meteorology-第3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！




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



direction　is　sideways　because　two　motions　are　involved；　a　compulsory



motion　downwards　and　a　natural　motion　upwards；　and　under　these



circumstances　an　object　always　moves　obliquely。　Hence　the　motion　of



'shooting…stars'　is　generally　oblique。



　　So　the　material　cause　of　all　these　phenomena　is　the　exhalation；



the　efficient　cause　sometimes　the　upper　motion；　sometimes　the



contraction　and　condensation　of　the　air。　Further；　all　these　things



happen　below　the　moon。　This　is　shown　by　their　apparent　speed；　which　is



equal　to　that　of　things　thrown　by　us；　for　it　is　because　they　are　close



to　us；　that　these　latter　seem　far　to　exceed　in　speed　the　stars；　the



sun；　and　the　moon。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　5







　　Sometimes　on　a　fine　night　we　see　a　variety　of　appearances　that



form　in　the　sky：　'chasms'　for　instance　and　'trenches'　and　blood…red



colours。　These；　too；　have　the　same　cause。　For　we　have　seen　that　the



upper　air　condenses　into　an　inflammable　condition　and　that　the



combustion　sometimes　takes　on　the　appearance　of　a　burning　flame；



sometimes　that　of　moving　torches　and　stars。　So　it　is　not　surprising



that　this　same　air　when　condensing　should　assume　a　variety　of　colours。



For　a　weak　light　shining　through　a　dense　air；　and　the　air　when　it　acts



as　a　mirror；　will　cause　all　kinds　of　colours　to　appear；　but　especially



crimson　and　purple。　For　these　colours　generally　appear　when



fire…colour　and　white　are　combined　by　superposition。　Thus　on　a　hot



day；　or　through　a　smoky；　medium；　the　stars　when　they　rise　and　set　look



crimson。　The　light　will　also　create　colours　by　reflection　when　the



mirror　is　such　as　to　reflect　colour　only　and　not　shape。



　　These　appearances　do　not　persist　long；　because　the　condensation　of



the　air　is　transient。



　　'Chasms'　get　their　appearance　of　depth　from　light　breaking　out　of



a　dark　blue　or　black　mass　of　air。　When　the　process　of　condensation



goes　further　in　such　a　case　we　often　find　'torches'　ejected。　When



the　'chasm'　contracts　it　presents　the　appearance　of　a　'trench'。



　　In　general；　white　in　contrast　with　black　creates　a　variety　of



colours；　like　flame；　for　instance；　through　a　medium　of　smoke。　But　by



day　the　sun　obscures　them；　and；　with　the　exception　of　crimson；　the



colours　are　not　seen　at　night　because　they　are　dark。



　　These　then　must　be　taken　to　be　the　causes　of　'shooting…stars'　and



the　phenomena　of　combustion　and　also　of　the　other　transient



appearances　of　this　kind。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　6







　　Let　us　go　on　to　explain　the　nature　of　comets　and　the　'milky　way'；



after　a　preliminary　discussion　of　the　views　of　others。



　　Anaxagoras　and　Democritus　declare　that　comets　are　a　conjunction　of



the　planets　approaching　one　another　and　so　appearing　to　touch　one



another。



　　Some　of　the　Italians　called　Pythagoreans　say　that　the　comet　is　one



of　the　planets；　but　that　it　appears　at　great　intervals　of　time　and



only　rises　a　little　above　the　horizon。　This　is　the　case　with　Mercury



too；　because　it　only　rises　a　little　above　the　horizon　it　often　fails



to　be　seen　and　consequently　appears　at　great　intervals　of　time。



　　A　view　like　theirs　was　also　expressed　by　Hippocrates　of　Chios　and



his　pupil　Aeschylus。　Only　they　say　that　the　tail　does　not　belong　to



the　comet　iself；　but　is　occasionally　assumed　by　it　on　its　course　in



certain　situations；　when　our　sight　is　reflected　to　the　sun　from　the



moisture　attracted　by　the　comet。　It　appears　at　greater　intervals



than　the　other　stars　because　it　is　slowest　to　get　clear　of　the　sun　and



has　been　left　behind　by　the　sun　to　the　extent　of　the　whole　of　its



circle　before　it　reappears　at　the　same　point。　It　gets　clear　of　the　sun



both　towards　the　north　and　towards　the　south。　In　the　space　between　the



tropics　it　does　not　draw　water　to　itself　because　that　region　is



dried　up　by　the　sun　on　its　course。　When　it　moves　towards　the　south



it　has　no　lack　of　the　necessary　moisture；　but　because　the　segment　of



its　circle　which　is　above　the　horizon　is　small；　and　that　below　it　many



times　as　large；　it　is　impossible　for　the　sun　to　be　reflected　to　our



sight；　either　when　it　approaches　the　southern　tropic；　or　at　the　summer



solstice。　Hence　in　these　regions　it　does　not　develop　a　tail　at　all。



But　when　it　is　visible　in　the　north　it　assumes　a　tail　because　the



arc　above　the　horizon　is　large　and　that　below　it　small。　For　under



these　circumstances　there　is　nothing　to　prevent　our　vision　from



being　reflected　to　the　sun。



　　These　views　involve　impossibilities；　some　of　which　are　common　to　all



of　them；　while　others　are　peculiar　to　some　only。



　　This　is　the　case；　first；　with　those　who　say　that　the　comet　is　one　of



the　planets。　For　all　the　planets　appear　in　the　circle　of　the　zodiac；



whereas　many　comets　have　been　seen　outside　that　circle。　Again　more



comets　than　one　have　often　appeared　simultaneously。　Besides；　if



their　tail　is　due　to　reflection；　as　Aeschylus　and　Hippocrates　say；



this　planet　ought　sometimes　to　be　visible　without　a　tail　since；　as



they　it　does　not　possess　a　tail　in　every　place　in　which　it　appears。



But；　as　a　matter　of　fact；　no　planet　has　been　observed　besides　the



five。　And　all　of　them　are　often　visible　above　the　horizon　together



at　the　same　time。　Further；　comets　are　often　found　to　appear；　as　well



when　all　the　planets　are　visible　as　when　some　are　not；　but　are



obscured　by　the　neighbourhood　of　the　sun。　Moreover　the　statement



that　a　comet　only　appears　in　the　north；　with　the　sun　at　the　summer



solstice；　is　not　true　either。　The　great　comet　which　appeared　at　the



time　of　the　earthquake　in　Achaea　and　the　tidal　wave　rose　due　west；　and



many　have　been　known　to　appear　in　the　south。　Again　in　the　archonship



of　Euclees；　son　of　Molon；　at　Athens　there　appeared　a　comet　in　the



north　in　the　month　Gamelion；　the　sun　being　about　the　winter



solstic