meteorology-第12节

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Make　a　vessel　of　wax　and　put　it　in　the　sea；　fastening　its　mouth　in



such　a　way　as　to　prevent　any　water　getting　in。　Then　the　water　that



percolates　through　the　wax　sides　of　the　vessel　is　sweet；　the　earthy



stuff；　the　admixture　of　which　makes　the　water　salt；　being　separated



off　as　it　were　by　a　filter。　It　is　this　stuff　which　make　salt　water



heavy　（it　weighs　more　than　fresh　water）　and　thick。　The　difference　in



consistency　is　such　that　ships　with　the　same　cargo　very　nearly　sink　in



a　river　when　they　are　quite　fit　to　navigate　in　the　sea。　This



circumstance　has　before　now　caused　loss　to　shippers　freighting　their



ships　in　a　river。　That　the　thicker　consistency　is　due　to　an



admixture　of　something　is　proved　by　the　fact　that　if　you　make　strong



brine　by　the　admixture　of　salt；　eggs；　even　when　they　are　full；　float



in　it。　It　almost　becomes　like　mud；　such　a　quantity　of　earthy　matter　is



there　in　the　sea。　The　same　thing　is　done　in　salting　fish。



　　Again　if；　as　is　fabled；　there　is　a　lake　in　Palestine；　such　that　if



you　bind　a　man　or　beast　and　throw　it　in　it　floats　and　does　not　sink；



this　would　bear　out　what　we　have　said。　They　say　that　this　lake　is　so



bitter　and　salt　that　no　fish　live　in　it　and　that　if　you　soak　clothes



in　it　and　shake　them　it　cleans　them。　The　following　facts　all　of　them



support　our　theory　that　it　is　some　earthy　stuff　in　the　water　which



makes　it　salt。　In　Chaonia　there　is　a　spring　of　brackish　water　that



flows　into　a　neighbouring　river　which　is　sweet　but　contains　no　fish。



The　local　story　is　that　when　Heracles　came　from　Erytheia　driving　the



oxen　and　gave　the　inhabitants　the　choice；　they　chose　salt　in



preference　to　fish。　They　get　the　salt　from　the　spring。　They　boil　off



some　of　the　water　and　let　the　rest　stand；　when　it　has　cooled　and　the



heat　and　moisture　have　evaporated　together　it　gives　them　salt；　not



in　lumps　but　loose　and　light　like　snow。　It　is　weaker　than　ordinary



salt　and　added　freely　gives　a　sweet　taste；　and　it　is　not　as　white　as



salt　generally　is。　Another　instance　of　this　is　found　in　Umbria。



There　is　a　place　there　where　reeds　and　rushes　grow。　They　burn　some



of　these；　put　the　ashes　into　water　and　boil　it　off。　When　a　little



water　is　left　and　has　cooled　it　gives　a　quantity　of　salt。



　　Most　salt　rivers　and　springs　must　once　have　been　hot。　Then　the



original　fire　in　them　was　extinguished　but　the　earth　through　which



they　percolate　preserves　the　character　of　lye　or　ashes。　Springs　and



rivers　with　all　kinds　of　flavours　are　found　in　many　places。　These



flavours　must　in　every　case　be　due　to　the　fire　that　is　or　was　in　them；



for　if　you　expose　earth　to　different　degrees　of　heat　it　assumes



various　kinds　and　shades　of　flavour。　It　becomes　full　of　alum　and　lye



and　other　things　of　the　kind；　and　the　fresh　water　percolates　through



these　and　changes　its　character。　Sometimes　it　becomes　acid　as　in



Sicania；　a　part　of　Sicily。　There　they　get　a　salt　and　acid　water



which　they　use　as　vinegar　to　season　some　of　their　dishes。　In　the



neighbourhood　of　Lyncus；　too；　there　is　a　spring　of　acid　water；　and



in　Scythia　a　bitter　spring。　The　water　from　this　makes　the　whole　of　the



river　into　which　it　flows　bitter。　These　differences　are　explained　by　a



knowledge　of　the　particular　mixtures　that　determine　different　savours。



But　these　have　been　explained　in　another　treatise。



　　We　have　now　given　an　account　of　waters　and　the　sea；　why　they



persist；　how　they　change；　what　their　nature　is；　and　have　explained



most　of　their　natural　operations　and　affections。







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　　Let　us　proceed　to　the　theory　of　winds。　Its　basis　is　a　distinction　we



