meteorology-第6部分

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the　water　falls　in　a　certain　time　and　the　cold　is　sufficient　to　freeze



it　in　less；　there　is　no　difficulty　about　its　having　frozen　in　the　air；



provided　that　the　freezing　takes　place　in　a　shorter　time　than　its



fall。　The　nearer　to　the　earth；　and　the　more　suddenly；　this　process



takes　place；　the　more　violent　is　the　rain　that　results　and　the



larger　the　raindrops　and　the　hailstones　because　of　the　shortness　of



their　fall。　For　the　same　reason　large　raindrops　do　not　fall　thickly。



Hail　is　rarer　in　summer　than　in　spring　and　autumn；　though　commoner



than　in　winter；　because　the　air　is　drier　in　summer；　whereas　in



spring　it　is　still　moist；　and　in　autumn　it　is　beginning　to　grow　moist。



It　is　for　the　same　reason　that　hailstorms　sometimes　occur　in　the



late　summer　as　we　have　said。



　　The　fact　that　the　water　has　previously　been　warmed　contributes　to



its　freezing　quickly：　for　so　it　cools　sooner。　Hence　many　people；



when　they　want　to　cool　hot　water　quickly；　begin　by　putting　it　in　the



sun。　So　the　inhabitants　of　Pontus　when　they　encamp　on　the　ice　to



fish　（they　cut　a　hole　in　the　ice　and　then　fish）　pour　warm　water



round　their　reeds　that　it　may　freeze　the　quicker；　for　they　use　the　ice



like　lead　to　fix　the　reeds。　Now　it　is　in　hot　countries　and　seasons



that　the　water　which　forms　soon　grows　warm。



　　It　is　for　the　same　reason　that　rain　falls　in　summer　and　not　in



winter　in　Arabia　and　Ethiopia　too；　and　that　in　torrents　and　repeatedly



on　the　same　day。　For　the　concentration　or　recoil　due　to　the　extreme



heat　of　the　country　cools　the　clouds　quickly。



　　So　much　for　an　account　of　the　nature　and　causes　of　rain；　dew；



snow；　hoar…frost；　and　hail。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　13







