essays-1-第69部分

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But　I　will　suppose　that　in　altering　such　a　Chimney　it　is　found　necessary；　in　order　to　accommodate　the　Fire…place　to　a　grate　or　stove　already　on　hand；　to　make　the　Fire…place　sixteen　inches　wide。　　In　that　case；　I　should　merely　increase　the　width　of　the　back；　to　the　dimensions　required；　without　altering　the　depth　of　the　Chimney；　or　increasing　the　width　of　the　opening　of　the　Chimney　in　front。　The　covings；　it　is　true；　would　be　somewhat　reduced　in　their　width；　by　this　alteration；　and　their　position　with　respect　to　the　plane　of　the　back　of　the　Chimney　would　be　a　little　changed；　but　these　alterations　would　produce　no　bad　effects　of　any　considerable　consequence；　and　would　be　much　less　likely　to　injure　the　Fire…place；　than　an　attempt　to　bring　the　proportions　of　its　parts　nearer　to　the　standard；　by　increasing　the　depth　of　the　Chimney；　and　the　width　of　its　opening　in　front；or　than　an　attempt　to　preserve　that　particular　obliquity　of　the　covings　which　is　recommended　as　the　best；　（135　degrees；）　by　increasing　the　width　of　the　opening　of　the　Fire…place；　without　increasing　its　depth。

In　order　to　illustrate　this　subject　more　fully；　we　will　suppose　one　case　more。We　will　suppose　that　in　the　Chimney　which　is　to　be　altered；　the　width　of　the　Fire…place　in　front　is　either　wider　or　narrower　than　it　ought　to　be；　in　order　that　the　different　parts　of　the　Fire…place；　after　it　is　altered；　may　be　of　the　proper　dimensions。　　In　this　case；　I　should　determine　the　depth　of　the　Fire…place；　and　the　width　of　the　back　of　it；　without　any　regard　to　the　width　of　the　opening　of　the　Fire…place　in　front；　and　when　this　is　done；　if　the　opening　of　Fire…place　should　be　only　two　or　three　inches　too　wide；　that　is　to　say；　only　two　or　three　inches　wider　than　is　necessary　in　order　that　the　covings　may　be　brought　into　their　proper　position　with　respect　to　the　back；　I　should　not　alter　the　width　of　this　opening；　but　should　accommodate　the　covings　to　this　width；　by　increasing　their　breadth；　and　increasing　the　angle　they　make　with　the　back　of　the　Fire…place；　but　if　the　opening　of　the　Fire…place　should　be　more　than　three　inches　too　wide；　I　should　reduce　it　to　the　proper　width　by　slips　of　stone；　or　by　bricks　and　mortar。

When　the　width　of　the　opening　of　the　Fire…place；　in　front；　is　very　great；　compared　with　the　depth　of　the　Fire…place；　and　with　the　width　of　the　back；　the　covings　in　that　case　being　very　wide；　and　consequently　very　oblique；　and　the　Fire…place　very　shallow；　any　sudden　motion　of　the　air　in　front　of　the　Fire…place；　（that　motion；　for　instance；　which　would　be　occasioned　by　the　clothes　of　a　woman　passing　hastily　before　the　fire；　and　very　near　it；）　would　be　apt　to　cause　eddies　in　the　air；　WITHIN　THE　OPENING　OF　THE　FIRE…PLACE；　by　which　puffs　of　smoke　might　easily　be　brought　into　the　room。　Should　the　opening　of　the　Chimney　be　too　narrow；　which　however　will　very　seldom　be　found　to　be　the　case；　it　will；　in　general；　be　advisable　to　let　it　remain　as　it　is；　and　to　accommodate　the　covings　to　it；　rather　to　attempt　to　increase　its　width；　which　would　be　attended　with　a　good　deal　of　trouble；　and　probably　a　considerable　expence。

