essays-1-第65部分

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cing　of　the　throat　of　a　Chimney　VERY　NEAR　the　burning　fuel。　In　introducing　the　improvements　proposed；　in　Chimnies　already　built；　there　can　be　no　question　in　regard　to　the　height　of　the　throat　of　the　Chimney；　for　its　place　will　be　determined　by　the　height　of　the　mantle。　　It　can　hardly　be　made　lower　than　the　mantle；　and　it　ought　always　to　be　brought　down　as　nearly　upon　the　level　with　the　bottom　of　it　as　possible。　　If　the　Chimney　is　apt　to　smoke；　it　will　sometimes　be　necessary　either　to　lower　the　mantle　or　to　diminish　the　height　of　the　opening　of　the　Fire…place；　by　throwing　over　a　flat　arch；　or　putting　in　a　straight　piece　of　stone　from　one　side　of　it　to　the　other；　or；　which　will　be　still　more　simple　and　easy　in　practice；　building　a　wall　of　bricks；　supported　by　a　flat　bar　of　iron；　immediately　under　the　mantle。

Nothing　is　so　effectual　to　prevent　Chimnies　from　smoking　as　diminishing　the　opening　of　the　Fire…place　in　the　manner　here　described；　and　lowering　and　diminishing　the　throat　of　the　Chimney；　and　I　have　always　found；　except　in　the　single　instance　already　mentioned；　that　a　perfect　cure　may　be　effected　by　THESE　MEANS　ALONE；　even　in　the　most　desperate　cases。　It　is　true；　that　when　the　construction　of　the　Chimney　is　very　bad　indeed；　or　its　situation　very　unfavourable　to　the　ascent　of　the　smoke；　and　especially　when　both　these　disadvantages　exist　at　the　same　time；　it　may　sometimes　be　necessary　to　diminish　the　opening　of　the　Fire…place；　and　particularly　to　lower　it；　and　also　to　lower　the　throat　of　the　Chimney；　more　than　might　be　wished：　but　still　I　think　this　can　produce　no　inconveniences　to　be　compared　with　that　greatest　of　all　plagues；　a　smoking　Chimney。

The　position　of　the　throat　of　a　Chimney　being　determined；　the　next　points　to　be　ascertained　are　its　size　and　form；　and　the　manner　in　which　it　ought　to　be　connected　with　the　Fire…place　below；　and　with　the　open　canal　of　the　Chimney　above。

But　as　these　investigations　are　intimately　connected　with　those　which　relate　to　the　form　proper　to　be　given　to　the　Fire…place　itself；　we　must　consider　them　all　together。

That　these　inquiries　may　be　pursued　with　due　method；　and　that　the　conclusions　drawn　from　them　may　be　clear　and　satisfactory；　it　will　be　necessary　to　consider；　first；　what　the　objects　are　which　ought　principally　to　be　had　in　view　in　the　construction　of　a　Fire…place；　and　secondly；　to　see　how　these　objects　can　best　be　attained。

Now　the　design　of　a　Chimney　Fire　being　simply　to　warm　a　room；　it　is　necessary；　first　of　all；　to　contrive　matters　so　that　the　room　shall　be　actually　warmed；　secondly；　that　it　be　warmed　with　the　smallest　expence　of　fuel　possible；　and；　thirdly；　that　in　warming　it；　the　air　of　the　room　be　preserved　perfectly　pure；　and　fit　for　respiration；　and　free　from　smoke　and　all　disagreeable　smells。

In　order　to　take　measures　with　certainty　for　warming　a　room　by　means　of　an　open　Chimney　Fire；　it　will　be　necessary　to　consider　HOW；　or　in　WHAT　MANNER；　such　a　Fire　communicates　heat　to　a　room。　This　question　may　perhaps；　at　the　first　view　of　it；　appear　to　be　superfluous　and　trifling；　but　a　more　careful　examination　of　the　matter　will　show　it　to　be　highly　deserving　of　the　most　attentive　investigation。

To　determine　in　what　manner　a　room　is　heated　by　an　open　Chimney　Fire；　it　will　be　necessary　first　of　all　to　find　out；　UNDER　WHAT　FORM　the　heat　generated　in　the　combustion　of　the　fuel　exists；　and　then　to　see　how　it　is　communicated　to　those　bodies　which　are　heated　by　it。

