essays-1-第42章

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erfluous　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　the　greatest　quantity　of　it　may　be　generated　in　the　combustion　of　the　fuel；　and　how　the　greatest　proportion　possible　of　that　generated　may　be　brought　into　the　room。

Now　the　quantity　of　radiant　heat　generated　in　the　combustion　of　a　given　quantity　of　any　kind　of　fuel　depends　very　much　upon　the　management　of　the　fire；　or　upon　the　manner　in　which　the　fuel　is　consumed。　　When　the　fire　burns　bright；　much　radiant　heat　will　be　sent　off　from　it；　but　when　it　is　SMOTHERED　UP；　very　little　will　be　generated；　and　indeed　very　little　combined　heat；　that　can　be　employed　to　any　useful　purpose：　most　of　the　heat　produced　will　be　immediately　EXPENDED　in　giving　elasticity　to　a　thick　dense　vapour　or　smoke　which　will　be　seen　rising　from　the　fire；　　and　the　combustion　being　very　incomplete；　a　great　part　of　the　inflammable　matter　of　the　fuel　being　merely　rarefied　and　driven　up　the　Chimney　without　being　inflamed；　the　fuel　will　be　wasted　to　little　purpose。　　And　hence　it　appears　of　how　much　importance　it　is；　whether　it　be　considered　with　a　view　to　economy；　or　to　cleanliness；　comfort；　and　elegance；　to　pay　due　attention　to　the　management　of　a　Chimney　Fire。

Nothing　can　be　more　perfectly　void　of　common　sense；　and　wasteful　and　slovenly　at　the　same　time；　than　the　manner　in　which　Chimney　Fires；　and　particularly　where　coals　are　burned；　are　commonly　managed　by　servants。　　They　throw　on　a　load　of　coals　at　once；　through　which　the　flame　is　hours　in　making　its　way；　and　frequently　it　is　not　without　much　trouble　that　the　fire　is　prevented　from　going　quite　out。　　During　this　time　no　heat　is　communicated　to　the　room；　and　what　is　still　worse；　the　throat　of　the　Chimney　being　occupied　merely　by　a　heavy　dense　vapour；　not　possessed　of　any　considerable　degree　of　heat；　and　consequently　not　having　much　elasticity；　the　warm　air　of　the　room　finds　less　difficulty　in　forcing　its　way　up　the　Chimney　and　escaping；　than　when　the　fire　burns　bright；and　it　happens　not　unfrequently；　especially　in　Chimneys　and　Fire…places　ill　constructed；　that　this　current　of　warm　air　from　the　room　which　presses　into　the　Chimney；　crossing　upon　the　current　of　heavy　smoke　which　rises　slowly　from　the　fire；　obstructs　it　in　its　ascent；　and　beats　it　back　into　the　room；　hence　it　is　that　Chimnies　so　often　smoke　when　too　large　a　quantity　of　fresh　coals　is　put　upon　the　fire。　　So　many　coals　should　never　be　put　on　the　fire　at　once　as　to　prevent　the　free　passage　of　the　flame　between　them。　　In　short；　a　fire　should　never　be　smothered；　and　when　proper　attention　is　paid　to　the　quantity　of　coals　put　on；　there　will　be　very　little　use　for　the　poker；　and　this　circumstance　will　contribute　very　much　to　cleanliness；　and　to　the　preservation　of　furniture。

Those　who　have　feeling　enough　to　be　made　miserable　by　any　thing　careless；　slovenly；　and　wasteful　which　happens　under　their　eyes；who　know　what　comfort　is；　and　consequence　are　worthy　of　the　enjoyments　of　a　CLEAN　HEARTH　and　a　CHEERFUL　FIRE；　should　really　either　take　the　trouble　themselves　to　manage　their　fires；　（which；　indeed；　would　rather　be　an　amusement　to　them　than　a　trouble；）　or　they　should　instruct　their　servants　to　manage　them　better。

But　to　return　to　the　subject　more　immediately　under　consideration。　As　we　have　seen　what　is　necessary　to　the　production　or　generation　of　radiant　heat；　it　remains　to　determine　how　the　greatest　proportion　of　that　generated　and　sent　off　from　the　fire　in　all　directions　may　be　made　to　enter　the　room；　and　assist　in　warming　it。　　How　as　the　rays　which　are　thrown　off　from　burning　fuel　have　this　property　in　common　with　light；　that　they　generate　heat　only　WHEN　and　WHERE　they　are　stopped　or　absorbed；　and　also　in　being　capable　of　being　reflected　WITHOUT　GENERATING　at　the　surfaces　of　various　bodies；　the　knowledge　of　these　properties　will　enable　us　to　take　measures；　with　the　utmost　certainty；　for　producing　the　effect　required；that　is　to　say；　for　bringing　as　much　radiant　heat　as　possible　into　the　room。

