meteorology-第10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！




advancing　in　one　place　and　receding　in　another　it　is　clear　that　the



same　parts　of　the　whole　earth　are　not　always　either　sea　or　land；　but



that　all　this　changes　in　course　of　time。



　　So　we　have　explained　that　the　same　parts　of　the　earth　are　not　always



land　or　sea　and　why　that　is　so：　and　also　why　some　rivers　are　perennial



and　others　not。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Book　II



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1







　　LET　us　explain　the　nature　of　the　sea　and　the　reason　why　such　a　large



mass　of　water　is　salt　and　the　way　in　which　it　originally　came　to　be。



　　The　old　writers　who　invented　theogonies　say　that　the　sea　has



springs；　for　they　want　earth　and　sea　to　have　foundations　and　roots



of　their　own。　Presumably　they　thought　that　this　view　was　grander　and



more　impressive　as　implying　that　our　earth　was　an　important　part　of



the　universe。　For　they　believed　that　the　whole　world　had　been　built　up



round　our　earth　and　for　its　sake；　and　that　the　earth　was　the　most



important　and　primary　part　of　it。　Others；　wiser　in　human　knowledge；



give　an　account　of　its　origin。　At　first；　they　say；　the　earth　was



surrounded　by　moisture。　Then　the　sun　began　to　dry　it　up；　part　of　it



evaporated　and　is　the　cause　of　winds　and　the　turnings　back　of　the



sun　and　the　moon；　while　the　remainder　forms　the　sea。　So　the　sea　is



being　dried　up　and　is　growing　less；　and　will　end　by　being　some　day



entirely　dried　up。　Others　say　that　the　sea　is　a　kind　of　sweat　exuded



by　the　earth　when　the　sun　heats　it；　and　that　this　explains　its



saltness：　for　all　sweat　is　salt。　Others　say　that　the　saltness　is　due



to　the　earth。　Just　as　water　strained　through　ashes　becomes　salt；　so



the　sea　owes　its　saltness　to　the　admixture　of　earth　with　similar



properties。



　　We　must　now　consider　the　facts　which　prove　that　the　sea　cannot



possibly　have　springs。　The　waters　we　find　on　the　earth　either　flow



or　are　stationary。　All　flowing　water　has　springs。　（By　a　spring；　as



we　have　explained　above；　we　must　not　understand　a　source　from　which



waters　are　ladled　as　it　were　from　a　vessel；　but　a　first　point　at　which



the　water　which　is　continually　forming　and　percolating　gathers。）



Stationary　water　is　either　that　which　has　collected　and　has　been



left　standing；　marshy　pools；　for　instance；　and　lakes；　which　differ



merely　in　size；　or　else　it　comes　from　springs。　In　this　case　it　is



always　artificial；　I　mean　as　in　the　case　of　wells；　otherwise　the



spring　would　have　to　be　above　the　outlet。　Hence　the　water　from



fountains　and　rivers　flows　of　itself；　whereas　wells　need　to　be



worked　artificially。　All　the　waters　that　exist　belong　to　one　or



other　of　these　classes。



　　On　the　basis　of　this　division　we　can　sec　that　the　sea　cannot　have



springs。　For　it　falls　under　neither　of　the　two　classes；　it　does　not



flow　and　it　is　not　artificial；　whereas　all　water　from　springs　must



belong　to　one　or　other　of　them。　Natural　standing　water　from　springs　is



never　found　on　such　a　large　scale。



　　Again；　there　are　several　seas　that　have　no　communication　with　one



another　at　all。　The　Red　Sea；　for　instance；　communicates　but　slightly



with　the　ocean　outside　the　straits；　and　the　Hyrcanian　and　Caspian　seas



are　distinct　from　this　ocean　and　people　dwell　all　round　them。　Hence；



if　these　seas　had　had　any　springs　anywhere　they　must　have　been



discovered。



　　It　is　true　that　in　straits；　where　the　land　on　either　side



contracts　an　open　sea　into　a　small　space；　the　sea　appears　to　flow。　But



this　is　because　it　is　swinging　to　and　fro。　In　the　open　sea　this　motion



is　not　observed；　but　where　the　land　narrows　and　contracts　the　sea



the　motion　that　was　imperceptible　in　the　open　necessarily　strikes



the　attention。



　　The　whole　of　the　Mediterranean　does　actually　flow。　The　direction



of　this　flow　is　determined　by　the　depth　of　the　basins　and　by　the



number　of　rivers。　Maeotis　flows　into　Pontus　and　Pontus　into　the



