Were　the　new　coving　to　range　with　the　front　edge　of　the　old　coving　o；　the　obliquity　of　the　new　coving　would　commonly　be　too　great；or　the　angle　d　c　o　would　exceed　135　degrees；　WHICH　IT　NEVER　SHOULD　DO；or　at　least　never　by　more　than　a　very　few　degrees。
  No　inconvenience　of　any　importance　will　arise　from　making　the　obliquity　of　the　covings　LESS　than　what　is　here　recommended；　but　many　cannot　fail　to　be　produced　by　making　it　much　greater；　and　as　I　know　from　experience　that　workmen　are　very　apt　to　do　this；　I　have　thought　it　necessary　to　warn　them　particularly　against　it。
  Fig。　11。　shows　how　the　width　and　obliquity　of　the　covings　of　a　Chimney　are　to　be　accommodated　to　the　width　of　the　back；　and　to　the　opening　in　front　and　depth　of　the　Fire…place；　where　the　width　of　the　opening　of　the　Fire…place　is　less　than　three　times　the　width　of　the　new　back。　As　all　those　who　may　be　employed　in　altering　Chimneys　may　not；　perhaps；　known　how　to　set　off　an　angle　of　any　certain　numbers　of　degrees；or　may　not　have　at　hand　the　instruments　necessary　for　doing　it；I　shall　here　show　how　an　instrument　may　be　made　which　will　be　found　to　be　very　useful　in　laying　out　the　work　for　the　bricklayers。
  Upon　a　board　about　18　inches　wide　and　four　feet　long；　or　upon　the　floor　or　a　table；　draw　three　equal　squares　A；　B；　C；　fig。　12。　of　about　12　or　14　inches　each　side；　placed　in　a　strait　line；　and　touching　each　other。From　the　back　corner　c　of　the　center　square　B；　draw　a　diagonal　line　across　the　square　A；　to　its　outward　front　corner　f；　and　the　adjoining　angle　formed　by　the　lines　d　c　and　c　f　will　be　equal　to　135　degrees；the　angle　which　the　plane　of　the　back　of　a　Chimney　Fire…place　ought　to　make　with　the　plane　of　its　covings。And　a　bevel　m；　n；　being　made　to　this　angle　with　thin　slips　of　hard　wood；　this　little　instrument　will　be　found　to　be　very　useful　in　marking　out　on　the　hearth；　with　chalk；　the　plans　of　the　walls　which　are　to　form　the　covings　of　Fire…places。
  As　Chimneys　which　are　apt　to　smoke　will　require　the　covings　to　be　placed　less　obliquely　in　respect　to　the　back　than　others　which　have　not　that　defect；　it　would　be　convenient　to　be　provided　with　several　bevels；three　or　four；　for　instance；　forming　different　angles。That　already　described；　which　may　be　called　No。　1。　will　measure　the　obliquity　of　the　covings　when　the　Fire…place　can　be　made　of　the　most　perfect　form：another　No。　2。　may　be　made　to　a　smaller　angle；　d　c　e；and　another　No。　3。　for　Chimnies　which　are　very　apt　to　smoke　at　the　still　smaller　angle　d　c　i。Or　a　bevel　may　be　so　contrived；　by　means　of　a　joint；　and　an　arch；　properly　graduated；　as　to　serve　for　all　the　different　degrees　of　obliquity　which　it　may　ever　be　necessary　to　give　to　the　covings　of　Fire…places。
  Another　point　of　much　importance；　and　particularly　in　Chimneys　which　are　apt　to　smoke；　is　to　form　the　throat　of　the　Chimney　properly；　by　carrying　up　the　back　and　covings　to　a　proper　height。　　This；　workmen　are　apt　to　neglect　to　do；　probably　on　account　of　the　difficulty　they　find　in　working　where　the　opening　of　the　canal　of　the　Chimney　is　so　much　reduced。But　it　is　absolutely　necessary　that　these　walls　should　be　carried　up　five　or　six　inches　at　least　above　the　upper　part　of　the　breast　of　the　Chimney；　or　to　that　point　where　the　wall　which　forms　the　front　of　the　throat　begins　to　rise　perpendicularly。　If　the　workman　has　intelligence　enough　to　avail　himself　of　the　opening　which　is　formed　in　the　back　of　the　Fire…place　to　give　a　passage　to　the　Chimney…sweeper；　he　will　find　little　difficulty　in　finishing　his　work　in　a　proper　manner。
  In　placing　the　plumb…line　against　the　breast　of　the　Chimney；　in　order　to　ascertain　how　far　the　new　back　is　to　be　brought　forward；　great　care　must　be　taken　to　place　it　at　the　very　top　of　the　breast；　where　the　canal　of　the　Chimney　BEGINS　TO　RISE　PERPENDICULARLY；　otherwise；　when　the　plumb…line　is　placed　too　low；　or　against　the　slope　of　the　breast；　when　the　new　back　comes　to　be　raised　to　its　proper　height；　the　throat　of　the　Chimney　will　found　to　be　too　narrow。
  Sometimes；　and　indeed　very　often　the　top　of　the　breast　of　a　Chimney　lies　very　high；　or　far　above　the　fire　（see　the　figures　13　and　14；　where　d　shows　the　top　of　the　breast　of　the　Chimney）；　when　this　is　the　case　it　must　be　brought　lower；　otherwise　the　Chimney　will　be　very　apt　to　smoke。So　much　has　been　said　in　the　First　Chapter　of　this　Essay　of　the　advantages　to　be　derived　from　bringing　the　throat　of　a　Chimney　near　to　the　burning　fuel；　that　I　do　not　think　it　necessary　to　enlarge　on　them　in　this　place；　taking　it　for　granted　that　the　utility　and　necessity　of　that　arrangement　have　already　been　made　sufficiently　evident；　but　a　few　directions　for　workmen；　to　show　them　how　the　breast　（and　consequently　the　throat）　of　a　Chimney　can　most　readily　be　lowered；　may　not　be　superfluous。
