meteorology-第14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！




but　winds　do　not。　For　earthquakes；　rains；　and　droughts　have　their



source　and　origin　inside　the　earth；　so　that　the　sun　is　not　equally



able　to　direct　all　the　evaporations　in　one　direction。　But　on　the



evaporations　in　the　air　the　sun　has　more　influence　so　that；　when



once　they　have　been　given　an　impulse　by　its　motion；　which　is



determined　by　its　various　positions；　they　flow　in　one　direction。



　　When　the　wind　is　present　in　sufficient　quantity　there　is　an



earthquake。　The　shocks　are　horizontal　like　a　tremor；　except



occasionally；　in　a　few　places；　where　they　act　vertically；　upwards　from



below；　like　a　throbbing。　It　is　the　vertical　direction　which　makes　this



kind　of　earthquake　so　rare。　The　motive　force　does　not　easily



accumulate　in　great　quantity　in　the　position　required；　since　the



surface　of　the　earth　secretes　far　more　of　the　evaporation　than　its



depths。　Wherever　an　earthquake　of　this　kind　does　occur　a　quantity　of



stones　comes　to　the　surface　of　the　earth　（as　when　you　throw　up



things　in　a　winnowing　fan）；　as　we　see　from　Sipylus　and　the



Phlegraean　plain　and　the　district　in　Liguria；　which　were　devastated　by



this　kind　of　earthquake。



　　Islands　in　the　middle　of　the　sea　are　less　exposed　to　earthquakes



than　those　near　land。　First；　the　volume　of　the　sea　cools　the



evaporations　and　overpowers　them　by　its　weight　and　so　crushes　them。



Then；　currents　and　not　shocks　are　produced　in　the　sea　by　the　action　of



the　winds。　Again；　it　is　so　extensive　that　evaporations　do　not



collect　in　it　but　issue　from　it；　and　these　draw　the　evaporations



from　the　earth　after　them。　Islands　near　the　continent　really　form　part



of　it：　the　intervening　sea　is　not　enough　to　make　any　difference；　but



those　in　the　open　sea　can　only　be　shaken　if　the　whole　of　the　sea



that　surrounds　them　is　shaken　too。



　　We　have　now　explained　earthquakes；　their　nature　and　cause；　and　the



most　important　of　the　circumstances　attendant　on　their　appearance。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　9







