essays-1- 第51部分

Thus；　for　example；　supposing　the　wall　of　the　Chimney　in　front；　from　the　upper　part　of　the　breast　of　the　Chimney　to　the　front　of　the　mantle；　to　be　only　four　inches；　（which　is　sometimes　the　case；　particularly　in　rooms　situated　near　the　top　of　a　house；）　in　this　case；　if　we　take　four　inches　for　the　width　of　the　throat；　this　will　give　eight　inches　only　for　the　depth　of　the　Fire…place；　which　would　be　too　little；　even　were　coals　to　be　burnt　instead　of　wood。In　this　case　I　should　increase　the　depth　of　the　Fire…place　at　the　hearth　to　12　or　13　inches；　and　should　build　the　back　perpendicular　to　the　height　of　the　top　of　the　burning　fuel；　（whether　it　be　wood　burnt　upon　the　hearth；　or　coals　in　a　grate；）　and　then；　sloping　the　back　by　a　gentle　inclination　forward；　bring　it　to　its　proper　place；　that　is　to　say；　PERPENDICULARLY　UNDER　THE　BACK　OF　THE　THROAT　OF　THE　CHIMNEY。　This　slope；　（which　will　bring　the　back　forward　four　or　five　inches；　or　just　as　much　as　the　depth　of　the　Fire…place　is　encreased；）　though　it　ought　not　to　be　too　abrupt；　yet　it　ought　to　be　quite　finished　at　the　height　of　eight　or　ten　inches　above　the　fire；　otherwise　it　may　perhaps　cause　the　Chimney　to　smoke；　but　when　it　is　very　near　the　fire；　the　heat　of　the　fire　will　enable　the　current　of　rising　smoke　to　overcome　the　obstacle　which　this　slope　will　oppose　to　its　ascent；　which　it　could　not　do　so　easily　were　the　slope　situated　at　a　greater　distance　from　the　burning　fuel＇2＇。

Fig。　7；　8；　and　9；　show　a　plan；　elevation；　and　section　of　a　Fire…place　constructed　or　altered　upon　this　principal。The　wall　of　the　Chimney　in　front　at　a；　fig。　9；　being　only　four　inches　thick；　four　inches　more　added　to　it　for　the　width　of　the　throat　would　have　left　the　depth　of　the　Fire…place　measured　upon　the　hearth　b　c　only　eight　inches；　which　would　have　been　too　little；a　niche　c　and　e；　was　therefore　made　in　the　new　back　of　the　Fire…place　for　receiving　the　grate；　which　niche　was　six　inches　deep　in　the　center　of　it；　below　13　inches　wide；　（or　equal　in　width　to　the　grate；）　and　23　inches　high；　finishing　above　with　a　semicirular　arch；　which；　in　its　highest　part；　rose　seven　inches　above　the　upper　part　of　the　grate。The　door…way　for　the　Chimney…sweeper；　which　begins　just　above　the　top　of　the　niche；　may　be　seen　distinctly　in　both　the　figures　8　and　9。The　space　marked　g；　fig。　9；　behind　this　door…way；　may　either　be　filled　with　loose　bricks；　or　may　be　left　void。The　manner　in　which　the　piece　of　stone　f；　fig。　9；　which　is　put　under　the　mantle　of　the　Chimney　to　reduce　the　height　of　the　opening　of　the　Fire…place；　is　rounded　off　on　the　inside　in　order　to　give　a　fair　run　to　the　column　of　smoke　in　its　ascent　through　the　throat　of　the　Chimney；　is　clearly　expressed　in　this　figure。

The　plan　fig。　7；　and　elevation　fig。　8；　show　how　much　the　width　of　the　opening　of　the　Fire…place　in　front　is　diminished；　and　how　the　covings　in　the　new　Fire…place　are　formed。

A　perfect　idea　of　the　form　and　dimension　of　the　Fire…place　in　its　original　state；　as　also　after　its　alteration；　may　be　had　by　careful　inspection　of　these　figures。

