meteorology-第13章

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　　It　also　explains　why　rain　that　comes　from　the　south；　and　the　first



rains　of　autumn；　are　brackish。　The　south　is　the　warmest　of　winds　and



it　blows　from　dry　and　hot　regions。　Hence　it　carries　little　moist



vapour　and　that　is　why　it　is　hot。　（It　makes　no　difference　even　if　this



is　not　its　true　character　and　it　is　originally　a　cold　wind；　for　it



becomes　warm　on　its　way　by　incorporating　with　itself　a　great



quantity　of　dry　evaporation　from　the　places　it　passes　over。）　The　north



wind；　on　the　other　hand；　comb　ing　from　moist　regions；　is　full　of



vapour　and　therefore　cold。　It　is　dry　in　our　part　of　the　world



because　it　drives　the　clouds　away　before　it；　but　in　the　south　it　is



rainy；　just　as　the　south　is　a　dry　wind　in　Libya。　So　the　south　wind



charges　the　rain　that　falls　with　a　great　quantity　of　this　stuff。



Autumn　rain　is　brackish　because　the　heaviest　water　must　fall　first；　so



that　that　which　contains　the　greatest　quantity　of　this　kind　of　earth



descends　quickest。



　　This；　too；　is　why　the　sea　is　warm。　Everything　that　has　been



exposed　to　fire　contains　heat　potentially；　as　we　see　in　the　case　of



lye　and　ashes　and　the　dry　and　liquid　excreta　of　animals。　Indeed



those　animals　which　are　hottest　in　the　belly　have　the　hottest　excreta。



　　The　action　of　this　cause　is　continually　making　the　sea　more　salt；



but　some　part　of　its　saltness　is　always　being　drawn　up　with　the



sweet　water。　This　is　less　than　the　sweet　water　in　the　same　ratio　in



which　the　salt　and　brackish　element　in　rain　is　less　than　the　sweet；



and　so　the　saltness　of　the　sea　remains　constant　on　the　whole。　Salt



water　when　it　turns　into　vapour　becomes　sweet；　and　the　vapour　does　not



form　salt　water　when　it　condenses　again。　This　I　know　by　experiment。



The　same　thing　is　true　in　every　case　of　the　kind：　wine　and　all



fluids　that　evaporate　and　condense　back　into　a　liquid　state　become



water。　They　all　are　water　modified　by　a　certain　admixture；　the



nature　of　which　determines　their　flavour。　But　this　subject　must　be



considered　on　another　more　suitable　occasion。



　　For　the　present　let　us　say　this。　The　sea　is　there　and　some　of　it



is　continually　being　drawn　up　and　becoming　sweet；　this　returns　from



above　with　the　rain。　But　it　is　now　different　from　what　it　was　when



it　was　drawn　up；　and　its　weight　makes　it　sink　below　the　sweet　water。



This　process　prevents　the　sea；　as　it　does　rivers；　from　drying　up



except　from　local　causes　（this　must　happen　to　sea　and　rivers　alike）。



On　the　other　hand　the　parts　neither　of　the　earth　nor　of　the　sea　remain



constant　but　only　their　whole　bulk。　For　the　same　thing　is　true　of



the　earth　as　of　the　sea：　some　of　it　is　carried　up　and　some　comes



down　with　the　rain；　and　both　that　which　remains　on　the　surface　and



that　which　comes　down　again　change　their　situations。



　　There　is　more　evidence　to　prove　that　saltness　is　due　to　the



admixture　of　some　substance；　besides　that　which　we　have　adduced。



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。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　4







　　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　wind