essays-1-第74部分

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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　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　thi