I　have　added　the　drawing　fig。　10；　merely　to　show　how　a　fault；　which　I　have　found　workmen　in　general　whom　I　have　employed　in　altering　Fire…places　are　very　apt　to　commit；　is　to　be　avoided。　In　Chimneys　like　that　represented　in　this　figure；　where　the　jambs　A　and　B　project　far　into　the　room；　and　where　the　front　edge　of　the　marble　slab；　o　which　forms　the　coving；　does　not　come　so　far　forward　as　the　front　of　the　jambs；　the　workmen　in　constructing　the　new　covings　are　very　apt　to　place　them；not　in　the　line　c　A；　which　they　ought　to　do；but　in　the　line　c　o；　which　is　a　great　fault。The　covings　of　a　Chimney　should　never　range　BEHIND　the　front　of　the　jambs；　however　those　jambs　may　project　into　the　room；but　it　is　not　absolutely　necessary　that　the　covings　should　MAKE　A　FINISH　with　the　internal　front　corners　of　the　jambs；　or　that　they　should　be　continues　from　the　back　c；　quite　to　the　front　of　the　jambs　at　A。They　may　finish　in　front　at　a　and　b；　and　small　corners　A；　o；　a；　may　be　left　for　placing　the　shovels；　tongs；　etc。

Were　the　new　coving　to　range　with　the　front　edge　of　the　old　coving　o；　the　obliquity　of　the　new　coving　would　commonly　be　too　great；or　the　angle　d　c　o　would　exceed　135　degrees；　WHICH　IT　NEVER　SHOULD　DO；or　at　least　never　by　more　than　a　very　few　degrees。

No　inconvenience　of　any　importance　will　arise　from　making　the　obliquity　of　the　covings　LESS　than　what　is　here　recommended；　but　many　cannot　fail　to　be　produced　by　making　it　much　greater；　and　as　I　know　from　experience　that　workmen　are　very　apt　to　do　this；　I　have　thought　it　necessary　to　warn　them　particularly　against　it。

Fig。　11。　shows　how　the　width　and　obliquity　of　the　covings　of　a　Chimney　are　to　be　accommodated　to　the　width　of　the　back；　and　to　the　opening　in　front　and　depth　of　the　Fire…place；　where　the　width　of　the　opening　of　the　Fire…place　is　less　than　three　times　the　width　of　the　new　back。　As　all　those　who　may　be　employed　in　altering　Chimneys　may　not；　perhaps；　known　how　to　set　off　an　angle　of　any　certain　numbers　of　degrees；or　may　not　have　at　hand　the　instruments　necessary　for　doing　it；I　shall　here　show　how　an　instrument　may　be　made　which　will　be　found　to　be　very　useful　in　laying　out　the　work　for　the　bricklayers。

Upon　a　board　about　18　inches　wide　and　four　feet　long；　or　upon　the　floor　or　a　table；　draw　three　equal　squares　A；　B；　C；　fig。　12。　of　about　12　or　14　inches　each　side；　placed　in　a　strait　line；　and　touching　each　other。From　the　back　corner　c　of　the　center　square　B；　draw　a　diagonal　line　across　the　square　A；　to　its　outward　front　corner　f；　and　the　adjoining　angle　formed　by　the　lines　d　c　and　c　f　will　be　equal　to　135　degrees；the　angle　which　the　plane　of　the　back　of　a　Chimney　Fire…place　ought　to　make　with　the　plane　of　its　covings。And　a　bevel　m；　n；　being　made　to　this　angle　with　thin　slips　of　hard　wood；　this　little　instrument　will　be　found　to　be　very　useful　in　marking　out　on　the　hearth；　with　chalk；　the　plans　of　the　walls　which　are　to　form　the　covings　of　Fire…places。

As　Chimneys　which　are　apt　to　smoke　will　require　the　covings　to　be　placed　less　obliquely　in　respect　to　the　back　than　others　which　have　not　that　defect；　it　would　be　convenient　to　be　provided　with　several　bevels；three　or　four；　for　instance；　forming　different　angles。That　already　described；　which　may　be　called　No。　1。　will　measure　the　obliquity　of　the　covings　when　the　Fire…place　can　be　made　of　the　most　perfect　form：another　No。　2。　may　be　made　to　a　smaller　angle；　d　c　e；and　another　No。　3。　for　Chimnies　which　are　very　apt　to　smoke　at　the　still　smaller　angle　d　c　i。Or　a　bevel　may　be　so　contrived；　by　means　of　a　joint；　and　an　arch；　properly　graduated；　as　to　serve　for　all　the　different　degrees　of　obliquity　which　it　may　ever　be　necessary　to　give　to　the　covings　of　Fire…places。

