essays-1-第70部分

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　ends　of　the　new　walls　form　a　flat　horizontal　surface；　it　will　be　much　more　difficult　for　any　wind　from　above；　to　find；　and　force　its　way　through　the　narrow　passage　of　the　throat　of　the　Chimney。

As　the　two　walls　which　form　the　new　covings　of　the　Chimney　are　not　parallel　to　each　other；　but　inclined；　presenting　an　oblique　surface　towards　the　front　of　the　Chimney；　and　as　they　are　built　perfectly　upright　and　quite　flat；　from　the　hearth　to　the　top　of　the　throat；　where　they　end；　it　is　evident　that　an　horizontal　section　of　the　throat　will　not　be　an　oblong　square；　but　its　deviation　from　that　form　is　a　matter　of　no　consequence；　and　no　attempts　should　ever　be　made；　by　twisting　the　covings　above；　where　they　approach　the　breast　of　the　Chimney；　to　bring　it　to　that　form。All　twists；　bends；　prominences；　excavations；　and　other　irregularities　of　form；　in　the　covings　of　a　Chimney；　never　fail　to　produce　eddies　in　the　current　of　air　which　is　continually　passing　into；　and　through　an　open　Fire…place　in　which　a　fire　is　burning；and　all　such　eddies　disturb；　either　the　fire；　or　the　ascending　currents　of　smoke；　or　both；　and　not　unfrequently　cause　the　smoke　to　be　thrown　back　into　the　room。Hence　it　appears；　that　the　covings　of　Chimneys　should　never　be　made　circular；　or　in　the　form　of　any　other　curve；　but　always　quite　flat。

For　the　same　reason；　that　is　to　say；　to　prevent　eddies；　the　breast　of　the　Chimney；　which　forms　that　side　of　the　throat　that　is　in　front；　or　nearest　to　the　room；　should　be　nearly　cleaned　off；　and　its　surface　made　quite　regular　and　smooth。

This　may　easily　be　done　by　covering　it　with　a　coat　of　plaster；　which　may　be　made　thicker　or　thinner　in　different　parts　as　may　be　necessary　in　order　to　bring　the　breast　of　the　Chimney　to　be　of　the　proper　form。

With　regard　to　the　form　of　the　breast　of　a　Chimney；　this　is　a　matter　of　very　great　importance；　and　which　ought　always　to　be　particularly　attended　to。The　worst　form　it　can　have　is　that　of　a　vertical　plane；　or　upright　flat；and　next　to　this　the　worst　form　is　an　inclined　plane。Both　these　forms　cause　the　current　of　warm　air　from　the　room；　which　will；　in　spite　of　every　precaution；　sometimes　find　its　way　into　the　Chimney；　to　cross　upon　the　current　of　smoke；　which　rises　from　the　fire；　in　a　manner　most　likely　to　embarrass　it　in　its　ascent；　and　drive　it　back。　The　inclined　plane　which　is　formed　by　a　flat　register　placed　in　the　throat　of　a　Chimney　produces　the　same　effects；　and　this　is　one　reason；　among　many　others；　which　have　induced　me　to　disapprove　of　register　stoves。

The　current　of　air；　which；　passing　under　the　mantle；　gets　into　the　Chimney；　should　be　made　GRADUALLY　TO　BEND　ITS　COURSE　UPWARDS；　by　which　means　it　will　be　QUIETLY　with　the　ascending　current　of　smoke；　and　will　be　less　likely　to　check　it；　or　force　it　back　into　the　room。Now　this　may　be　effected　with　the　greatest　ease　and　certainty；　merely　by　ROUNDING　OFF　the　breast　of　the　Chimney　or　back　part　of　the　mantle；　instead　of　leaving　it　flat；　or　full　of　holes　and　corners；　and　this　of　course　ought　always　to　be　done。

I　have　hitherto　given　no　precise　directions　in　regard　to　the　height　to　which　the　new　back　and　covings　ought　to　be　carried：　This　will　depend　not　only　on　the　height　of　the　mantle；　but　also；　and　more　especially；　on　the　height　of　the　breast　of　the　Chimney；　or　of　that　part　of　the　Chimney　where　the　breast　ends　and　the　upright　canal　begins。The　back　and　covings　must　rise　a　few　inches；　five　or　six　for　instance；　higher　than　this　part；　otherwise　the　throat　of　the　Chimney　will　not　be　properly　formed：but　I　know　of　no　advantages　that　would　be　gained　by　carrying　them　up　still　higher。