　　Let　us　explain　the　nature　of　winds；　and　all　windy　vapours；　also　of



rivers　and　of　the　sea。　But　here；　too；　we　must　first　discuss　the



difficulties　involved：　for；　as　in　other　matters；　so　in　this　no



theory　has　been　handed　down　to　us　that　the　most　ordinary　man　could　not



have　thought　of。



　　Some　say　that　what　is　called　air；　when　it　is　in　motion　and　flows；　is



wind；　and　that　this　same　air　when　it　condenses　again　becomes　cloud　and



water；　implying　that　the　nature　of　wind　and　water　is　the　same。　So　they



define　wind　as　a　motion　of　the　air。　Hence　some；　wishing　to　say　a



clever　thing；　assert　that　all　the　winds　are　one　wind；　because　the



air　that　moves　is　in　fact　all　of　it　one　and　the　same；　they　maintain



that　the　winds　appear　to　differ　owing　to　the　region　from　which　the　air



may　happen　to　flow　on　each　occasion；　but　really　do　not　differ　at



all。　This　is　just　like　thinking　that　all　rivers　are　one　and　the　same



river；　and　the　ordinary　unscientific　view　is　better　than　a



scientific　theory　like　this。　If　all　rivers　flow　from　one　source；　and



the　same　is　true　in　the　case　of　the　winds；　there　might　be　some　truth



in　this　theory；　but　if　it　is　no　more　true　in　the　one　case　than　in



the　other；　this　ingenious　idea　is　plainly　false。　What　requires



investigation　is　this：　the　nature　of　wind　and　how　it　originates；　its



efficient　cause　and　whence　they　derive　their　source；　whether　one　ought



to　think　of　the　wind　as　issuing　from　a　sort　of　vessel　and　flowing



until　the　vessel　is　empty；　as　if　let　out　of　a　wineskin；　or；　as



painters　represent　the　winds；　as　drawing　their　source　from　themselves。



　　We　find　analogous　views　about　the　origin　of　rivers。　It　is　thought



that　the　water　is　raised　by　the　sun　and　descends　in　rain　and　gathers



below　the　earth　and　so　flows　from　a　great　reservoir；　all　the　rivers



from　one；　or　each　from　a　different　one。　No　water　at　all　is



generated；　but　the　volume　of　the　rivers　consists　of　the　water　that



is　gathered　into　such　reservoirs　in　winter。　Hence　rivers　are　always



fuller　in　winter　than　in　summer；　and　some　are　perennial；　others　not。



Rivers　are　perennial　where　the　reservoir　is　large　and　so　enough



water　has　collected　in　it　to　last　out　and　not　be　used　up　before　the



winter　rain　returns。　Where　the　reservoirs　are　smaller　there　is　less



water　in　the　rivers；　and　they　are　dried　up　and　their　vessel　empty



before　the　fresh　rain　comes　on。



　　But　if　any　one　will　picture　to　himself　a　reservoir　adequate　to　the



water　that　is　continuously　flowing　day　by　day；　and　consider　the　amount



of　the　water；　it　is　obvious　that　a　receptacle　that　is　to　contain　all



the　water　that　flows　in　the　year　would　be　larger　than　the　earth；　or；



at　any　rate；　not　much　smaller。



　　Though　it　is　evident　that　many　reservoirs　of　this　kind　do　exist　in



many　parts　of　the　earth；　yet　it　is　unreasonable　for　any　one　to



refuse　to　admit　that　air　becomes　water　in　the　earth　for　the　same



reason　as　it　does　above　it。　If　the　cold　causes　the　vaporous　air　to



condense　into　water　above　the　earth　we　must　suppose　the　cold　in　the



earth　to　produce　this　same　effect；　and　recognize　that　there　not　only



exists　in　it　and　flows　out　of　it　actually　formed　water；　but　that　water



is　continually　forming　in　it　too。



　　Again；　even　in　the　case　of　the　water　that　is　not　being　formed　from



day　to　day　but　exists　as　such；　we　must　not　suppose　as　some　do　that



rivers　have　their　source　in　definite　subterranean　lakes。　On　the



contrary；　just　as　above　the　earth　small　drops　form　and　these　join



others；　till　finally　the　water　descends　in　a　body　as　rain；　so　too　we



must　suppose　that　in　the　earth　the　water　at　first　trickles　together



little　by　little；　and　that　the　sources　of　the　rivers　drip；　as　it　were；



out　of　the　earth　and　then　unite。　This　is　proved　by　facts。　When　men



construct　an　aqueduct　they　collect　the　water　in　pipes　and　trenches；　as



if　the　earth　in　the　higher　ground　were　sweating　the　water　out。



Hence；　too；　the　head…waters　of　rivers　are　found　to　flow　from



mountains；　and　from　the　greatest　mountains　there　flow　the　most



