essays-1-第71部分

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e…place；　but　as　many　of　the　grates　now　in　common　use　will　be　found　to　be　too　large；　when　the　Fire…places　are　altered　and　improved；　it　will　be　necessary　to　diminish　their　capacities　by　filling　them　up　at　the　back　and　the　sides　with　pieces　of　fire…stone。　　When　this　is　done；　it　is　the　front　of　the　flat　piece　of　fire…stone　which　is　made　to　form　a　new　back　to　the　grate；　which　must　be　made　to　coincide　with；　and　make　part　of　the　back；　of　the　Fire…place。　But　in　diminishing　the　capacities　of　grates　with　pieces　of　fire…stone；　care　must　be　taken　not　to　make　them　TOO　NARROW。

The　proper　width　for　grates　destined　for　rooms　of　a　middling　size　will　be　from　six　to　eight　inches；　and　their　length　may　be　diminished　more　or　less；　according　as　the　room　is　heated　with　more　or　less　difficulty；　or　as　the　weather　is　more　or　less　severe。　But　where　the　width　of　a　grate　is　not　more　than　five　inches；　it　will　be　very　difficult　to　prevent　the　fire　from　going　out。

It　goes　out　for　the　same　reason　that　a　live　coal　from　the　grate　that　falls　upon　the　hearth　soon　ceases　to　be　red　hot；it　is　cooled　by　the　surrounding　cold　air　of　the　atmosphere。　The　knowledge　of　the　cause　which　produces　this　effect　is　important；　as　it　indicates　the　means　which　may　be　used　for　preventing　it。　But　of　this　subject　I　shall　treat　more　fully　hereafter。

It　frequently　happens　that　the　iron　backs　of　grates　are　not　vertical；　or　upright；　but　inclined　backwards。When　these　grates　are　so　much　too　wide　as　to　render　it　necessary　to　fill　them　up　behind　with　fire…stone；　the　inclination　of　the　back　will　be　of　little　consequence；　for　by　making　the　piece　of　stone　with　which　the　width　of　the　grate　is　to　be　diminished　in　the　form　of　a　wedge；　or　thicker　above　than　below；　the　front　of　this　stone；　which　in　effect　will　become　the　back　of　the　grate；　may　be　made　perfectly　vertical；　and　the　iron　back　of　the　grate　being　hid　in　the　solid　work　of　the　back　of　the　Fire…place；　will　produce　no　effect　whatever；　but　if　the　grate　be　already　so　narrow　as　not　to　admit　of　any　diminution　of　its　width；　in　that　case　it　will　be　best　to　take　away　the　iron　back　of　the　grate　entirely；　and　fixing　the　grate　firmly　in　the　brick…work；　cause　the　back　of　the　Fire…place　to　serve　as　a　back　to　the　grate。This　I　have　very　frequently　done；　and　have　always　found　it　to　answer　perfectly　well。

Where　it　is　necessary　that　the　fire　in　a　grate　should　be　very　small；　it　will　be　best；　in　reducing　the　grate　with　fire…stone；　to　bring　its　cavity；　destined　for　containing　the　fuel；　to　the　form　of　one　half　of　a　hollow　hemisphere；　the　two　semicircular　openings　being　one　above；　to　receive　the　coals；　and　the　other　in　front；　or　towards　the　bars　of　the　grate；　for　when　the　coals　are　burnt　in　such　a　confined　space；　and　surrounded　on　all　sides；　except　in　the　front　and　above；　by　fire…stone；　（a　substance　peculiarly　well　adapted　for　confining　heat；）　the　heat　of　the　fire　will　be　concentrated；　and　the　cold　air　of　the　atmosphere　being　kept　at　a　distance；　a　much　smaller　quantity　of　coals　will　burn；　than　could　possibly　be　made　to　burn　in　a　grate　where　they　would　be　more　exposed　to　be　cooled　by　the　surrounding　air；　or　to　have　their　heat　carried　off　by　being　in　contact　with　iron；　or　with　any　other　substance　through　which　heat　passes　with　greater　facility　than　through　fire…stone。

Being　persuaded　that　if　the　improvements　in　Chimney　Fire…places　here　recommended　should　be　generally　adopted；　（which　I　cannot　help　flattering　myself　will　be　the　case；）　that　it　will　become　necessary　to　reduce；　very　considerably；　the　sizes　of　grates；　I　was　desirous　of　showing　how　this　may；　with　the　greatest　safety　and　facility；　be　done。