Another　point　of　much　importance；　and　particularly　in　Chimneys　which　are　apt　to　smoke；　is　to　form　the　throat　of　the　Chimney　properly；　by　carrying　up　the　back　and　covings　to　a　proper　height。　　This；　workmen　are　apt　to　neglect　to　do；　probably　on　account　of　the　difficulty　they　find　in　working　where　the　opening　of　the　canal　of　the　Chimney　is　so　much　reduced。But　it　is　absolutely　necessary　that　these　walls　should　be　carried　up　five　or　six　inches　at　least　above　the　upper　part　of　the　breast　of　the　Chimney；　or　to　that　point　where　the　wall　which　forms　the　front　of　the　throat　begins　to　rise　perpendicularly。　If　the　workman　has　intelligence　enough　to　avail　himself　of　the　opening　which　is　formed　in　the　back　of　the　Fire…place　to　give　a　passage　to　the　Chimney…sweeper；　he　will　find　little　difficulty　in　finishing　his　work　in　a　proper　manner。

In　placing　the　plumb…line　against　the　breast　of　the　Chimney；　in　order　to　ascertain　how　far　the　new　back　is　to　be　brought　forward；　great　care　must　be　taken　to　place　it　at　the　very　top　of　the　breast；　where　the　canal　of　the　Chimney　BEGINS　TO　RISE　PERPENDICULARLY；　otherwise；　when　the　plumb…line　is　placed　too　low；　or　against　the　slope　of　the　breast；　when　the　new　back　comes　to　be　raised　to　its　proper　height；　the　throat　of　the　Chimney　will　found　to　be　too　narrow。

Sometimes；　and　indeed　very　often　the　top　of　the　breast　of　a　Chimney　lies　very　high；　or　far　above　the　fire　（see　the　figures　13　and　14；　where　d　shows　the　top　of　the　breast　of　the　Chimney）；　when　this　is　the　case　it　must　be　brought　lower；　otherwise　the　Chimney　will　be　very　apt　to　smoke。So　much　has　been　said　in　the　First　Chapter　of　this　Essay　of　the　advantages　to　be　derived　from　bringing　the　throat　of　a　Chimney　near　to　the　burning　fuel；　that　I　do　not　think　it　necessary　to　enlarge　on　them　in　this　place；　taking　it　for　granted　that　the　utility　and　necessity　of　that　arrangement　have　already　been　made　sufficiently　evident；　but　a　few　directions　for　workmen；　to　show　them　how　the　breast　（and　consequently　the　throat）　of　a　Chimney　can　most　readily　be　lowered；　may　not　be　superfluous。

Where　the　too　great　height　of　the　breast　of　a　Chimney　is　owing　to　the　great　height　of　the　mantle；　（see　fig。　13；）　or；　which　is　the　same　thing；　of　the　opening　of　the　Fire…place　in　front；　which　will　commonly　be　found　to　be　the　case；　the　only　remedy　for　the　evil　will　be　to　bring　down　the　mantle　lower；or　rather；　to　make　the　opening　of　the　Fire…place　in　front　lower；　by　throwing　across　the　top　of　this　opening；　from　one　jamb　to　the　other；　and　immediately　under　the　mantle；　a　very　flat　arch；a　wall　of　bricks　and　mortar；　supported　on　straight　bars　of　iron；or　a　piece　of　stone　（h；　fig。　13）。When　this　is　done；　the　slope　of　the　old　throat　of　the　Chimney；　or　of　the　back　side　of　the　mantle；　is　to　be　filled　up　with　plaster；　so　as　to　form　one　continued　flat；　vertical；　or　upright　plane　surface　with　the　lower　part　of　the　wall　of　the　canal　of　the　Chimney；　and　a　new　breast　is　to　be　formed　lower　down；　care　being　taken　to　round　it　off　properly；　and　make　it　finish　at　the　lower　surface　of　the　new　wall　built　under　the　mantle；which　wall　forms　in　fact　a　new　mantle。

The　annexed　drawing　fig。　13；　which　represents　the　section　of　a　Chimney　in　which　the　breast　has　been　lowered　according　to　the　method　here　described；　will　show　these　various　alterations　in　a　clear　and　satisfactory　manner。　　In　this　figure；　as　well　as　in　most　of　the　others　in　this　Essay；　the　old　walls　are　distinguished　from　the　new　ones　by　the　manner　in　which　they　are　shaded；　the　old　walls　b