numerous　and　greatest　rivers。　Again；　most　springs　are　in　the



neighbourhood　of　mountains　and　of　high　ground；　whereas　if　we　except



rivers；　water　rarely　appears　in　the　plains。　For　mountains　and　high



ground；　suspended　over　the　country　like　a　saturated　sponge；　make　the



water　ooze　out　and　trickle　together　in　minute　quantities　but　in　many



places。　They　receive　a　great　deal　of　water　falling　as　rain　（for　it



makes　no　difference　whether　a　spongy　receptacle　is　concave　and



turned　up　or　convex　and　turned　down：　in　either　case　it　will　contain



the　same　volume　of　matter）　and；　they　also　cool　the　vapour　that　rises



and　condense　it　back　into　water。



　　Hence；　as　we　said；　we　find　that　the　greatest　rivers　flow　from　the



greatest　mountains。　This　can　be　seen　by　looking　at　itineraries：　what



is　recorded　in　them　consists　either　of　things　which　the　writer　has



seen　himself　or　of　such　as　he　has　compiled　after　inquiry　from　those



who　have　seen　them。



　　In　Asia　we　find　that　the　most　numerous　and　greatest　rivers　flow　from



the　mountain　called　Parnassus；　admittedly　the　greatest　of　all



mountains　towards　the　south…east。　When　you　have　crossed　it　you　see　the



outer　ocean；　the　further　limit　of　which　is　unknown　to　the　dwellers



in　our　world。　Besides　other　rivers　there　flow　from　it　the　Bactrus；　the



Choaspes；　the　Araxes：　from　the　last　a　branch　separates　off　and　flows



into　lake　Maeotis　as　the　Tanais。　From　it；　too；　flows　the　Indus；　the



volume　of　whose　stream　is　greatest　of　all　rivers。　From　the　Caucasus



flows　the　Phasis；　and　very　many　other　great　rivers　besides。　Now　the



Caucasus　is　the　greatest　of　the　mountains　that　lie　to　the　northeast；



both　as　regards　its　extent　and　its　height。　A　proof　of　its　height　is



the　fact　that　it　can　be　seen　from　the　so…called　'deeps'　and　from　the



entrance　to　the　lake。　Again；　the　sun　shines　on　its　peaks　for　a　third



part　of　the　night　before　sunrise　and　again　after　sunset。　Its　extent　is



proved　by　the　fact　that　thought　contains　many　inhabitable　regions



which　are　occupied　by　many　nations　and　in　which　there　are　said　to　be



great　lakes；　yet　they　say　that　all　these　regions　are　visible　up　to　the



last　peak。　From　Pyrene　（this　is　a　mountain　towards　the　west　in



Celtice）　there　flow　the　Istrus　and　the　Tartessus。　The　latter　flows



outside　the　pillars；　while　the　Istrus　flows　through　all　Europe　into



the　Euxine。　Most　of　the　remaining　rivers　flow　northwards　from　the



Hercynian　mountains；　which　are　the　greatest　in　height　and　extent　about



that　region。　In　the　extreme　north；　beyond　furthest　Scythia；　are　the



mountains　called　Rhipae。　The　stories　about　their　size　are　altogether



too　fabulous：　however；　they　say　that　the　most　and　（after　the　Istrus）



the　greatest　rivers　flow　from　them。　So；　too；　in　Libya　there　flow



from　the　Aethiopian　mountains　the　Aegon　and　the　Nyses；　and　from　the



so…called　Silver　Mountain　the　two　greatest　of　named　rivers；　the



river　called　Chremetes　that　flows　into　the　outer　ocean；　and　the　main



source　of　the　Nile。　Of　the　rivers　in　the　Greek　world；　the　Achelous



flows　from　Pindus；　the　Inachus　from　the　same　mountain；　the　Strymon；



the　Nestus；　and　the　Hebrus　all　three　from　Scombrus；　many　rivers；



too；　flow　from　Rhodope。



　　All　other　rivers　would　be　found　to　flow　in　the　same　way；　but　we　have



mentioned　these　as　examples。　Even　where　rivers　flow　from　marshes；



the　marshes　in　almost　every　case　are　found　to　lie　below　mountains　or



gradually　rising　ground。



　　It　is　clear　then　that　we　must　not　suppose　rivers　to　originate　from



definite　reservoirs：　for　the　whole　earth；　we　might　almost　say；　would



not　be　sufficient　（any　more　than　the　region　of　the　clouds　would　be）　if



we　were　to　suppose　that　they　were　fed　by　actually　existing　water



only　and　it　were　not　the　case　that　as　some　water　passed　out　of



existence　some　more　came　into　existence；　but　rivers　always　drew



their　stream　from　an　existing　store。　Secondly；　the　fact　that　rivers



rise　at　the　foot　of　mountains　proves　that　a　place　transmits　the



water　it　contains　by　gradual　percolation　of　many　drops；　little　by



little；　and　that　th