　　Let　us　go　on　to　explain　lightning　and　thunder；　and　further



whirlwind；　fire…wind；　and　thunderbolts：　for　the　cause　of　them　all　is



the　same。



　　As　we　have　said；　there　are　two　kinds　of　exhalation；　moist　and　dry；



and　the　atmosphere　contains　them　both　potentially。　It；　as　we　have　said



before；　condenses　into　cloud；　and　the　density　of　the　clouds　is　highest



at　their　upper　limit。　（For　they　must　be　denser　and　colder　on　the



side　where　the　heat　escapes　to　the　upper　region　and　leaves　them。



This　explains　why　hurricanes　and　thunderbolts　and　all　analogous



phenomena　move　downwards　in　spite　of　the　fact　that　everything　hot



has　a　natural　tendency　upwards。　Just　as　the　pips　that　we　squeeze



between　our　fingers　are　heavy　but　often　jump　upwards：　so　these



things　are　necessarily　squeezed　out　away　from　the　densest　part　of



the　cloud。）　Now　the　heat　that　escapes　disperses　to　the　up　region。



But　if　any　of　the　dry　exhalation　is　caught　in　the　process　as　the　air



cools；　it　is　squeezed　out　as　the　clouds　contract；　and　collides　in



its　rapid　course　with　the　neighbouring　clouds；　and　the　sound　of　this



collision　is　what　we　call　thunder。　This　collision　is　analogous；　to



compare　small　with　great；　to　the　sound　we　hear　in　a　flame　which　men



call　the　laughter　or　the　threat　of　Hephaestus　or　of　Hestia。　This



occurs　when　the　wood　dries　and　cracks　and　the　exhalation　rushes　on　the



flame　in　a　body。　So　in　the　clouds；　the　exhalation　is　projected　and　its



impact　on　dense　clouds　causes　thunder：　the　variety　of　the　sound　is　due



to　the　irregularity　of　the　clouds　and　the　hollows　that　intervene　where



their　density　is　interrupted。　This　then；　is　thunder；　and　this　its



cause。



　　It　usually　happens　that　the　exhalation　that　is　ejected　is　inflamed



and　burns　with　a　thin　and　faint　fire：　this　is　what　we　call



lightning；　where　we　see　as　it　were　the　exhalation　coloured　in　the



act　of　its　ejection。　It　comes　into　existence　after　the　collision　and



the　thunder；　though　we　see　it　earlier　because　sight　is　quicker　than



hearing。　The　rowing　of　triremes　illustrates　this：　the　oars　are　going



back　again　before　the　sound　of　their　striking　the　water　reaches　us。



　　However；　there　are　some　who　maintain　that　there　is　actually　fire



in　the　clouds。　Empedocles　says　that　it　consists　of　some　of　the　sun's



rays　which　are　intercepted：　Anaxagoras　that　it　is　part　of　the　upper



ether　（which　he　calls　fire）　which　has　descended　from　above。　Lightning；



then；　is　the　gleam　of　this　fire；　and　thunder　the　hissing　noise　of



its　extinction　in　the　cloud。



　　But　this　involves　the　view　that　lightning　actually　is　prior　to



thunder　and　does　not　merely　appear　to　be　so。　Again；　this



intercepting　of　the　fire　is　impossible　on　either　theory；　but



especially　it　is　said　to　be　drawn　down　from　the　upper　ether。　Some



reason　ought　to　be　given　why　that　which　naturally　ascends　should



descend；　and　why　it　should　not　always　do　so；　but　only　when　it　is



cloudy。　When　the　sky　is　clear　there　is　no　lightning：　to　say　that　there



is；　is　altogether　wanton。



　　The　view　that　the　heat　of　the　sun's　rays　intercepted　in　the　clouds



is　the　cause　of　these　phenomena　is　equally　unattractive：　this；　too；　is



a　most　careless　explanation。　Thunder；　lightning；　and　the　rest　must



have　a　separate　and　determinate　cause　assigned　to　them　on　which　they



ensue。　But　this　theory　does　nothing　of　the　sort。　It　is　like



supposing　that　water；　snow；　and　hail　existed　all　along　and　were



produced　when　the　time　came　and　not　generated　at　all；　as　if　the



atmosphere　brought　each　to　hand　out　of　its　stock　from　time　to　time。



They　are　concretions　in　the　same　way　as　thunder　and　lightning　are



discretions；　so　that　if　it　is　true　of　either　that　they　are　not



generated　but　pre…exist；　the　same　must　be　true　of　the　other。　Again；



how　can　any　distinction　be　made　about　the　intercepting　between　this



case　and　that　of　interception　in　denser　substances　such　as　water？



Water；　too；　is　heated　by　the　sun　and　by　fire：　yet　when　it　contracts



again　and　grows　cold　and　freezes　no　such　ejection　as　they　describe



occurs；　though　it　ought　on　their　the。　to　take　place　on　a　proportionate



scale。　Boiling　is　due　to　the　exhalation　generated　by　fire：　but　it　is



impossible　for　it　to　exist　in　the　water　beforehand；　and　besides　they



call　the　noise　'hissing'；　not　'boiling'。　But　hissing　is　really　boiling



on　a　small　scale：　for　when　that　which　is　brought　into　contact　with



