適合高中生拓展閱讀的英語美文

v1.0可編輯可修改v1.0可編輯可修改PAGEPAGE74v1.0可編輯可修改PAGE新東方優(yōu)美背誦短文50篇Unit50:CellsandTemperature50.CellsandTemperatureCellscannotremainaliveoutsidecertainlimitsoftemperatureandmuchnarrowerlimitsmarktheboundariesofeffectivefunctioning.Enzymesystemsofmammalsandbirdsaremostefficientonlywithinanarrowrangearound37C;adepartureofafewdegreesfromthisvalueseriouslyimpairstheirfunctioning.Eventhoughcellscansurvivewiderfluctuationstheintegratedactionsofbodilysystemsareimpaired.Otheranimalshaveawidertoleranceforchangesofbodilytemperature.Forcenturiesithasbeenrecognizedthatmammalsandbirdsdifferfromotheranimalsinthewaytheyregulatebodytemperature.Waysofcharacterizingthedifferencehavebecomemoreaccurateandmeaningfulovertime,butpopularterminologystillreflectstheolddivisioninto“warm-blooded”and“cold-blooded”species;warm-bloodedincludedmammalsandbirdswhereasallothercreatureswereconsideredcold-blooded.Asmorespecieswerestudied,itbecameevidentthatthisclassificationwasinadequate.Afencelizardoradesertiguana—eachcold-bloodedusuallyhasabodytemperatureonlyadegreeortwobelowthatofhumansandsoisnotcold.Thereforethenextdistinctionwasmadebetweenanimalsthatmaintainaconstantbodytemperature,calledhome0therms,andthosewhosebodytemperaturevarieswiththeirenvironments,calledpoikilotherms.Butthisclassificationalsoprovedinadequate,becauseamongmammalstherearemanythatvarytheirbodytemperaturesduringhibernation.Furthermore,manyinvertebratesthatliveinthedepthsoftheoceanneverexperiencechangeinthedepthsoftheoceanneverexperiencechangeinthechillofthedeepwater,andtheirbodytemperaturesremainconstant.細(xì)胞與溫度細(xì)胞只能在一定的溫度范圍內(nèi)存活，而進(jìn)一步保證它們有效工作的溫度范圍就更小了。哺乳動(dòng)物和鳥類的酶系統(tǒng)只能在37℃左右的很小范圍內(nèi)才能有效工作。與此相差僅幾度的溫度都會(huì)大大削弱它們的工作效率。盡管溫度變化更大時(shí)細(xì)胞仍能存活，但機(jī)體系統(tǒng)的整體運(yùn)行能力卻被削弱了。其它動(dòng)物對(duì)體溫的變化有更強(qiáng)的適應(yīng)性。幾個(gè)世紀(jì)以來，人們就認(rèn)識(shí)到哺乳動(dòng)物和鳥類調(diào)節(jié)體溫的方式與其它動(dòng)物不同。隨著時(shí)間的推移，人們對(duì)這種差異的描述越來越精確和有意義，但是"暖血?jiǎng)游?和"冷血?jiǎng)游?這一古老的分類方式至今仍在大眾詞匯中有所反映。暖血?jiǎng)游锇ú溉閯?dòng)物和鳥類，其它動(dòng)物統(tǒng)統(tǒng)被視為冷血?jiǎng)游?。但是?duì)更多物種進(jìn)行的研究表明這種分類顯然是不適當(dāng)?shù)摹Ｃ乐抟环N小型蜥蜴和沙漠鬣蜥同屬冷血?jiǎng)游?，但?shí)際上它們的體溫通常只比人類的體溫低1～2度，因此并不是真正的冷血。因此又出現(xiàn)了恒溫動(dòng)物(即保持恒定體溫的動(dòng)物)和變溫動(dòng)物(即體溫隨外界環(huán)境的變化而改變的動(dòng)物)這一區(qū)分方式。但這種分類也不恰當(dāng)。因?yàn)橛胁簧俨溉閯?dòng)物在冬眠期間會(huì)改變體溫，而許多生活在深海的無脊椎動(dòng)物在寒冷的深海水域中體溫并不變化，而是恒定的。