essays-1-第47部分

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very　ill　contrived　for　throwing　into　the　room　by　reflection　the　rays　from　the　fire　which　fall　on　them。

To　have　a　clear　and　perfect　idea　of　the　alterations　I　propose　in　the　forms　of　Fire…places；　the　reader　need　only　observe；　that；　whereas　the　backs　of　Fire…places；　as　they　are　now　commonly　constructed；　are　as　wide　as　the　opening　of　the　Fire…place　in　front；　and　the　sides　of　it　are　of　course　perpendicular　to　it；　and　parallel　to　each　other；in　the　Fire…places　I　recommend；　the　back　（i　k；　Fig。　3）　is　only　about　one…third　of　the　width　of　the　opening　of　the　Fire…place　in　front　（a；b）；　and　consequently　that　the　two　sides　of　covings　of　the　Fire…place　（a　i　and　b　k）；　instead　of　being　perpendicular　to　the　back；　are　inclined　to　it　at　an　angle　of　about　135　degrees；　and　in　consequence　of　this　position；　instead　of　being　parallel　to　each　other；　each　of　them　presents　an　oblique　front　towards　the　opening　of　the　Chimney；　by　means　of　which　the　rays　which　they　reflect　are　thrown　into　the　room。　A　bare　inspection　of　the　annexed　drawings　（Fig。　1。　and　Fig。　3。）　will　render　this　matter　perfectly　clear　and　intelligible。

In　regard　to　the　materials　which　it　will　be　most　advantageous　to　employ　in　the　construction　of　Fire…places；　so　much　light　has；　I　flatter　myself；　already　been　thrown　on　the　subject　we　are　investigating；　and　the　principles　adopted　have　been　established　on　such　clear　and　obvious　facts；　that　no　great　difficulty　will　attend　the　determination　of　that　point。As　the　object　in　view　is　to　bring　radiant　heat　into　the　room；　it　is　clear　that　that　material　is　best　for　the　construction　of　a　Fire…place　which　reflects　the　most；　or　which　ABSORBS　THE　LEAST　of　it；　for　that　heat　which　is　ABSORBED　cannot　be　REFLECTEDNow　as　bodies　which　absorb　radiant　heat　are　necessarily　heated　in　consequence　of　that　absorption；　to　discover　which　of　the　various　materials　that　can　be　employed　for　constructing　Fire…places　are　best　adapted　for　that　purpose；　we　have　only　to　find　out　by　an　experiment；　very　easy　to　be　made；　what　bodies　acquire　LEAST　HEAT　when　exposed　to　the　direct　rays　of　a　clear　fire；for　those　which　are　least　heated；　evidently　absorb　the　least；　and　consequently　reflect　the　most　radiant　heat。　　And　hence　it　appears　that　iron；　and；　in　general；　metals　of　all　kinds；　which　are　well　known　to　GROW　VERY　HOT　when　exposed　to　the　rays　projected　by　burning　fuel；　are　to　be　reckoned　among　the　VERY　WORST　materials　that　it　is　possible　to　employ　in　the　construction　of　Fire…places。

The　best　materials　I　have　hitherto　been　able　to　discover　are　fire…stone；　and　common　bricks　and　mortar。　　Both　these　materials　are；　fortunately；　very　cheap；　and　as　to　their　comparative　merits；　I　hardly　know　to　which　of　them　the　preference　ought　to　be　given。

When　bricks　are　used　they　should　be　covered　with　a　thin　coating　of　plaster；　which；　when　it　is　become　perfectly　dry；　should　be　white…washed。　　The　fire…stone　should　likewise　be　white　washed；　when　that　is　used；　and　every　part　of　the　Fire…place；　which　is　not　exposed　to　being　soiled　and　made　black　by　the　smoke；　should　be　kept　as　white　and　clean　as　possible。　　As　WHITE　reflects　more　heat；　as　well　as　more　light　than　any　other　colour；　it　ought　always　to　be　preferred　for　the　inside　of　a　Chimney　Fire…place；　and　BLACK；　which　reflects　neither　light　nor　heat　should　be　most　avoided。

I　am　well　aware　how　much　the　opinion　I　have　have　ventured　to　give；　respecting　the　unfitness　of　iron　and　other　metals　to　be　employed　in　the　construction　of　open　Fire…places；　differs　from　the　opinion　generally　received　upon　that　subject；and　I　even　know　that　the　very　reason　which；　according　to　my　ideas　of　the　matter；　renders　them　totally　unfit　for　the　purpose；　is　commonly　assigned　for　making　use　of　them；　namely；　that　they　soon　grow　very　hot。　　But　I　would　beg　leave　to　ask　what　advantage　is　derived　from　heating　them？

I　have　shown　the　disadvantage　of　it；　namely；　that　the　quantity　of　radiant　heat　thrown　into　the　room　is　diminished；and　it　is　easy　to　show　that　almost　the　whole　of　that　absorbed　by　the　metal　is　ultimately　carried　up　the　Chimney　by　the　air；　which；　coming　into　contact　with　this　hot　metal；　is　heated　and　rarefied　by　it；　and　forcing　its　way　upwards；　goes　off　with　the　smoke；　and　as　no　current　of　air　ever　sets　from　any　part　of　the　opening　of　a　Fire…place　into　the　room；　it　is　impossible　to　conceive　how　the　heat　existing　in　the　metal　composing　any　part　of　the　apparatus　of　the　Fire…place；　and　situated　within　its　cavity；　can　come；　or　be　brought　into　the　room。