In　regard　to　the　first　of　these　subjects　of　inquiry；　it　is　quite　certain　that　the　heat　which　is　generated　in　the　combustion　of　the　fuel　exists　under　TWO　perfectly　distinct　and　very　different　forms。　One　part　of　it　is　COMBINED　with　the　smoke；　vapour；　and　heated　air　which　rise　from　the　burning　fuel；　and　goes　off　with　them　into　the　upper　regions　of　the　atmosphere；　while　the　other　part；　which　appears　to　be　UNCOMBINED；　or；　as　some　ingenious　philosophers　have　supposed；　combined　only　with　light；　is　sent　off　from　the　fire　in　rays　in　all　possible　directions。

With　respect　to　the　second　subject　of　inquiry；　namely；　how　this　heat；　existing　under　these　two　different　forms；　is　communicated　to　other　bodies；　it　is　highly　probable　that　the　combined　heat　can　only　be　communicated　to　other　bodies　by　ACTUAL　CONTACT　with　the　body　with　which　it　is　combined；　and　with　regard　to　the　rays　which　are　sent　off　by　burning　fuel；　it　is　certain　that　THEY　communicate　or　generate　heat　only　WHEN　and　WHERE　they　are　stopped　or　absorbed。　　In　passing　through　air；　which　is　transparent；　they　certainly　do　not　communicate　any　heat　to　it；　and　it　seems　highly　probable　that　they　do　not　communicate　heat　to　solid　bodies　by　which　they　are　reflected。

In　these　respects　they　seem　to　bear　a　great　resemblance　to　the　solar　rays。　　But　in　order　not　to　distract　the　attention　of　my　reader；　or　carry　him　too　far　away　from　the　subject　more　immediately　under　consideration；　I　must　not　enter　too　deeply　into　these　inquiries　respecting　the　nature　and　properties　of　what　has　been　called　RADIANT　HEAT。　　It　is　certainly　a　most　curious　subject　of　philosophical　investigation；　but　more　time　would　be　required　to　do　it　justice　than　we　now　have　to　spare。　　We　must　therefore　content　ourselves　with　such　a　partial　examination　of　it　as　will　be　sufficient　for　our　present　purpose。

A　question　which　naturally　presents　itself　here　is。　What　proportion　does　the　radiant　heat　bear　to　the　combined　heat？Though　that　point　has　not　yet　been　determined　with　any　considerable　degree　of　precision；　it　is；　however；　quite　certain；　that　the　quantity　of　heat　which　goes　off　combined　with　the　smoke；　vapour；　and　heated　air　is　much　more　considerable；　perhaps　three　of　four　times　greater　at　least；　than　that　which　is　sent　off　from　the　fire　in　rays。And　yet；　small　as　the　quantity　is　of　this　radiant　heat；　it　is　the　only　part　of　the　heat　generated　in　the　combustion　of　fuel　burnt　in　an　open　Fire…place　which　is　ever　employed；　or　which　can　ever　be　employed；　in　heating　a　room。

The　whole　of　the　combined　heat　escapes　by　the　Chimney；　and　is　totally　lost；　and；　indeed；　no　part　of　it　could　ever　be　brought　into　a　room　from　an　open　Fire…place；　without　bringing　along　with　it　the　smoke　with　which　it　is　combined；　which；　of　course；　would　render　it　impossible　for　the　room　to　be　inhabited。　　There　is；　however；　one　method　by　which　combining　heat；　and　even　that　which　arises　from　an　open　Fire…place；　may　be　made　to　assist　in　warming　a　room；　and　that　is　by　making　it　pass　through　something　analogous　to　a　German　stove；　placed　in　the　Chimney　above　the　fire。But　of　this　contrivance　I　shall　take　occasion　to　treat　more　fully　hereafter；　in　the　mean　time　I　shall　continue　to　investigate　the　properties　of　open　Chimney　Fire…places；　constructed　upon　the　most　simple　principles；　such　as　are　now　in　common　use；　and　shall　endeavour　to　point　out　and　explain　all　those　improvements　of　which　THEY　appear　to　me　to　be　capable。　　When　fuel　is　burnt　in　Fire…places　upon　this　simple　construction；　where　the　smoke　escapes　immediately　by　the　open　canal　of　the　Chimney；　it　is　quite　evident　that　all　the　combined　heat　must　of　necessity　be　lost；　and　as　it　is　the　radiant　heat　alone　which　can　be　employed　in　heating　a　room；　it　becomes　an　object　of　much　importance　to　determine　how　t