  Where　the　too　great　height　of　the　breast　of　a　Chimney　is　owing　to　the　great　height　of　the　mantle；　（see　fig。　13；）　or；　which　is　the　same　thing；　of　the　opening　of　the　Fire…place　in　front；　which　will　commonly　be　found　to　be　the　case；　the　only　remedy　for　the　evil　will　be　to　bring　down　the　mantle　lower；or　rather；　to　make　the　opening　of　the　Fire…place　in　front　lower；　by　throwing　across　the　top　of　this　opening；　from　one　jamb　to　the　other；　and　immediately　under　the　mantle；　a　very　flat　arch；a　wall　of　bricks　and　mortar；　supported　on　straight　bars　of　iron；or　a　piece　of　stone　（h；　fig。　13）。When　this　is　done；　the　slope　of　the　old　throat　of　the　Chimney；　or　of　the　back　side　of　the　mantle；　is　to　be　filled　up　with　plaster；　so　as　to　form　one　continued　flat；　vertical；　or　upright　plane　surface　with　the　lower　part　of　the　wall　of　the　canal　of　the　Chimney；　and　a　new　breast　is　to　be　formed　lower　down；　care　being　taken　to　round　it　off　properly；　and　make　it　finish　at　the　lower　surface　of　the　new　wall　built　under　the　mantle；which　wall　forms　in　fact　a　new　mantle。
  The　annexed　drawing　fig。　13；　which　represents　the　section　of　a　Chimney　in　which　the　breast　has　been　lowered　according　to　the　method　here　described；　will　show　these　various　alterations　in　a　clear　and　satisfactory　manner。　　In　this　figure；　as　well　as　in　most　of　the　others　in　this　Essay；　the　old　walls　are　distinguished　from　the　new　ones　by　the　manner　in　which　they　are　shaded；　the　old　walls　being　shaded　by　diagonal　lines；　and　the　new　ones　by　vertical　lines。　　The　additions；　which　are　formed　of　plaster；　are　shaded　by　dots　instead　of　lines。
  Where　the　too　great　height　of　the　breast　of　a　Chimney　is　occasioned；　not　by　the　height　of　the　mantle；　but　by　the　too　great　width　of　the　breast；　in　that　case；　（which　however　will　seldom　be　found　to　occur；）　this　defect　may　be　remedied　by　covering　the　lower　part　of　the　breast　with　a　thick　coating　of　plaster；　supported；　if　necessary；　by　nails　or　studs；　driven　into　the　wall　which　forms　the　breast；　and　properly　rounded　off　at　the　lower　part　of　the　mantle。See　fig。　14。
  CHAPTER。　III。
  Of　the　cause　of　the　ascent　of　smoke。　　Illustration　of　the　subject　by　familiar　comparisons　and　　　　experiments。　　Of　chimnies　which　affect　and　cause　each　other　to　smoke。　　Of　chimnies　which　smoke　from　want　of　air。　　Of　the　eddies　of　wind　which　sometimes　blow　down　chimnies；　　　　and　cause　them　to　smoke。　　Explanation　of　the　figures。
  Though　it　was　my　wish　to　avoid　all　abstruse　philosophical　investigations　in　this　Essay；　yet　I　feel　that　it　is　necessary　to　say　a　few　words　upon　a　subject　generally　considered　as　difficult　to　be　explained；　which　is　too　intimately　connected　with　the　matter　under　consideration　to　be　passed　over　in　silence。　A　knowledge　of　the　cause　of　the　ascent　of　Smoke　being　indispensably　necessary　to　those　who　engage　in　the　improvement　of　Fire…places；　or　who　are　desirous　of　forming　just　ideas　relative　to　the　operations　of　fire；　and　the　management　of　heat；　I　shall　devote　a　few　pages　to　the　investigation　of　that　curious　and　interesting　subject。And　as　many　of　those　who　may　derive　advantage　from　these　inquiries　are　not　much　accustomed　to　philosophical　disquisitions；　and　would　not　readily　comprehend　either　the　language　or　the　diagrams　commonly　used　by　scientific　writers　to　explain　the　phaenomena　in　question；　I　shall　take　pains　to　express　myself　in　the　most　familiar　manner；　and　to　use　such　comparisons　for　illustration　as　may　easily　be　understood。
  If　small　leaden　bullets；　or　large　goose　shot；　be　mixed　with　peas；　and　the　whole　well　shaken　in　a　bushel；　the　shot　will　separate　from　the　peas；　and　will　take　its　place　at　the　bottom　of　the　bushel；　forcing　by　its　greater　weight　the　peas　which　are　lighter；　to　move　upwards；　contrary　to　their　natural　tendency；　and　take　their　places　above。
  If　water　and　linseed　oil；　which　is　lighter　than　water；　be　mixed　in　a　vessel　by　shaking　them　together；　upon　suffering　this　mixture　to　remain　quite；　the　water　will　descend　and　occupy　the　bottom　of　the　vessel；　and　the　oil；　being　forced　out　of　its　place　by　the　greater　pressure　downwards　of　the　heavier　liquid；　will　be　obliged　to　rise　and　swim　on　the　surface　of　the　water。
  If　a　bottle　containing　linseed　oil　be　plunged　in　water　with　its　mouth　upwards；　and　open；　the　oil　will　ascent　out　of　the　bottle；　and　passing　upwards　through　the　mass　of　water；　in　a　continued　stream；　will　spread　itself　over　its　surface。
  In　like　manner　when　two　fluids　of　any　kind；　of　different　densities；　come　into　contact；　or　are　mixed　w