This　difficulty　may　be　in　part　removed；　by　supposing；　what　indeed　seems　to　be　true　in　a　certain　degree；　that　the　heated　metal　sends　off　rays；　the　heat　it　acquires　from　the　fire；　even　when　it　is　not　heated　red　hot；　but　still；　as　it　never　can　be　admitted　that　the　heat；　absorbed　by　the　metal　and　afterwards　thrown　off　by　it　in　rays；　is　INCREASED　by　this　operation；　nothing　can　be　gained　by　it；　and　as　much　must　necessary　be　lost　in　consequence　of　the　great　quantity　of　heat　communicated　by　the　hot　metal　to　the　air　in　contact　with　it；　which；　as　has　already　been　shown；　always　makes　its　way　up　the　Chimney；　and　flies　off　into　the　atmosphere；　the　loss　of　heat　attending　the　use　of　it　is　too　evident　to　require　being　farther　insisted　on。

There　is；　however；　in　Chimney　Fire…places　destined　for　burning　coals；　one　essential　part；　the　grate；　which　cannot　well　be　made　of　any　thing　else　but　iron；　but　there　is　no　necessity　whatever　for　that　immense　quantity　of　iron　which　surrounds　grates　as　they　are　now　commonly　constructed　and　fitted　up；　and　which　not　only　renders　them　very　expensive；　but　injures　very　essentially　the　Fire…place。　　If　it　should　be　necessary　to　diminish　the　opening　of　a　large　Chimney　in　order　to　prevent　its　smoking；　it　is　much　more　simple；　economical；　and　better　in　all　respects；　to　do　this　with　marble；　fire…stone；　or　even　with　bricks　and　mortar；　than　to　make　use　of　iron；　which；　as　has　already　been　shown；　is　the　very　worst　material　that　can　possibly　be　employed　for　that　purpose；　and　as　to　registers；　they　not　only　are　quite　unnecessary；　where　the　throat　of　a　Chimney　is　properly　constructed；　and　of　proper　dimensions；　but　in　that　case　would　do　much　harm。　　If　they　act　at　all；　it　must　be　by　opposing　their　flat　surfaces　to　the　current　of　rising　smoke　in　a　manner　which　cannot　fail　to　embarrass　and　impede　its　motion。　　But　we　have　shown　that　the　passage　of　the　smoke　through　the　throat　of　a　Chimney　ought　to　be　facilitated　as　much　as　possible；　in　order　that　it　may　be　enabled　to　pass　by　a　small　aperture。

Register…stoves　have　often　been　found　to　be　of　use；　but　it　is　because　the　great　fault　of　all　Fire…places　constructed　upon　the　common　principles　being　the　enormous　dimensions　of　the　throat　of　the　Chimney；　this　fault　has　been　in　some　measure　corrected　by　them；　but　I　will　venture　to　affirm；　that　there　never　was　a　Fire…place　so　corrected　that　would　not　have　been　much　more　improved；　and　with　infinitely　less　expence；　by　the　alterations　here　recommended；　and　which　will　be　more　particularly　explained　in　the　next　Chapter。


CHAPTER。　II。

　Practical　directions　designed　for　the　use　of　workmen；　showing　　　　how　they　are　to　proceed　in　making　the　alterations　necessary　to　　　　improve　chimney　fire…places；　and　effectually　to　cure　smoking　　　　chimnies。

All　Chimney　Fire…places；　without　exception；　whether　they　are　designed　for　burning　wood　or　coals；　and　even　those　which　do　not　smoke；　as　well　as　those　which　do；　may　be　greatly　improved　by　making　the　alterations　in　them　here　recommended；　for　it　is　by　no　means　MERELY　to　prevent　Chimnies　from　smoking　that　these　improvements　are　recommended；　but　it　is　also　to　make　them　better　in　all　other　respects　as　Fire…places；　and　when　the　alterations　proposed　are　properly　executed；　which　may　be　very　easily　be　done　with　the　assistance　of　the　following　plain　and　simple　directions；　the　Chimnies　will　never　fail　to　answer；　I　will　venture　to　say；　even　beyond　expectation。　　The　room　will　be　heated　much　more　equally　and　more　pleasantly　with　LESS　THAN　HALF　THE　FUEL　used　before；　the　fire　will　be　more　cheerful　and　more　agreeable；　and　the　general　appearance　of　the　Fire…place　more　neat　and　elegant；　and　the　Chimney　WILL　NEVER　SMOKE。

The　advantages　which　are　derived　from　mechanical　inventions　and　contrivances　are；　I　know；　frequently　accompanied　by　disadvantages　which　it　is　not　always　possible　to　avoid；　but　in　the　case　in　question；　I　can　say　with　truth；　that　I　know　of　no　disadvantage　whatever　that　attends　the　Fire…places　constructed　upon　the　principles　here　recommended。　But　to　proceed　in　giving　directions　for　the　construction　of　these　Fire…places。

That　what　I　have　to　offer　on　this　subject　may　be　the　more　easily　understood；　it　will　be　proper　to　begin　by　explaining　the　precise　meaning　of　all　those　technical　words　and　expressions　which　I　may　find　it　necessary　or　convenient　to　use。

By　the　THROAT　of　a　Chimney；　I　mean　the　lower　extremity　of　its　canal；　where　it　unites　with　the　upper　part　of　its　open　Fire…place。　This　throat　is　commonly　found　about　a　foot　above　the　level　of　the　lower　part　of　the　mantle；　and　it　is　sometimes　contracted　to　a　smaller　size　than　the　rest　of　the　canal　of　the　Chimney；　and　sometimes　not。

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　subjec