In　like　manner　when　two　fluids　of　any　kind；　of　different　densities；　come　into　contact；　or　are　mixed　with　each　other；　that　which　is　the　lightest　will　be　forced　upwards　by　that　which　is　the　heaviest。
  And　as　heat　rarefies　all　bodies；　fluids　as　well　as　solids；　air　as　well　as　water；　or　mercury；it　follows　that　two　portions　of　the　same　fluid；　at　different　temperatures；　being　brought　into　contact　with　each　other；　that　portion　which　is　the　hottest　being　more　rarefied　or　specifically　LIGHTER　than　that　which　is　colder；　must　be　forced　upwards　by　this　last。And　this　is　what　always　happens　in　fact。
  When　hot　water　and　cold　water　are　mixed；　the　hottest　part　of　the　mixture　will　be　found　to　be　at　the　surface　above；and　when　cold　air　is　admitted　into　a　warmed　room；　it　will　always　be　found　to　take　its　place　at　the　bottom　of　the　room；　the　warmer　air　being　in　part　expelled；　and　in　part　forced　upwards　to　the　top　of　the　room。
  Both　air　and　water　being　transparent　and　colourless　fluids；　their　internal　motions　are　not　easily　discovered　by　the　sight；　and　when　these　motions　are　very　slow；　they　make　no　impression　whatever　on　any　of　our　senses；　consequently　they　cannot　be　detected　by　us　without　the　aid　of　some　mechanical　contrivance：　But　where　we　have　reason　to　think　that　those　motions　exist；　means　should　be　sought；　and　may　often　be　found；　for　rendering　them　perceptible。
  If　a　bottle　containing　hot　water　tinged　with　log…wood；　or　any　other　colouring　drug；　be　immersed；　with　its　mouth　open；　and　upwards；　into　a　deep　glass　jar　filled　with　cold　water；　the　ascent　of　the　hot　water　from　the　bottle　through　the　mass　of　cold　water　will　be　perfectly　visible　through　the　glass。　Now　nothing　can　be　more　evident　than　that　both　of　these　fluids　are　forced；　or　PUSHED；　and　not　DRAWN　upwards。Smoke　is　frequently　said　to　be　drawn　up　the　Chimney；and　that　a　Chimney　draws　well；　or　ill；but　these　are　careless　expressions；　and　lead　to　very　erroneous　ideas　respecting　the　cause　of　the　ascent　of　Smoke；　and　consequently　tend　to　prevent　the　progress　of　improvements　in　the　management　of　fires。The　experiment　just　mentioned　with　the　coloured　water　is　very　striking　and　beautiful；　and　it　is　well　calculated　to　give　a　just　idea　of　the　cause　of　the　ascent　of　Smoke。　　The　cold　water　in　the　jar；　which；　in　consequence　of　its　superior　weight　or　density；　forces　the　heated　and　rarefied　water　in　the　bottle　to　give　place　to　it；　and　to　move　upwards　out　of　its　way；　may　represent　the　cold　air　of　the　atmosphere；　while　the　rising　column　of　coloured　water　will　represent　the　column　of　Smoke　which　ascends　from　a　fire。
  If　Smoke　required　a　Chimney　to　DRAW　it　upwards；　how　happens　it　that　Smoke　rises　from　a　fire　which　is　made　in　the　open　air；　where　there　is　no　Chimney？
  If　a　tube；　open　at　both　ends；　and　of　such　a　length　that　its　upper　end　be　below　the　surface　of　the　cold　water　in　the　jar；　be　held　vertically　over　the　mouth　of　the　bottle　which　contains　the　hot　coloured　water；　the　hot　water　will　rise　up　through　it；　just　a　smoke　rises　in　a　Chimney。
  If　the　tube　be　previously　heated　before　it　is　plunged　into　the　cold　water；　the　ascent　of　the　hot　coloured　water　will　be　facilitated　and　accelerated；　in　like　manner　as　Smoke　is　known　to　rise　with　greater　facility　in　a　Chimney　which　is　hot；　than　in　one　in　which　no　fire　has　been　made　for　a　long　time。But　in　neither　of　these　cases　can　it；　with　any　propriety；　be　said；　that　the　hot　water　is　DRAWN　up　the　tube。The　hotter　the　water　in　the　bottle　is；　and　the　colder　that　in　the　jar；　the　greater　will　be　the　velocity　with　which　the　hot　water　will　be　forced　up　through　the　tube；　and　the　same　holds　of　the　ascent　of　hot　Smoke　in　a　Chimney。When　the　fire　is　intense；　and　the　weather　very　cold；　the　ascent　of　the　Smoke　is　very　rapid；　and　under　such　circumstances　Chimneys　seldom　smoke。
  As　the　cold　water　of　the　jar　immediately　　surrounding　the　bottle　which　contains　the　hot　water；　will　be　heated　by　the　bottle　while　the　other　parts　of　the　water　in　the　jar　will　remain　cold；　this　water　so　heated；　becoming　specifically　lighter　than　that　which　surrounds　it；　will　be　forced　upwards；　and　if　it　finds　its　way　into　the　tube　will　rise　up　through　it　with　the　coloured　hot　water。The　warmed　air　of　a　room　heated　by　an　open　Chimney　Fire…place　has　always　a　tendency　to　rise；　（if　I　may　use　that　inaccurate　expression；）　and　finding　its　way　into　the　Chimney　frequently　goes　off　with　the　Smoke。
