meteorology-第9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！




whether　it　was　its　weight；　as　some　even　of　those　who　hold　these



views　say　（for　it　is　easy　to　see　the　cause　here）；　or　some　other



reason…clearly　the　same　thing　must　make　it　persist　for　ever。　They　must



either　deny　that　the　water　raised　by　the　sun　will　return　at　all；　or；



if　it　does；　they　must　admit　that　the　sea　persists　for　ever　or　as



long　as　this　process　goes　on；　and　again；　that　for　the　same　period　of



time　that　sweet　water　must　have　been　carried　up　beforehand。　So　the　sea



will　never　dry　up：　for　before　that　can　happen　the　water　that　has



gone　up　beforehand　will　return　to　it：　for　if　you　say　that　this　happens



once　you　must　admit　its　recurrence。　If　you　stop　the　sun's　course　there



is　no　drying　agency。　If　you　let　it　go　on　it　will　draw　up　the　sweet



water　as　we　have　said　whenever　it　approaches；　and　let　it　descend　again



when　it　recedes。　This　notion　about　the　sea　is　derived　from　the　fact



that　many　places　are　found　to　be　drier　now　than　they　once　were。　Why



this　is　so　we　have　explained。　The　phenomenon　is　due　to　temporary



excess　of　rain　and　not　to　any　process　of　becoming　in　which　the



universe　or　its　parts　are　involved。　Some　day　the　opposite　will　take



place　and　after　that　the　earth　will　grow　dry　once　again。　We　must



recognize　that　this　process　always　goes　on　thus　in　a　cycle；　for　that



is　more　satisfactory　than　to　suppose　a　change　in　the　whole　world　in



order　to　explain　these　facts。　But　we　have　dwelt　longer　on　this　point



than　it　deserves。



　　To　return　to　the　saltness　of　the　sea：　those　who　create　the　sea



once　for　all；　or　indeed　generate　it　at　all；　cannot　account　for　its



saltness。　It　makes　no　difference　whether　the　sea　is　the　residue　of　all



the　moisture　that　is　about　the　earth　and　has　been　drawn　up　by　the　sun；



or　whether　all　the　flavour　existing　in　the　whole　mass　of　sweet　water



is　due　to　the　admixture　of　a　certain　kind　of　earth。　Since　the　total



volume　of　the　sea　is　the　same　once　the　water　that　evaporated　has



returned；　it　follows　that　it　must　either　have　been　salt　at　first



too；　or；　if　not　at　first；　then　not　now　either。　If　it　was　salt　from　the



very　beginning；　then　we　want　to　know　why　that　was　so；　and　why；　if　salt



water　was　drawn　up　then；　that　is　not　the　case　now。



　　Again；　if　it　is　maintained　that　an　admixture　of　earth　makes　the



sea　salt　（for　they　say　that　earth　has　many　flavours　and　is　washed　down



by　the　rivers　and　so　makes　the　sea　salt　by　its　admixture）；　it　is



strange　that　rivers　should　not　be　salt　too。　How　can　the　admixture　of



this　earth　have　such　a　striking　effect　in　a　great　quantity　of　water



and　not　in　each　river　singly？　For　the　sea；　differing　in　nothing　from



rivers　but　in　being　salt；　is　evidently　simply　the　totality　of　river



water；　and　the　rivers　are　the　vehicle　in　which　that　earth　is　carried



to　their　common　destination。



　　It　is　equally　absurd　to　suppose　that　anything　has　been　explained



by　calling　the　sea　'the　sweat　of　the　earth'；　like　Empedicles。



Metaphors　are　poetical　and　so　that　expression　of　his　may　satisfy　the



requirements　of　a　poem；　but　as　a　scientific　theory　it　is



unsatisfactory。　Even　in　the　case　of　the　body　it　is　a　question　how



the　sweet　liquid　drunk　becomes　salt　sweat　whether　it　is　merely　by



the　departure　of　some　element　in　it　which　is　sweetest；　or　by　the



admixture　of　something；　as　when　water　is　strained　through　ashes。



Actually　the　saltness　seems　to　be　due　to　the　same　cause　as　in　the　case



of　the　residual　liquid　that　gathers　in　the　bladder。　That；　too；　becomes



bitter　and　salt　though　the　liquid　we　drink　and　that　contained　in　our



food　is　sweet。　If　then　the　bitterness　is　due　in　these　cases　（as　with



the　water　strained　through　lye）　to　the　presence　of　a　certain　sort　of



stuff　that　is　carried　along　by　the　urine　（as　indeed　we　actually　find　a



salt　deposit　settling　in　chamber…pots）　and　is　secreted　from　the



