meteorology-第2节

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air　and　the　water　that　surrounds　the　whole　earth…vapour　being　water



dissolved。



　　After　this　exposition　of　the　difficulties　involved；　let　us　go　on



to　lay　down　the　truth；　with　a　view　at　once　to　what　follows　and　to　what



has　already　been　said。　The　upper　region　as　far　as　the　moon　we　affirm



to　consist　of　a　body　distinct　both　from　fire　and　from　air；　but　varying



degree　of　purity　and　in　kind；　especially　towards　its　limit　on　the　side



of　the　air；　and　of　the　world　surrounding　the　earth。　Now　the　circular



motion　of　the　first　element　and　of　the　bodies　it　contains　dissolves；



and　inflames　by　its　motion；　whatever　part　of　the　lower　world　is



nearest　to　it；　and　so　generates　heat。　From　another　point　of　view　we



may　look　at　the　motion　as　follows。　The　body　that　lies　below　the



circular　motion　of　the　heavens　is；　in　a　sort；　matter；　and　is



potentially　hot；　cold；　dry；　moist；　and　possessed　of　whatever　other



qualities　are　derived　from　these。　But　it　actually　acquires　or



retains　one　of　these　in　virtue　of　motion　or　rest；　the　cause　and



principle　of　which　has　already　been　explained。　So　at　the　centre　and



round　it　we　get　earth　and　water；　the　heaviest　and　coldest　elements；　by



themselves；　round　them　and　contiguous　with　them；　air　and　what　we



commonly　call　fire。　It　is　not　really　fire；　for　fire　is　an　excess　of



heat　and　a　sort　of　ebullition；　but　in　reality；　of　what　we　call　air；



the　part　surrounding　the　earth　is　moist　and　warm；　because　it



contains　both　vapour　and　a　dry　exhalation　from　the　earth。　But　the　next



part；　above　that；　is　warm　and　dry。　For　vapour　is　naturally　moist　and



cold；　but　the　exhalation　warm　and　dry；　and　vapour　is　potentially



like　water；　the　exhalation　potentially　like　fire。　So　we　must　take



the　reason　why　clouds　are　not　formed　in　the　upper　region　to　be　this：



that　it　is　filled　not　with　mere　air　but　rather　with　a　sort　of　fire。



　　However；　it　may　well　be　that　the　formation　of　clouds　in　that　upper



region　is　also　prevented　by　the　circular　motion。　For　the　air　round　the



earth　is　necessarily　all　of　it　in　motion；　except　that　which　is　cut　off



inside　the　circumference　which　makes　the　earth　a　complete　sphere。　In



the　case　of　winds　it　is　actually　observable　that　they　originate　in



marshy　districts　of　the　earth；　and　they　do　not　seem　to　blow　above



the　level　of　the　highest　mountains。　It　is　the　revolution　of　the　heaven



which　carries　the　air　with　it　and　causes　its　circular　motion；　fire



being　continuous　with　the　upper　element　and　air　with　fire。　Thus　its



motion　is　a　second　reason　why　that　air　is　not　condensed　into　water。



　　But　whenever　a　particle　of　air　grows　heavy；　the　warmth　in　it　is



squeezed　out　into　the　upper　region　and　it　sinks；　and　other　particles



in　turn　are　carried　up　together　with　the　fiery　exhalation。　Thus　the



one　region　is　always　full　of　air　and　the　other　of　fire；　and　each　of



them　is　perpetually　in　a　state　of　change。



　　So　much　to　explain　why　clouds　are　not　formed　and　why　the　air　is



not　condensed　into　water；　and　what　account　must　be　given　of　the



space　between　the　stars　and　the　earth；　and　what　is　the　body　that　fills



it。



　　As　for　the　heat　derived　from　the　sun；　the　right　place　for　a



special　and　scientific　account　of　it　is　in　the　treatise　about　sense；



since　heat　is　an　affection　of　sense；　but　we　may　now　explain　how　it　can



be　produced　by　the　heavenly　bodies　which　are　not　themselves　hot。



　　We　see　that　motion　is　able　to　dissolve　and　inflame　the　air；



indeed；　moving　bodies　are　often　actually　found　to　melt。　Now　the



sun's　motion　alone　is　sufficient　to　account　for　the　origin　of



terrestrial　warmth　and　heat。　For　a　motion　that　is　to　have　this



effect　must　be　rapid　and　near；　and　that　of　the　stars　is　rapid　but



distant；　while　that　of　the　moon　is　near　but　slow；　whereas　the　sun's



motion　combines　both　conditions　in　a　sufficient　degree。　That　most　heat



should　be　generated　where　the　sun　is　present　is　easy　to　understand



if　we　consider　the　analogy　of　terrestrial　phenomena；　for　here；　too；　it



is　the　air　that　is　nearest　to　a　thing　in　rapid　motion　which　is



heated　most。　This　is　just　what　we　should　expect；　as　it　is　the



nearest　air　that　is　most　dissolved　by　the　motion　of　a　solid　body。



　　This　then　is　one　reason　why　heat　reaches　our　world。　Another　is



that　the　fire　surrounding　the　air　is　often　scattered　by　the　motion



of　the　heavens　and　driven　downwards　in　spite　of　itself。



　　Shooting…stars　further　suffix　to　prove　that　the　celestial　sphere



is　not　hot　or　fiery：　for　they　do　not　occur　in　that　upper　region　but



below：　yet　the　more　and　the　faster　a　thing　moves；　the　more　apt　it　is



to　take　fire。　Besides；　the　sun；　which　most　of　all　the　stars　is



considered　to　be　hot；　is　really　white　and　not　fiery　in　colour。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　4