  What　has　been　said；　will；　I　flatter　myself；　be　sufficient　to　explain　and　illustrate；　in　a　clear　and　satisfactory　manner；　the　cause　of　the　ascent　of　Smoke；　and　just　ideas　upon　that　subject　are　absolutely　necessary　in　order　to　judge；　with　certainty；　of　the　merit　of　any　scheme　proposed　for　the　improvement　of　Fire…places；　or　to　take　effectual　measures；　in　all　cases；　for　curing　smoking　Chimnies。For　though　the　perpetual　changes　and　alterations　which　are　produced　by　accident；　whim；　and　caprice；　do　sometimes　lead　to　useful　discoveries；　yet　the　progress　of　improvement　under　such　guidance　must　be　exceedingly　slow；　fluctuating；　and　uncertain。
  As　to　the　causes　of　the　smoking　of　Chimnies；　they　are　very　numerous；　and　various；　but　as　a　general　idea　of　them　may　be　acquired　from　what　has　already　been　said　upon　that　subject　in　various　parts　of　this　Essay；　and　as　they　may；　in　all　cases；　（a　very　few　only　excepted；）　be　completely　remedied　by　making　the　alterations　in　Fire…places　here　pointed　out；　I　do　not　think　it　necessary　to　enumerate　them　all　in　this　place；　or　to　enter　into　those　long　details　and　investigations　which　would　be　required　to　show　the　precise　manner　in　which　each　of　them　operates；　either　alone；　or　in　conjunction　with　others。
  There　is　however　one　cause　of　smoking　Chimnies　which　I　think　it　is　necessary　to　mention　more　particularly。In　modern　built　houses；　where　the　doors　and　windows　are　generally　made　to　close　with　such　accuracy　that　no　crevice　is　left　for　the　passage　of　the　air　from　without；　the　Chimnies　in　rooms　adjoining　to　each　other；　or　connected　by　close　passages；　are　frequently　found　to　affect　each　other；　and　this　is　easy　to　be　accounted　for。When　there　is　a　fire　burning　in　one　of　the　Chimnies；　as　the　air　necessary　to　supply　the　current　up　the　Chimney　where　the　fire　burns　cannot　be　had　in　sufficient　quantities　from　without；　through　the　very　small　crevices　of　the　doors　and　windows；　the　air　in　the　room　becomes　rarefied；　not　by　heat；　but　by　subtraction　of　that　portion　of　air　which　is　employed　in　keeping　up　the　fire；　or　supporting　the　combustion　of　the　fuel；　and　in　consequence　of　this　rarefaction；　its　elasticity　is　diminished；　and　being　at　last　overcome　by　the　pressure　of　the　external　air　of　the　atmosphere；　this　external　air　rushes　into　the　room　by　the　only　passage　left　for　it；　namely；　by　the　open　Chimney　of　the　neighbouring　room：And　the　flow　of　air　into　the　Fire…place；　and　up　the　Chimney　where　the　fire　is　burning　being　constant；　this　expence　of　air　is　supplied　by　a　continued　current　down　the　other　Chimney。
  If　an　attempt　be　made　to　light　fires　in　both　Chimnies　at　the　same　time；　it　will　be　found　to　be　very　difficult　to　get　the　fires　to　burn；　and　the　rooms　will　both　be　filled　with　Smoke。
  One　of　the　fires；that　which　is　made　in　the　Chimney　where　the　construction　of　the　Fire…place　is　best　adapted　to　facilitate　the　ascent　of　the　Smoke；or　if　both　Fire…places　are　on　the　same　construction；that　which　has　the　wind　most　favourable；　or　in　which　the　fire　happens　to　be　soonest　kindled；will　overcome　the　other；　and　cause　its　Smoke　to　be　beat　back　into　the　room　by　the　cold　air　which　descends　through　the　Chimney。The　most　obvious　remedy　in　this　case　is　to　provide　for　the　supply　of　fresh　air　necessary　for　keeping　up　the　fires　by　opening　a　passage　for　the　external　air　into　the　room　by　a　shorter　road　than　down　one　of　the　Chimnies；　and　when　this　is　done；　both　Chimnies　will　be　found　to　be　effectually　cured。
  But　Chimnies　so　circumstanced　may　very　frequently　be　prevented　from　smoking　even　without　opening　any　new　passage　for　the　external　air；　merely　by　diminishing　the　draught；　（as　it　is　called；）　up　the　Chimnies；　which　can　best　be　done　by　altering　both　Fire…places　upon　the　principles　recommended　and　fully　explained　in　the　foregoing　Chapters　of　this　Essay。
  Should　the　doors　and　windows　of　a　room　be　closed　with　so　much　nicety　as　to　leave　no　crevices　by　which　a　supply　of　air　can　enter　sufficient　for　maintaining　the　fire；　AFTER　THE　CURRENT　OF　AIR　UP　THE　CHIMNEY　HAS　BEEN　DIMINISHED　AS　MUCH　AS　POSSIBLE　BY　DIMINISHING　THE　THROAT　OF　THE　FIRE…PLACE；　in　that　case　there　would　be　no　other　way　of　preventing　the　Chimney　from　smoking　but　by　opening　a　passage　for　the　admission　of　fresh　air　from　without；but　this；　I　believe；　will　very　seldom　be　found　to　be　the　case。
  A　case　more　frequently　to　be　met　with　is　where　currents　of　air　set　down　Chimnies　in　consequence　of　a　diminution　and　rarefaction　of　the　air　in　a　room；　occasioned　by　the　doors　of　the　room　opening　into　passages　or　courts　where　the　air　is　rarefied　by　the　action　of　some　particular　winds。　　In　such　cases　the　evil　may　be　remedied；　either　by　causing　the　doors　in　question　to　close　more　accurately；or；　（which　will　be　still　more　effectual；）　by　giving　a　supply　of　air　to　the　passage　or　court　which　wants　it；　by　some　other　way。
  Where　the　top　of　a　Chimney　is　commanded　by　high　buildings；　by　clifts；　or　by　high　ground