flesh　in　sweat　（as　if　the　departing　moisture　were　washing　the　stuff



out　of　the　body）；　then　no　doubt　the　admixture　of　something　earthy　with



the　water　is　what　makes　the　sea　salt。



　　Now　in　the　body　stuff　of　this　kind；　viz。　the　sediment　of　food；　is



due　to　failure　to　digest：　but　how　there　came　to　be　any　such　thing　in



the　earth　requires　explanation。　Besides；　how　can　the　drying　and



warming　of　the　earth　cause　the　secretion　such　a　great　quantity　of



water；　especially　as　that　must　be　a　mere　fragment　of　what　is　left　in



the　earth？　Again；　waiving　the　question　of　quantity；　why　does　not　the



earth　sweat　now　when　it　happens　to　be　in　process　of　drying？　If　it



did　so　then；　it　ought　to　do　so　now。　But　it　does　not：　on　the



contrary；　when　it　is　dry　it　graws　moist；　but　when　it　is　moist　it



does　not　secrete　anything　at　all。　How　then　was　it　possible　for　the



earth　at　the　beginning　when　it　was　moist　to　sweat　as　it　grew　dry？



Indeed；　the　theory　that　maintains　that　most　of　the　moisture　departed



and　was　drawn　up　by　the　sun　and　that　what　was　left　over　is　the　sea



is　more　reasonable；　but　for　the　earth　to　sweat　when　it　is　moist　is



impossible。



　　Since　all　the　attempts　to　account　for　the　saltness　of　the　sea　seem



unsuccessful　let　us　explain　it　by　the　help　of　the　principle　we　have



used　already。



　　Since　we　recognize　two　kinds　of　evaporation；　one　moist；　the　other



dry；　it　is　clear　that　the　latter　must　be　recognized　as　the　source　of



phenomena　like　those　we　are　concerned　with。



　　But　there　is　a　question　which　we　must　discuss　first。　Does　the　sea



always　remain　numerically　one　and　consisting　of　the　same　parts；　or



is　it；　too；　one　in　form　and　volume　while　its　parts　are　in　continual



change；　like　air　and　sweet　water　and　fire？　All　of　these　are　in　a



constant　state　of　change；　but　the　form　and　the　quantity　of　each　of



them　are　fixed；　just　as　they　are　in　the　case　of　a　flowing　river　or　a



burning　flame。　The　answer　is　clear；　and　there　is　no　doubt　that　the



same　account　holds　good　of　all　these　things　alike。　They　differ　in　that



some　of　them　change　more　rapidly　or　more　slowly　than　others；　and



they　all　are　involved　in　a　process　of　perishing　and　becoming　which　yet



affects　them　all　in　a　regular　course。



　　This　being　so　we　must　go　on　to　try　to　explain　why　the　sea　is　salt。



There　are　many　facts　which　make　it　clear　that　this　taste　is　due　to　the



admixture　of　something。　First；　in　animal　bodies　what　is　least



digested；　the　residue　of　liquid　food；　is　salt　and　bitter；　as　we　said



before。　All　animal　excreta　are　undigested；　but　especially　that　which



gathers　in　the　bladder　（its　extreme　lightness　proves　this；　for



everything　that　is　digested　is　condensed）；　and　also　sweat；　in　these



then　is　excreted　（along　with　other　matter）　an　identical　substance　to



which　this　flavour　is　due。　The　case　of　things　burnt　is　analogous。　What



heat　fails　to　assimilate　becomes　the　excrementary　residue　in　animal



bodies；　and；　in　things　burnt；　ashes。　That　is　why　some　people　say



that　it　was　burnt　earth　that　made　the　sea　salt。　To　say　that　it　was



burnt　earth　is　absurd；　but　to　say　that　it　was　something　like　burnt



earth　is　true。　We　must　suppose　that　just　as　in　the　cases　we　have



described；　so　in　the　world　as　a　whole；　everything　that　grows　and　is



naturally　generated　always　leaves　an　undigested　residue；　like　that



of　things　burnt；　consisting　of　this　sort　of　earth。　All　the　earthy



stuff　in　the　dry　exhalation　is　of　this　nature；　and　it　is　the　dry



exhalation　which　accounts　for　its　great　quantity。　Now　since；　as　we



have　said；　the　moist　and　the　dry　evaporations　are　mixed；　some　quantity



of　this　stuff　must　always　be　included　in　the　clouds　and　the　water　that



are　formed　by　condensation；　and　must　redescend　to　the　earth　in　rain。



This　process　must　always　go　on　with　such　regularity　as　the　sublunary



world　admits　of。　and　it　is　the　answer　to　the　question　how　the　sea



comes　to　be　salt。



　　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