Aegean。　After　that　the　flow　of　the　remaining　seas　is　not　so　easy　to



observe。　The　current　of　Maeotis　and　Pontus　is　due　to　the　number　of



rivers　（more　rivers　flow　into　the　Euxine　and　Maeotis　than　into　the



whole　Mediterranean　with　its　much　larger　basin）；　and　to　their　own



shallowness。　For　we　find　the　sea　getting　deeper　and　deeper。　Pontus



is　deeper　than　Maeotis；　the　Aegean　than　Pontus；　the　Sicilian　sea



than　the　Aegean；　the　Sardinian　and　Tyrrhenic　being　the　deepest　of　all。



（Outside　the　pillars　of　Heracles　the　sea　is　shallow　owing　to　the



mud；　but　calm；　for　it　lies　in　a　hollow。）　We　see；　then；　that　just　as



single　rivers　flow　from　mountains；　so　it　is　with　the　earth　as　a　whole：



the　greatest　volume　of　water　flows　from　the　higher　regions　in　the



north。　Their　alluvium　makes　the　northern　seas　shallow；　while　the　outer



seas　are　deeper。　Some　further　evidence　of　the　height　of　the　northern



regions　of　the　earth　is　afforded　by　the　view　of　many　of　the　ancient



meteorologists。　They　believed　that　the　sun　did　not　pass　below　the



earth；　but　round　its　northern　part；　and　that　it　was　the　height　of　this



which　obscured　the　sun　and　caused　night。



　　So　much　to　prove　that　there　cannot　be　sources　of　the　sea　and　to



explain　its　observed　flow。



　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　2







　　We　must　now　discuss　the　origin　of　the　sea；　if　it　has　an　origin；



and　the　cause　of　its　salt　and　bitter　taste。



　　What　made　earlier　writers　consider　the　sea　to　be　the　original　and



main　body　of　water　is　this。　It　seems　reasonable　to　suppose　that　to



be　the　case　on　the　analogy　of　the　other　elements。　Each　of　them　has　a



main　bulk　which　by　reason　of　its　mass　is　the　origin　of　that　element；



and　any　parts　which　change　and　mix　with　the　other　elements　come　from



it。　Thus　the　main　body　of　fire　is　in　the　upper　region；　that　of　air



occupies　the　place　next　inside　the　region　of　fire；　while　the　mass　of



the　earth　is　that　round　which　the　rest　of　the　elements　are　seen　to



lie。　So　we　must　clearly　look　for　something　analogous　in　the　case　of



water。　But　here　we　can　find　no　such　single　mass；　as　in　the　case　of　the



other　elements；　except　the　sea。　River　water　is　not　a　unity；　nor　is



it　stable；　but　is　seen　to　be　in　a　continuous　process　of　becoming



from　day　to　day。　It　was　this　difficulty　which　made　people　regard　the



sea　as　the　origin　and　source　of　moisture　and　of　all　water。　And　so　we



find　it　maintained　that　rivers　not　only　flow　into　the　sea　but



originate　from　it；　the　salt　water　becoming　sweet　by　filtration。



　　But　this　view　involves　another　difficulty。　If　this　body　of　water



is　the　origin　and　source　of　all　water；　why　is　it　salt　and　not　sweet？



The　reason　for　this；　besides　answering　this　question；　will　ensure



our　having　a　right　first　conception　of　the　nature　of　the　sea。



　　The　earth　is　surrounded　by　water；　just　as　that　is　by　the　sphere　of



air；　and　that　again　by　the　sphere　called　that　of　fire　（which　is　the



outermost　both　on　the　common　view　and　on　ours）。　Now　the　sun；　moving　as



it　does；　sets　up　processes　of　change　and　becoming　and　decay；　and　by



its　agency　the　finest　and　sweetest　water　is　every　day　carried　up　and



is　dissolved　into　vapour　and　rises　to　the　upper　region；　where　it　is



condensed　again　by　the　cold　and　so　returns　to　the　earth。　This；　as　we



have　said　before；　is　the　regular　course　of　nature。



　　Hence　all　my　predecessors　who　supposed　that　the　sun　was　nourished　by



moisture　are　absurdly　mistaken。　Some　go　on　to　say　that　the　solstices



are　due　to　this；　the　reason　being　that　the　same　places　cannot　always



supply　the　sun　with　nourishment　and　that　without　it　he　must　perish。



For　the　fire　we　are　familiar　with　lives　as　long　as　it　is　fed；　and



the　only　food　for　fire　is　moisture。　As　if　the　moisture　that　is



raised　could　reach　the　sun！　or　this　ascent　were　really　like　that



performed　by　flame　as　it　comes　into　being；　and　to　which　they



supposed　the　case　of　the　sun　to　be　analogous！　Really　there　is　no



similarity。　A　flame　is　a　process　of　becoming；　i