essays-1-第68部分

快捷操作: 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! 如果本书没有阅读完，想下次继续接着阅读，可使用上方 "收藏到我的浏览器" 功能 和 "加入书签" 功能！



Fig。　5。　shows　the　section　of　a　Chimney　on　the　common　construction；　in　which　d　e　is　the　throat。

Fig。　6。　shows　the　section　of　the　same　Chimney　altered　and　improved；　in　which　d　i　is　the　reduced　throat。

The　BREAST　of　a　Chimney；　is　that　part　of　it　which　is　immediately　behind　the　mantle。It　is　the　wall　which　forms　the　entrance　from　below　into　the　throat　of　the　Chimney　in　front；　or　towards　the　room。It　is　opposite　to　the　upper　extremity　of　the　back　of　the　open　Fire…place；　and　parallel　to　it；　in　short　it　may　said　to　be　the　back　part　of　the　mantle　itself。In　the　figures　5　and　6；　it　is　marked　by　the　letter　d。　　The　WIDTH　of　the　throat　of　Chimney　（d　e　fig。　5；　and　d　i　fig。　6；）　is　taken　from　the　breast　of　the　Chimney　to　the　back；　and　its　LENGTH　is　taken　at　right　angles　to　its　width；　or　in　a　line　parallel　to　the　mantle　（a　fig。　5。　and　6。）。

Before　I　proceed　to　give　particular　directions　respecting　the　exact　forms　and　dimensions　of　the　different　parts　of　a　Fire…place；　it　may　be　useful　to　make　such　general　an　practical　observations　upon　the　subject　as　can　be　clearly　understood　without　the　assistance　of　drawings；　for　the　more　complete　the　knowledge　of　any　subject　is　which　can　be　acquired　without　drawings；　the　more　easy　will　it　be　to　understand　the　drawings　when　it　becomes　necessary　to　have　recourse　to　them。

The　bringing　forward　of　the　Fire　into　the　room；　or　rather　bringing　it　nearer　to　the　front　of　the　opening　of　the　Fire…place；and　the　diminishing　of　the　throat　of　the　Chimney；　being　two　objects　principally　had　in　view　in　the　alterations　in　Fire…places　here　recommended；　it　is　evident　that　both　these　may　be　attained　merely　by　bringing　forward　the　back　of　the　Chimney。　The　only　question　therefore　is；　how　far　it　should　be　brought　forward？The　answer　is　short；　and　easy　to　be　understood；bring　it　forward　as　far　as　possible；　without　diminishing　too　much　the　passage　which　must　be　left　for　the　smoke。　　Now　as　this　passage；　which；　in　its　narrowest　part；　I　have　called　the　THROAT　OF　THE　CHIMNEY；　ought；　for　reasons　which　are　fully　explained　in　the　foregoing　Chapter；　to　be　immediately；　or　perpendicularly　over　the　Fire；　it　is　evident　that　the　back　of　the　Chimney　must　always　be　built　perfectly　upright。To　determine　therefore　the　place　for　the　new　back；　or　how　far　precisely　it　ought　to　be　brought　forward；　nothing　more　is　necessary　than　to　ascertain　how　wide　the　throat　of　the　Chimney　ought　to　be　left；　or　what　space　must　be　left；　between　the　top　of　the　breast　of　the　Chimney；　where　the　upright　canal　of　the　Chimney　begins；　and　the　new　back　of　the　Fire…place　carried　up　perpendicularly　to　that　height。

In　the　course　of　my　numerous　experiments　upon　Chimnies；　I　have　taken　much　pains　to　determine　the　width　proper　to　be　given　to　this　passage；　and　I　have　found；　that；　when　the　back　of　the　Fire…place　is　of　a　proper　width；　the　best　width　for　the　throat　of　a　Chimney；　when　the　Chimney　and　the　Fire…place　are　at　the　usual　form　and　size；　is　FOUR　INCHES。Three　inches　might　sometimes　answer；　especially　where　the　Fire…place　is　very　small；　and　the　Chimney　good；　and　well　situated：　but　as　it　is　always　of　much　importance　to　prevent　those　accidental　puffs　of　smoke　which　are　sometimes　thrown　into　rooms　by　the　carelessness　of　servants　in　putting　on　suddenly　too　many　coals　at　once　upon　the　fire；　and　as　I　found　these　accidents　sometimes　happened　when　the　throats　of　Chimneys　were　made　very　narrow；　I　found　that；　upon　the　whole；　all　circumstances　being　well　considered；　and　advantages　and　disadvantages　compared　and　balanced；　FOUR　INCHES　is　the　best　width　that　can　be　given　to　the　throat　of　a　chimney；　and　this；　whether　the　Fire…place　be　destined　to　burn　wood；　coals；　turf；　or　any　other　fuel　commonly　used　for　heating　rooms　by　an　open　fire。