have　already　made。　We　recognize　two　kinds　of　evaporation；　one　moist；



the　other　dry。　The　former　is　called　vapour：　for　the　other　there　is



no　general　name　but　we　must　call　it　a　sort　of　smoke；　applying　to　the



whole　of　it　a　word　that　is　proper　to　one　of　its　forms。　The　moist



cannot　exist　without　the　dry　nor　the　dry　without　the　moist：　whenever



we　speak　of　either　we　mean　that　it　predominates。　Now　when　the　sun　in



its　circular　course　approaches；　it　draws　up　by　its　heat　the　moist



evaporation：　when　it　recedes　the　cold　makes　the　vapour　that　had　been



raised　condense　back　into　water　which　falls　and　is　distributed　through



the　earth。　（This　explains　why　there　is　more　rain　in　winter　and　more　by



night　than　by　day：　though　the　fact　is　not　recognized　because　rain　by



night　is　more　apt　to　escape　observation　than　by　day。）　But　there　is　a



great　quantity　of　fire　and　heat　in　the　earth；　and　the　sun　not　only



draws　up　the　moisture　that　lies　on　the　surface　of　it；　but　warms　and



dries　the　earth　itself。　Consequently；　since　there　are　two　kinds　of



evaporation；　as　we　have　said；　one　like　vapour；　the　other　like　smoke；



both　of　them　are　necessarily　generated。　That　in　which　moisture



predominates　is　the　source　of　rain；　as　we　explained　before；　while



the　dry　evaporation　is　the　source　and　substance　of　all　winds。　That



things　must　necessarily　take　this　course　is　clear　from　the　resulting



phenomena　themselves；　for　the　evaporation　that　is　to　produce　them　must



necessarily　differ；　and　the　sun　and　the　warmth　in　the　earth　not　only



can　but　must　produce　these　evaporations。



　　Since　the　two　evaporations　are　specifically　distinct；　wind　and



rain　obviously　differ　and　their　substance　is　not　the　same；　as　those



say　who　maintain　that　one　and　the　same　air　when　in　motion　is　wind；　but



when　it　condenses　again　is　water。　Air；　as　we　have　explained　in　an



earlier　book；　is　made　up　of　these　as　constituents。　Vapour　is　moist



and　cold　（for　its　fluidity　is　due　to　its　moistness；　and　because　it



derives　from　water　it　is　naturally　cold；　like　water　that　has　not



been　warmed）：　whereas　the　smoky　evaporation　is　hot　and　dry。　Hence　each



contributes　a　part；　and　air　is　moist　and　hot。　It　is　absurd　that　this



air　that　surrounds　us　should　become　wind　when　in　motion；　whatever　be



the　source　of　its　motion　on　the　contrary　the　case　of　winds　is　like



that　of　rivers。　We　do　not　call　water　that　flows　anyhow　a　river；　even



if　there　is　a　great　quantity　of　it；　but　only　if　the　flow　comes　from



a　spring。　So　too　with　the　winds；　a　great　quantity　of　air　might　be



moved　by　the　fall　of　some　large　object　without　flowing　from　any　source



or　spring。



　　The　facts　bear　out　our　theory。　It　is　because　the　evaporation　takes



place　uninterruptedly　but　differs　in　degree　and　quantity　that　clouds



and　winds　appear　in　their　natural　proportion　according　to　the



season；　and　it　is　because　there　is　now　a　great　excess　of　the　vaporous；



now　of　the　dry　and　smoky　exhalation；　that　some　years　are　rainy　and



wet；　others　windy　and　dry。　Sometimes　there　is　much　drought　or　rain；



and　it　prevails　over　a　great　and　continuous　stretch　of　country。　At



other　times　it　is　local；　the　surrounding　country　often　getting



seasonable　or　even　excessive　rains　while　there　is　drought　in　a　certain



part；　or；　contrariwise；　all　the　surrounding　country　gets　little　or



even　no　rain　while　a　certain　part　gets　rain　in　abundance。　The　reason



for　all　this　is　that　while　the　same　affection　is　generally　apt　to



prevail　over　a　considerable　district　because　adjacent　places　（unless



there　is　something　special　to　differentiate　them）　stand　in　the　same



relation　to　the　sun；　yet　on　occasion　the　dry　evaporation　will



prevail　in　one　part　and　the　moist　in　another；　or　conversely。　Again　the



reason　for　this　latter　is　that　each　evaporation　goes　over　to　that　of



the　neighbouring　district：　for　instance；　the　dry　evaporation



circulates　in　its　own　place　while　the　moist　migrates　to　the　next



district　or　is　even　driven　by　winds　to　some　distant　place：　or　else　the



moist　evaporation　remains　and　the　dry　moves　away。　Just　as　in　the



case　of　the　body　when　the　stomach　is　dry　the　lower　belly　is　often　in



the　contrary　state；　and　when　it　is　dry　the　stomach　is　moist　and



cold；　so　it　often　happens　that　the　evaporations　reciprocally　take



one　another's　place　and　interchange。



　　Further；　after　rain　wind　generally　rises　in　those　places　where　the



rain　fell；　and　when　rain　has　come　on　the　wind　ceases。　These　are



necessary　effects　of　the　principles　we　have　explained。　After　rain



the　earth　is　being　dried　by　its　own　heat　and　that　from　above　and　gives



off　the　evaporation　which　we　saw　to　be　the　material　cause　of。　wind。



Again；　suppose　this　secretion　is　present　and　wind　prevails；　the　heat



is　continually　being　thrown　off；　rising　to　the　upper　region；　and　so



the　wind　ceases；　then　the　fall　in　temperature　makes　vapour　form　and



condense　into　water。　Water　also　forms　and　cools　the　dry　evaporation



when　the　clouds　are　driven　together　and　the　cold　concentrated　in　them。



These　are　the　causes　that　make　wind　cease　on　the　advent　of　rain；　and



rain　fall　on　the　cessation　of　wind。



　　The　cause　of　the　predominance　of　winds　from　the　north　and　from　the



south　is　the　same