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…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