Where　grates；　which　are　designed　for　rooms　of　a　middling　size；　are　longer　than　14　or　15　inches；　it　will　always　be　best；　not　merely　to　diminish　their　lengths；　by　filling　them　up　at　their　two　ends　with　fire…stone；　but；　forming　the　back　of　the　Chimney　of　a　proper　width；　without　paying　any　regard　to　the　length　of　the　grate；　to　carry　the　covings　through　the　two　ends　of　the　grate　in　such　a　manner　as　to　conceal　them；　or　at　least　to　conceal　the　back　corners　of　them　in　the　walls　of　the　covings。

I　cannot　help　flattering　myself　that　the　directions　here　given　in　regard　to　the　alterations　which　it　may　be　necessary　to　make　in　Fire…places；　in　order　to　introduce　the　improvements　proposed；　will　be　found　to　be　so　perfectly　plain　and　intelligible　that　no　one　who　reads　them　will　be　at　any　loss　respecting　the　manner　in　which　the　work　is　to　be　performed；　　but　as　order　and　arrangement　tend　much　to　facilitate　all　mechanical　operations；　I　shall　here　give　a　few　short　directions　respecting　the　manner　of　LAYING　OUT　THE　WORK；　which　may　be　found　useful；　and　particularly　to　gentlemen　who　may　undertake　to　be　their　own　architects；　in　ordering　and　directing　the　alterations　to　be　made　for　the　improvement　of　their　Fire…places。

　　　　　　　　　　Directions　for　laying　out　the　Work。

If　there　be　a　grate　in　the　Chimney　which　is　to　be　altered；　it　will　always　be　best　to　take　it　away；　and　when　this　is　done；　the　rubbish　must　be　removed；　and　the　hearth　swept　perfectly　clean。

Suppose　the　annexed　figure　No。　1。　to　represent　the　ground　plan　of　such　a　Fire…place；　A　B　being　the　opening　of　it　in　front；　A　C　and　B　D　the　two　sides　or　covings；　and　C　D　the　back。

Figure　2。　shows　the　elevation　of　this　Fire…place。

First　draw　a　strait　line；　with　chalk；　or　with　a　lead　pencil；　upon　the　hearth；　from　one　jamb　to　the　other；even　with　the　front　of　the　jambs。　　The　dotted　line　A　B；　figure　3；　may　represent　this　line。

From　the　middle　C　of　this　line；　（A　B）　another　line　c　d；　is　to　be　drawn　perpendicular　to　it；　across　the　hearth；　to　the　middle　d；　of　the　back　of　the　Chimney。

A　person　must　now　stand　upright　in　the　Chimney；　with　his　back　to　the　back　of　the　Chimney；　and　hold　a　plumb…line　to　the　middle　of　the　upper　part　of　the　breast　of　the　Chimney　（d；　fig。　5；）　or　where　the　canal　of　the　Chimney　begins　to　rise　perpendicularly；　taking　care　to　place　the　line　above　in　such　a　manner　that　the　plumb　may　fall　on　the　line　c　d；　draw　on　the　hearth　from　the　middle　of　the　opening　of　the　Chimney　in　front　to　the　middle　of　the　back；　and　an　assistant　must　mark　the　precise　place　e；　on　that　line　where　the　plumb　falls。

This　being　done；　and　the　person　in　the　Chimney　having　quitted　his　station；　four　inches　are　to　be　set　off　the　line　c　d；　from　e；　towards　d；　and　the　point　f；　where　these　four　inches　end；　（which　must　be　marked　with　chalk；　or　with　a　pencil；）　will　show　how　far　the　new　back　is　to　be　brought　forward。

Through　f；　draw　the　line　g　h；　parallel　to　the　line　A　B；　and　this　line　g　h　will　show　the　direction　of　the　new　back；　or　the　ground　line　upon　which　it　is　to　be　built。

The　line　c　f　will　show　the　depth　of　the　new　Fire…place；　and　if　it　should　happen　that　c　f　is　equal　to　about　ONE…THIRD　of　the　line　A　B；　and　if　the　grate　can　be　accommodated　to　the　Fire…place　instead　of　its　being　necessary　to　accommodate　the　Fire…place　to　the　grate；　in　that　case；　half　the　length　of　the　line　c　f；　is　to　be　set　off　from　f　on　the　line　g　f　h；　on　one　side　to　k；　and　on　the　other　to　i；　and　the　line　i　k　will　show　the　ground　line　of　the　fore　part　of　the　back　of　the　Chimney。

In　all　cases　where　the　width　of　the　opening　of　the　Fire…place　in　front　（A　B）　happens　to　be　n