　　Having　determined　these　principles　let　us　explain　the　cause　of　the



appearance　in　the　sky　of　burning　flames　and　of　shooting…stars；　and



of　'torches'；　and　'goats'；　as　some　people　call　them。　All　these



phenomena　are　one　and　the　same　thing；　and　are　due　to　the　same　cause；



the　difference　between　them　being　one　of　degree。



　　The　explanation　of　these　and　many　other　phenomena　is　this。　When



the　sun　warms　the　earth　the　evaporation　which　takes　place　is



necessarily　of　two　kinds；　not　of　one　only　as　some　think。　One　kind　is



rather　of　the　nature　of　vapour；　the　other　of　the　nature　of　a　windy



exhalation。　That　which　rises　from　the　moisture　contained　in　the



earth　and　on　its　surface　is　vapour；　while　that　rising　from　the　earth



itself；　which　is　dry；　is　like　smoke。　Of　these　the　windy　exhalation；



being　warm；　rises　above　the　moister　vapour；　which　is　heavy　and　sinks



below　the　other。　Hence　the　world　surrounding　the　earth　is　ordered　as



follows。　First　below　the　circular　motion　comes　the　warm　and　dry



element；　which　we　call　fire；　for　there　is　no　word　fully　adequate　to



every　state　of　the　fumid　evaporation：　but　we　must　use　this　terminology



since　this　element　is　the　most　inflammable　of　all　bodies。　Below　this



comes　air。　We　must　think　of　what　we　just　called　fire　as　being　spread



round　the　terrestrial　sphere　on　the　outside　like　a　kind　of　fuel；　so



that　a　little　motion　often　makes　it　burst　into　flame　just　as　smoke



does：　for　flame　is　the　ebullition　of　a　dry　exhalation。　So　whenever　the



circular　motion　stirs　this　stuff　up　in　any　way；　it　catches　fire　at　the



point　at　which　it　is　most　inflammable。　The　result　differs　according　to



the　disposition　and　quantity　of　the　combustible　material。　If　this　is



broad　and　long；　we　often　see　a　flame　burning　as　in　a　field　of　stubble：



if　it　burns　lengthwise　only；　we　see　what　are　called　'torches'　and



'goats'　and　shooting…stars。　Now　when　the　inflammable　material　is



longer　than　it　is　broad　sometimes　it　seems　to　throw　off　sparks　as　it



burns。　（This　happens　because　matter　catches　fire　at　the　sides　in　small



portions　but　continuously　with　the　main　body。）　Then　it　is　called　a



'goat'。　When　this　does　not　happen　it　is　a　'torch'。　But　if　the　whole



length　of　the　exhalation　is　scattered　in　small　parts　and　in　many



directions　and　in　breadth　and　depth　alike；　we　get　what　are　called



shooting…stars。



　　The　cause　of　these　shooting…stars　is　sometimes　the　motion　which



ignites　the　exhalation。　At　other　times　the　air　is　condensed　by　cold



and　squeezes　out　and　ejects　the　hot　element；　making　their　motion



look　more　like　that　of　a　thing　thrown　than　like　a　running　fire。　For



the　question　might　be　raised　whether　the　'shooting'　of　a　'star'　is　the



same　thing　as　when　you　put　an　exhalation　below　a　lamp　and　it　lights



the　lower　lamp　from　the　flame　above。　For　here　too　the　flame　passes



wonderfully　quickly　and　looks　like　a　thing　thrown；　and　not　as　if　one



thing　after　another　caught　fire。　Or　is　a　'star'　when　it　'shoots'　a



single　body　that　is　thrown？　Apparently　both　cases　occur：　sometimes



it　is　like　the　flame　from　the　lamp　and　sometimes　bodies　are



projected　by　being　squeezed　out　（like　fruit　stones　from　one's　fingers）



and　so　are　seen　to　fall　into　the　sea　and　on　the　dry　land；　both　by



night　and　by　day　when　the　sky　is　clear。　They　are　thrown　downwards



because　the　condensation　which　propels　them　inclines　downwards。



Thunderbolts　fall　downwards　for　the　same　reason：　their　origin　is　never



combustion　but　ejection　under　pressure；　since　naturally　all　heat　tends



upwards。



　　When　the　phenomenon　is　formed　in　the　upper　region　it　is　due　to　the



combustion　of　the　exhalation。　When　it　takes　place　at　a　lower　level



it　is　due　to　the　ejection　of　the　exhalation　by　the　condensing　and



cooling　of　the　moister　evaporation：　for　this　latter　as　it　condenses



and　inclines　downward　contracts；　and　thrusts　out　the　hot　element　and



causes　it　to　be　thrown　downwards。　The　motion　is　upwards　or　downwards



or　sideways　according　to　the　way　in　which　the　evaporation　lies；　and



its　disposition　in　respect　of　breadth　and　depth。　In　most　cases　the



dir