I　mentioned　above；　that　the　space　between　the　walls　which　form　the　new　back　and　covings；　and　the　old　back　and　sides　of　the　Fire…place；　should　be　filled　up：but　this　must　not　be　understood　to　apply　to　the　space　between　the　wall　of　dry　bricks；　or　the　tile　which　closes　the　passage　for　the　Chimney…sweeper；　and　the　old　back　of　the　Chimney；　for　that　space　must　be　left　void；　otherwise；　though　this　tile　（which　at　most　will　not　be　more　than　two　inches　in　thickness；）　were　taken　away；　there　would　not　be　any　room　sufficient　for　him　to　pass。


In　forming　this　door…way；　the　best　method　of　proceeding　is　to　place　the　tile　or　flat　piece　of　stone　destined　for　closing　it；　in　its　proper　place；　and　to　build　round　it；　or　rather　by　the　sides　of　it；　taking　care　not　to　bring　any　mortar　near　it；　in　order　that　it　may　be　easily　removed　when　the　door…way　is　finished。With　regard　to　the　rabbet　which　should　be　made　in　the　door…way　to　receive　it　and　fix　it　more　firmly　in　its　place；　this　may　either　be　formed　at　the　same　time　when　the　door…way　is　built；　or　it　may　be　made　after　it　is　finished；　by　attaching　to　its　bottom　and　sides；　with　strong　mortar；　pieces　of　thin　roof　tiles。　　Such　as　are　about　half　an　inch　in　thickness　will　be　best　for　this　use；　if　they　are　thicker；　they　will　diminish　too　much　the　opening　of　the　door…way；　and　will　likewise　be　more　liable　to　be　torn　away　by　the　Chimney…sweeper　in　passing　up　and　down　the　Chimney。

It　will　hardly　be　necessary　for　me　to　add；　that　the　tile；　or　flat　stone；　or　wall　of　dry　bricks；　which　is　used　for　closing　up　the　door…way；　must　be　of　sufficient　height　to　reach　quite　up　to　a　level　with　the　top　of　the　walls　which　form　the　new　back　and　covings　of　the　Chimnies。

I　ought；　perhaps；　to　apologize　for　having　been　so　very　particular　in　these　description　and　explanations；　but　it　must　be　remembered　that　this　chapter　is　written　principally　for　the　information　of　those　who；　having　had　few　opportunities　of　employing　their　attention　in　abstruse　philosophical　researches；　are　not　sufficiently　practised　in　these　intricate　investigations；　to　seize；　with　facility；　new　ideas；and　consequently；　that　I　have　frequently　been　obliged　TO　LABOUR　to　make　myself　understood。

I　have　only　to　express　my　wishes　that　my　reader　may　not　be　more　FATIGUED　with　this　labour　than　I　have　been；for　we　shall　them　most　certainly　be　satisfied　with　each　other。But　to　return　once　more　to　the　charge。

There　is　one　important　circumstance　respecting　Chimney　Fire…places；　destined　for　burning　coals；　which　still　remains　to　be　farther　examined；and　that　is　the　Grate。

Although　there　are　few　grates　that　may　not　be　used　in　Chimneys　constructed　or　altered　upon　the　principles　here　recommended；　yet　they　are　not；　by　any　means；　all　equally　well　adapted　for　that　purpose。Those　whose　construction　is　the　most　simple；　and　which　of　course　are　the　cheapest；　are　beyond　comparison　the　best；　ON　ALL　ACCOUNTS。Nothing　being　wanted　in　these　Chimnies　but　merely　a　grate　for　containing　the　coals；　and　in　which　they　will　burn　with　a　clear　fire；and　all　additional　apparatus　being；　not　only　useless；　but　very　pernicious；　all　complicated　and　expensive　grates　should　be　laid　aside；　and　such　as　more　simple　substituted　in　the　room　of　them。And　in　the　choice　of　a　grate；　as　in　every　thing　else；　BEAUTY　and　ELEGANCE　may　easily　be　united　with　the　MOST　PERFECT　SIMPLICITY。Indeed　they　are　incompatible　with　every　thing　else。

In　placing　the　grate；　the　thing　principally　to　be　attended　to　is；　to　make　the　back　of　it　coincide　with　the　back　of　the　Fire…place；　but　as　many　of　the　grates　now　in　common　use　will　be　found　to　be　too　large；　when　the　Fire…pl