In　Fire…places　destined　for　heating　very　large　halls；　and　where　very　great　fires　are　kept　up；　the　throat　of　the　Chimney　may；　if　it　should　be　thought　necessary；　be　made　four　inches　and　an　half；　or　five　inches　wide；but　I　have　frequently　made　Fire…places　for　halls　which　have　answered　perfectly　well　where　the　throats　of　the　Chimnies　have　not　been　wider　than　four　inches。

It　may　perhaps　appear　extraordinary；　upon　the　first　view　of　the　matter；　that　Fire…places　of　such　different　sizes　should　all　require　the　throat　of　the　Chimney　to　be　of　the　same　width；　but　when　it　is　considered　that　the　CAPACITY　of　the　throat　of　a　Chimney　does　not　depend　on　its　width　alone；　but　on　its　width　and　LENGTH　taken　together；　and　that　in　large　Fire…places；　the　width　of　the　back；　and　consequently　the　length　of　the　throat　of　the　Chimney；　is　greater　than　in　those　which　are　smaller；　this　difficulty　vanishes。

And　this　leads　us　to　consider　another　important　point　respecting　open　Fire…places；　and　that　is；　the　width　which　it　will；　in　each　case；　be　proper　to　give　to　the　back。In　Fire…places　as　they　are　now　commonly　constructed；　the　back　is　of　equal　width　with　the　opening　of　the　Fire…place　in　front；but　this　construction　is　faulty　on　two　accounts。First；　in　a　Fire…place；　so　constructed；　the　sides　of　the　Fire…place；　or　COVINGS；　as　they　are　called；　are　parallel　to　each　other；　and　consequently　ill…contrived　to　throw　out　into　the　room　the　heat　they　receive　from　the　fire　in　the　form　of　rays；and　secondly；　the　large　open　corners　which　are　formed　by　making　the　back　as　wide　as　the　opening　of　the　Fire…place　in　front　occasion　eddies　of　wind；　which　frequently　disturb　the　fire；　and　embarrass　the　smoke　in　its　ascent　in　such　a　manner　as　often　to　bring　it　into　the　room。Both　these　defects　may　be　entirely　remedied　by　diminishing　the　width　of　the　back　of　the　Fire…place。　The　width　which；　in　most　cases；　it　will　be　best　to　give　it；　is　ONE　THIRD　of　the　width　of　the　opening　of　the　Fire…place　in　front。But　it　is　not　absolutely　necessary　to　conform　rigorously　to　this　decision；　nor　will　it　always　be　possible。It　will　frequently　happen　that　the　back　of　a　Chimney　must　be　made　wider　than；　according　to　the　rule　here　given；　it　ought　to　be。This　may　be；　either　to　accommodate　the　Fire…place　to　a　stove；　which　being　already　on　hand；　must；　to　avoid　the　expense　of　purchasing　a　new　one；　be　employed；　or　for　other　reasons；and　any　small　deviation　from　the　general　rule　will　be　attended　with　no　considerable　inconvenience。It　will　always　be　best；　however；　to　conform　to　it　as　far　as　circumstances　will　allow。

Where　a　Chimney　is　designed　for　warming　a　room　of　a　middling　size；　and　where　the　thickness　of　the　wall　of　the　Chimney　in　front；　measured　from　the　front　of　the　mantle　to　the　breast　of　the　Chimney；　is　nine　inches；　I　should　set　off　four　inches　more　for　the　width　of　the　throat　of　the　Chimney；　which；　supposing　the　back　of　the　Chimney　to　be　built　upright；　as　it　always　ought　to　be；　will　give　thirteen　inches　for　the　depth　of　the　Fire…place；　measured　upon　the　hearth；　from　the　opening　of　the　Fire…place　in　front；　to　the　back。In　this　case　thirteen　inches　would　be　a　good　size　for　the　width　of　the　back；　and　three　times　thirteen　inches；　or　thirty…nine　inches；　for　the　width　of　the　opening　of　the　Fire…place　in　front；　and　the　angle　made　by　the　back　of　the　Fire…place　and　the　sides　of　it；　or　covings；　would　be　just　135　degrees；　which　is　the　best　position　they　can　have　for　throwing　heat　into　the　room。

But　I　will　suppose　that　in　altering　such　a　Chimney　it　is　fo