This　must　be　done；　first；　by　causing　as　many　as　possible　of　the　rays；　as　they　are　sent　off　from　the　fire　in　straight　lines；　to　come　DIRECTLY　into　the　room；　which　can　only　be　effected　by　bringing　the　fire　as　far　forward　as　possible；　and　leaving　the　opening　of　the　Fire…place　as　wide　and　as　high　as　can　be　done　without　inconveniences；　and　secondly；　by　making　the　sides　and　back　of　the　Fire…place　of　such　form；　and　constructing　them　of　such　materials；　as　to　cause　the　direct　rays　from　the　fire；　which　strike　against　them；　to　be　sent　into　the　room　BY　REFLECTION　in　the　greatest　abundance。

Now　it　will　be　found；　upon　examination；　that　the　best　form　for　the　vertical　sides　of　a　Fire…place；　or　the　COVINGS；　（as　they　are　called；）　is　that　of　an　upright　plane；　making　an　angle　with　the　plane　of　the　back　of　the　Fire…place；　of　about　135　degrees。　According　to　the　present　construction　of　Chimnies　this　angle　is　90　degrees；　or　forms　a　right　angle；　but　as　in　this　case　the　two　sides　or　covings　of　the　Fire…place　（AC；　BD；　Fig。　1。）　are　parallel　to　each　other；　it　is　evident　that　they　are　very　ill　contrived　for　throwing　into　the　room　by　reflection　the　rays　from　the　fire　which　fall　on　them。

To　have　a　clear　and　perfect　idea　of　the　alterations　I　propose　in　the　forms　of　Fire…places；　the　reader　need　only　observe；　that；　whereas　the　backs　of　Fire…places；　as　they　are　now　commonly　constructed；　are　as　wide　as　the　opening　of　the　Fire…place　in　front；　and　the　sides　of　it　are　of　course　perpendicular　to　it；　and　parallel　to　each　other；in　the　Fire…places　I　recommend；　the　back　（i　k；　Fig。　3）　is　only　about　one…third　of　the　width　of　the　opening　of　the　Fire…place　in　front　（a；b）；　and　consequently　that　the　two　sides　of　covings　of　the　Fire…place　（a　i　and　b　k）；　instead　of　being　perpendicular　to　the　back；　are　inclined　to　it　at　an　angle　of　about　135　degrees；　and　in　consequence　of　this　position；　instead　of　being　parallel　to　each　other；　each　of　them　presents　an　oblique　front　towards　the　opening　of　the　Chimney；　by　means　of　which　the　rays　which　they　reflect　are　thrown　into　the　room。　A　bare　inspection　of　the　annexed　drawings　（Fig。　1。　and　Fig。　3。）　will　render　this　matter　perfectly　clear　and　intelligible。

In　regard　to　the　materials　which　it　will　be　most　advantageous　to　employ　in　the　construction　of　Fire…places；　so　much　light　has；　I　flatter　myself；　already　been　thrown　on　the　subject　we　are　investigating；　and　the　principles　adopted　have　been　established　on　such　clear　and　obvious　facts；　that　no　great　difficulty　will　attend　the　determination　of　that　point。As　the　object　in　view　is　to　bring　radiant　heat　into　the　room；　it　is　clear　that　that　material　is　best　for　the　construction　of　a　Fire…place　which　reflects　the　most；　or　which　ABSORBS　THE　LEAST　of　it；　for　that　heat　which　is　ABSORBED　cannot　be　REFLECTEDNow　as　bodies　which　absorb　radiant　heat　are　necessarily　heated　in　consequence　of　that　absorption；　to　discover　which　of　the　various　materials　that　can　be　employed　for　constructing　Fire…places　are　best　adapted　for　that　purpose；　we　have　only　to　find　out　by　an　experiment；　very　easy　to　be　made；　what　bodies　acquire　LEAST　HEAT　when　exposed　to　the　direct　rays　of　a　clear　fire；for　those　which　are　least　heated；　evidently　absorb　the　least；　and　c