From　all　that　has　been　said　it　is　evident；　that　the　points　of　the　greatest　importance；　and　which　ought　most　particularly　to　be　attended　to；　in　altering　Fire…places　upon　the　principles　here　recommended；　are；　the　bringing　forward　the　back　to　its　proper　place；　and　making　it　of　a　proper　width。But　it　is　time　that　I　should　mention　another　matter　upon　which　it　is　probable　that　my　reader　is　already　impatient　to　receive　information。Provision　must　be　made　for　the　passage　of　the　Chimney…sweeper　up　the　Chimney。This　may　easily　be　done　in　the　following　manner：　In　building　up　the　new　back　of　the　Fire…place；　when　this　wall；　（which　need　never　be　more　than　the　width　of　a　single　brick　in　thickness；）　is　brought　up　so　high　that　there　remains　no　more　than　about　ten　or　eleven　inches　between　what　is　then　the　top　of　it；　and　the　inside　of　the　mantle；　or　lower　extremity　of　the　breast　of　the　Chimney；　an　opening；　or　door…way；　eleven　or　twelve　inches　wide；　must　be　begun　in　the　middle　of　the　back；　and　continued　quite　to　the　top　of　it；　which；　according　to　the　height　to　which　it　will　commonly　be　necessary　to　carry　up　the　back；　will　make　the　opening　about　twelve　or　fourteen　inches　high；　which　will　be　quite　sufficient　to　allow　the　Chimney…sweeper　to　pass。　　When　the　Fire…place　is　finished；　this　door…way　is　to　be　closed　by　a　few　bricks；　by　a　tile；　or　a　fit　piece　of　stone；　placed　in　it；　dry；　or　without　mortar；　and　confined　in　its　place　by　means　of　a　rabbet　made　for　that　purpose　in　the　brick…work。As　often　as　the　Chimney　is　swept；　the　Chimney…sweeper　takes　down　this　temporary　wall；　which　is　very　easily　done；　and　when　he　has　finished　his　work；　he　puts　it　again　into　its　place。The　annexed　drawing　（No。　6。）　will　give　a　clear　idea　of　this　contrivance；　and　the　experience　I　have　had　of　it　has　proved　that　it　answers　perfectly　well　the　purpose　for　which　it　is　designed。

I　observed　above；　that　the　new　back；　which　it　will　always　be　found　necessary　to　build　in　order　to　bring　the　fire　sufficiently　forward；　in　altering　a　Chimney　constructed　on　the　common　principles；　need　never　be　thicker　than　the　width　of　a　common　brick。I　may　say　the　same　of　the　thickness　necessary　to　be　given　to　the　new　sides；　or　covings；　of　the　Chimney；　or　if　the　new　back　and　covings　are　constructed　of　stone；　one　inch　and　three　quarters；　or　two　inches　in　thickness　will　be　sufficient。Care　should　be　taken　in　building　up　these　new　walls　to　unite　the　back　to　the　covings　in　a　solid　manner。

Whether　the　new　back　and　covings　are　constructed　of　stone；　or　built　of　bricks；　the　space　between　them；　and　the　old　back　and　covings　of　the　Chimney　ought　to　be　filled　up；　to　give　greater　solidity　to　the　structure。This　may　be　done　with　loose　rubbish；　or　pieces　of　broken　bricks；　or　stones　provided　the　work　be　strengthened　by　a　few　layers　or　courses　of　bricks　laid　in　mortar；　but　it　will　be　indispensably　necessary　to　finish　the　work；　where　these　new　walls　end；　that　is　to　say；　at　the　top　of　the　throat　of　the　Chimney；　where　it　ends　abruptly　in　the　open　canal　of　the　Chimney　by　a　horizontal　course　of　bricks　well　secured　with　mortar。　This　course　of　bricks　will　be　upon　a　level　with　the　top　of　the　door…way　left　for　the　Chimney…sweeper。

From　these　descriptions　it　is　clear　that　where　the　throat　of　the　Chimney　has　an　end；　that　is　to　say；　where　it　enters　into　the　lower　part　of　the　open　canal　of　the　Chimney；　THERE　the　three　walls　which　form　the　two　covings　and　the　back　of　the　Fire…place　all　end　abruptly。It　is　of　much　importance　that　they　should　end　in　this　manner；　for　were　they　to　be　sloped　outward　and　raised　in　such　a　manner　as　to　swell　out　the　upper　extremity　of　the　throat　of　the　Chimney　in　the　form　of　a　trumpet；　and　increase　it　by　degrees　to　the　size　of　the　canal　of　the　Chimney；　this　manner　of　uniting　the　lower　extremity　of　the　canal　of　the　Chimney　with　the　throat　would　tend　to　assist　the　winds　which　may　attempt　to　blow　down　the　Chimney；　in　forcing　their　way　through　the　throat；　and　throwing　the　smoke　backward　into　the　room；　but　when　the　throat　of　the　Chimney　ends　abruptly；　and　the　ends　of　the　new　walls　form　a　flat　horizontal　surface；　it　will　be　much　more　difficult　for　any　w