essays-1-第47章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


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　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