moisture　and　is　in　process　of　being　extinguished　gets　the　better　of



it；　then　it　boils　and　makes　the　noise　in　question。　Some…Cleidemus　is



one　of　them…say　that　lightning　is　nothing　objective　but　merely　an



appearance。　They　compare　it　to　what　happens　when　you　strike　the　sea



with　a　rod　by　night　and　the　water　is　seen　to　shine。　They　say　that



the　moisture　in　the　cloud　is　beaten　about　in　the　same　way；　and　that



lightning　is　the　appearance　of　brightness　that　ensues。



　　This　theory　is　due　to　ignorance　of　the　theory　of　reflection；　which



is　the　real　cause　of　that　phenomenon。　The　water　appears　to　shine



when　struck　because　our　sight　is　reflected　from　it　to　some　bright



object：　hence　the　phenomenon　occurs　mainly　by　night：　the　appearance　is



not　seen　by　day　because　the　daylight　is　too　in；　tense　and　obscures　it。



　　These　are　the　theories　of　others　about　thunder　and　lightning：　some



maintaining　that　lightning　is　a　reflection；　the　others　that



lightning　is　fire　shining　through　the　cloud　and　thunder　its



extinction；　the　fire　not　being　generated　in　each　case　but　existing



beforehand。　We　say　that　the　same　stuff　is　wind　on　the　earth；　and



earthquake　under　it；　and　in　the　clouds　thunder。　The　essential



constituent　of　all　these　phenomena　is　the　same：　namely；　the　dry



exhalation。　If　it　flows　in　one　direction　it　is　wind；　in　another　it



causes　earthquakes；　in　the　clouds；　when　they　are　in　a　process　of



change　and　contract　and　condense　into　water；　it　is　ejected　and



causes　thunder　and　lightning　and　the　other　phenomena　of　the　same



nature。



　　So　much　for　thunder　and　lightning。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Book　III



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1







　　LET　us　explain　the　remaining　operations　of　this　secretion　in　the



same　way　as　we　have　treated　the　rest。　When　this　exhalation　is　secreted



in　small　and　scattered　quantities　and　frequently；　and　is　transitory；



and　its　constitution　rare；　it　gives　rise　to　thunder　and　lightning。　But



if　it　is　secreted　in　a　body　and　is　denser；　that　is；　less　rare；　we



get　a　hurricane。　The　fact　that　it　issues　in　body　explains　its



violence：　it　is　due　to　the　rapidity　of　the　secretion。　Now　when　this



secretion　issues　in　a　great　and　continuous　current　the　result



corresponds　to　what　we　get　when　the　opposite　development　takes　place



and　rain　and　a　quantity　of　water　are　produced。　As　far　as　the　matter



from　which　they　are　developed　goes　both　sets　of　phenomena　are　the



same。　As　soon　as　a　stimulus　to　the　development　of　either



potentiality　appears；　that　of　which　there　is　the　greater　quantity



present　in　the　cloud　is　at　once　secreted　from　it；　and　there　results



either　rain；　or；　if　the　other　exhalation　prevails；　a　hurricane。



　　Sometimes　the　exhalation　in　the　cloud；　when　it　is　being　secreted；



collides　with　another　under　circumstances　like　those　found　when　a　wind



is　forced　from　an　open　into　a　narrow　space　in　a　gateway　or　a　road。



It　often　happens　in　such　cases　that　the　first　part　of　the　moving



body　is　deflected　because　of　the　resistance　due　either　to　the



narrowness　or　to　a　contrary　current；　and　so　the　wind　forms　a　circle



and　eddy。　It　is　prevented　from　advancing　in　a　straight　line：　at　the



same　time　it　is　pushed　on　from　behind；　so　it　is　compelled　to　move



sideways　in　the　direction　of　least　resistance。　The　same　thing



happens　to　the　next　part；　and　the　next；　and　so　on；　till　the　series



becomes　one；　that　is；　till　a　circle　is　formed：　for　if　a　figure　is



described　by　a　single　motion　that　figure　must　itself　be　one。　This　is



how　eddies　are　generated　on　the　earth；　and　the　case　is　the　same　in　the



clouds　as　far　as　the　beginning　of　them　goes。　Only　here　（as　in　the　case



of　the　hurricane　which　shakes　off　the　cloud　without　cessation　and



becomes　a　continuous　wind）　the　cloud　follows　the　exhalation



unbroken；　and　the　exhalation；　failing　to　break　away　from　the　cloud



because　of　its　density；　first　moves　in　a　circle　for　the　reason　given



and　then　descends；　because　clouds　are　always　densest　on　the　side　where



the　heat　escapes。　This　phenomenon　is　called　a　whirlwind　when　it　is



colourless；　and　it　is　a　sort　of　undigested　hurricane。　There　is　never　a



whirlwind　when　the　weather　is　northerly；　nor　a　hurricane　when　there　is



snow。　The　reason　is　that　all　these　phenomena　are　'wind'；　and　wind　is　a



dry　and　warm　evaporation。　Now　frost　and　cold　prevail　over　this



principle　and　quench　it　at　its　birth：　that　they　d