　NOT　BEFORE；　more　coals　should　be　added　to　increase　the　fire　to　its　proper　size＇3＇。

The　enormous　waste　of　fuel　in　London　may　be　estimated　by　the　vast　dark　cloud　which　continually　hangs　over　this　great　metropolis；　and　frequently　overshadows　the　whole　country；　far　and　wide；　for　this　dense　cloud　is　certainly　composed　almost　entirely　of　UNCONSUMED　COAL；　which　having　stolen　wings　from　the　innumerable　fires　of　this　great　city　has　escaped　by　the　Chimnies；　and　continues　to　sail　about　in　the　air；　till　having　lost　the　heat　which　gave　it　volatility；　it　falls　in　a　dry　shower　of　extremely　fine　black　dust　to　the　ground；　obscuring　the　atmosphere　in　its　descent；　and　frequently　changing　the　brightest　day　into　more　than　Egyptian　darkness。

I　never　view　from　a　distance；　as　I　come　into　town；　this　black　cloud　which　hangs　over　London；　without　wishing　to　be　able　to　compute　the　immense　number　of　chaldrons　of　coals　of　which　it　is　composed；　for　could　this　be　ascertained；　I　am　persuaded　so　striking　a　fact　would　awaken　the　curiosity；　and　excite　the　astonishment　of　all　ranks　of　the　inhabitants；　and　PERHAPS　turn　their　minds　to　an　object　of　economy　to　which　they　have　hitherto　paid　little　attention。

　　　　　　　　　　Conclusion。

Though　the　saving　of　fuel　which　will　result　from　the　improvements　in　the　forms　of　CHIMNEY　FIRE…PLACES　here　recommended　will　be　very　considerable；　yet　I　hope　to　be　able　to　show　in　a　future　Essay；　that　still　greater　savings　may　be　made；　and　more　important　advantages　derived　from　the　introduction　of　improvements　I　shall　propose　in　KITCHEN　FIRE…PLACES。

I　hope　likewise　to　be　able　to　show　in　an　Essay　on　COTTAGE　FIRE…PLACES；　which　I　am　now　preparing　for　publication；　that　THREE　QUARTERS；　at　least；　of　the　fuel　which　cottagers　now　consume　in　cooking　their　victuals；　and　in　warming　their　dwellings；　may　with　great　ease；　and　without　any　expensive　apparatus；　be　saved。

END　OF　THE　FOURTH　ESSAY。


　　　　　　　　　　EXPLANATION　OF　THE　FIGURES

＇　IMAGE　＇

Fig。　1。　The　plan　of　a　Fire…place　on　the　common　construction。　A　B；　the　opening　of　the　Fire…place　in　front。　C　D；　the　back　of　the　Fire…place。　A　C　and　B　D；　the　covings。　See　page　341。

＇　IMAGE　＇

Fig。　2。　This　figure　shows　the　elevation；　or　front　view　of　a　Fire…place　on　the　common　construction。　See　page　341。

＇　IMAGE　＇

Fig。　3。　This　Figure　shows　how　the　Fire…place　represented　by　the　Fig。　1；　is　to　be　altered　in　order　to　its　being　improved。

A　B　is　the　opening　in　front；C　D；　the　back；　and　A　C　and　B　D；　the　covings　of　the　Fire…place　in　its　original　state。

a　b；　its　opening　in　front；i　k；　its　back；and　a　i　and　b　k；　its　covings　after　it　has　been　altered；　e　is　a　point　upon　the　hearth　upon　which　a　plum　suspended　from　the　middle　of　the　upper　part　of　the　breast　of　the　Chimney　falls。　　The　situation　for　the　new