新東方優(yōu)美背誦短文50篇Unit49:Sleep49SleepSleetispartofaperson’sdailyactivitycycle.Thereareseveraldifferentstagesofsleep,andtheytoooccurincycles.Ifyouareanaveragesleeper,yoursleepcycleisasfollows.Whenyoufistdriftoffintoslumber,youreyeswillrollaboutabit,youtemperaturewilldropslightly,yourmuscleswillrelax,andyourbreathingwellslowandbecomequiteregular.Yourbrainwavesslowandbecomequiteregular.Yourbrainwavesslowdownabittoo,withthealpharhythmofratherfastwaves1sleep.Forthenexthalfhourorso,asyourelaxmoreandmore,youwilldriftdownthroughstage2andstage3sleep.Theloweryourstageofsleep.sloweryourbrainwaveswillbe.Thenabout40to69minutesafteryouloseconsciousnessyouwillhavereachedthedeepestsleepofall.Yourbrainwillshowthelargeslowwavesthatareknownasthedeltarhythm.Thisisstage4sleep.Youdonotremainatthisdeepfourthstageallnightlong,butinsteadabout80minutesafteryoufallintoslumber,yourbrainactivitylevelwillincreaseagainslightly.Thedeltarhythmwilldisappear,tobereplacedbytheactivitypatternofbrainwaves.Youreyeswillbegintodartaroundunderyourclosedeyelidsasifyouwerelookingatsomethingoccurringinfrontofyou.Thisperiodofrapideyemovementlastsforsome8to15minutesandiscalledREMsleep.ItisduringREMsleepperiod,yourbodywillsoonrelaxagain,yourbreathingwillslipgentlybackfromstage1tostage4sleeponlytoriseonceagaintothesurfaceofnearconsciousnesssome80minuteslater.睡眠睡眠是人每天日?；顒?dòng)循環(huán)的一部分。人的睡眠分幾個(gè)階段，而這些階段也是循環(huán)發(fā)生的。如果你是一個(gè)正常的睡眠者，你的睡眠循環(huán)會(huì)這樣進(jìn)行。在你開始昏昏入睡時(shí)，你的眼睛會(huì)滾動(dòng)幾下，體溫略有下降，肌肉放松，呼吸變得緩慢而有節(jié)奏。除了開始幾分鐘比較快的α節(jié)奏外，腦電波也稍有減緩。這被稱為第一階段睡眠。在隨后約半小時(shí)內(nèi)，你進(jìn)一步放松，進(jìn)入第二和第三階段睡眠。睡眠越深入，腦電波就越緩慢。大約在開始睡眠后的40到60分鐘，你將進(jìn)入沉睡狀態(tài)。這時(shí)的腦電波表現(xiàn)為巨大的緩波，被稱為δ節(jié)奏。這就是第四階段睡眠。但你并不是整夜都保持這種沉睡狀態(tài)。入睡后約80分鐘左右，你的大腦運(yùn)動(dòng)水平會(huì)再度略有提高。δ節(jié)奏消失，并被腦電波的運(yùn)動(dòng)圖形取代。你的眼睛會(huì)在閉著的眼瞼下迅速轉(zhuǎn)動(dòng)，就好象你在看著眼前發(fā)生的什么事情。這種迅速的眼球運(yùn)動(dòng)持續(xù)約8～15分鐘，這一階段睡眠被稱之為快速眼動(dòng)(REM)睡眠。在REM睡眠階段，你的肢體會(huì)很快再度放松，呼吸也再次放慢并變得有節(jié)奏，你會(huì)輕松地從第一階段滑入第四階段睡眠-直到大約80分鐘后重新接近清醒狀態(tài)。新東方優(yōu)美背誦短文50篇Unit48:Bacteria48BacteriaBacteriaareextremelysmalllivingthings.Whilewemeasureourownsizesininchesorcentimeters,bacterialsizeismeasuredinmicrons.Onemicronisathousandthofamillimeter:apinheadisaboutamillimeteracross.Rod-shapedbacteriaareusuallyfromtwotofourmicronslong,whileroundedonesaregenerallyonemicronindiameter.Thusifyouenlargedaroundedbacteriumathousandtimes,itwouldbejustaboutthesizeofapinhead.Anadulthumanmagnifiedbythesameamountwouldbeoveramilekilometer)tall.Evenwithanordinarymicroscope,youmustlookcloselytoseebacteria.Usingamagnificationof100times,onefindsthatbacteriaarebarelyvisibleastinyrodsordots.Onecannotmakeoutanythingoftheirstructure.Usingspecialstains,onecanseethatsomebacteriahaveattachedtothemwavy-looking“hairs”calledflagella.Othershaveonlyoneflagellum.Theflagellarotate,pushingthebacteriathroughthewater.Manybacterialackflagellaandcannotmoveaboutbytheirownpower,whileotherscanglidealongoversurfacesbysomelittle-understoodmechanism.Fromthebacteriapointofview,theworldisaverydifferentplacefromwhatitistohumans.Toabacteriumwaterisasthickasmolassesistous.Bacteriaaresosmallthattheyareinfluencedbythemovementsofthechemicalmoleculesaroundthem.Bacteriaunderthemicroscope,eventhosewithnoflagella,oftenbounceaboutinthewater.Thisisbecausetheycollidewiththewaterymoleculesandarepushedthiswayandthat.Moleculesmovesorapidlythatwithinatenthofasecondthemoleculesaroundabacteriahaveallbeenreplacedbynewones;evenbacteriawithoutflagellaarethusconstantlyexposedtoachangingenvironment.細(xì)菌細(xì)菌是極其微小的生物體。我們用英寸或厘米來測(cè)量自己的大小，而測(cè)量細(xì)菌卻要用微米。一微米等于千分之一毫米。針頭直徑大約一毫米。棒狀細(xì)菌通常有2～4微米長(zhǎng)，而圓形細(xì)菌的直徑一般只有1微米。因此，即使你把一個(gè)圓形細(xì)菌放大1000倍，它也不過一個(gè)針頭那么大。可是如果把一個(gè)成年人放大1000倍，就會(huì)變成1英里(或公里)多高。用一般的顯微鏡觀察細(xì)菌時(shí)，你必須仔細(xì)觀察才能看見它們。使用100倍的顯微鏡時(shí)，你會(huì)發(fā)現(xiàn)細(xì)菌不過是隱約可見的小細(xì)棒或小點(diǎn)點(diǎn)，而它們的結(jié)構(gòu)你卻根本看不出來。使用特殊的著色劑后，你會(huì)發(fā)現(xiàn)有的細(xì)菌上長(zhǎng)著不少波狀的"毛發(fā)"即鞭毛，而有的細(xì)菌只有一根鞭毛。鞭毛的旋轉(zhuǎn)可以推動(dòng)細(xì)菌在水中行進(jìn)。不少細(xì)菌沒有鞭毛，因而不能自己行進(jìn)。還有些細(xì)菌卻能通過某些鮮為人知的機(jī)制沿物體表面滑動(dòng)。我們所熟知的世界在細(xì)菌眼中完全是另一個(gè)樣子。對(duì)于細(xì)菌來說，水就同糖漿之于人類一樣稠密。細(xì)菌是如此的微小，周圍化學(xué)分子的一舉一動(dòng)都會(huì)對(duì)它們產(chǎn)生影響。在顯微鏡下，細(xì)菌，甚至包括那些沒有鞭毛的細(xì)菌，經(jīng)常在水中跳來跳去。這是因?yàn)樗鼈兣c水分子相撞后，被彈向各個(gè)方向。分子移動(dòng)很迅速，僅秒之隔，一個(gè)細(xì)菌周圍的分子就會(huì)完全更新。因此，即使是沒有鞭毛的細(xì)菌也暴露在一個(gè)不斷變化的環(huán)境中新東方優(yōu)美背誦短文50篇Unit47:FolkCulture47FolkCulturesAfolkcultureisasmallisolated,cohesive,conservative,nearlyself-sufficientgroupthatishomogeneousincustomandracewithastrongfamilyorclanstructureandhighlydevelopedrituals.Orderismaintainedthroughsanctionsbasedinthereligionorfamilyandinterpersonal.Relationshipsarestrong.Traditionisparamount,andchangecomesinfrequentlyandslowly.Thereisrelativelylittledivisionoflaborintospecializedduties.Rather,eachpersonisexpectedtoperformagreatvarietyoftasks,thoughdutiesmaydifferbetweenthesexes.Mostgoodsarehandmadeandsubsistenceeconomyprevails.Individualismisweaklydevelopedinfolkculturesasaresocialclasses.UnalteredfolkculturesnolongerexistinindustrializedcountriessuchastheUnitedStatesandCanada.PerhapsthenearestmodernequivalentinAngloAmericaistheAmish,aGermanAmericanfarmingsectthatlargelyrenouncestheproductsandlaborsavingdevicesoftheindustrialage.InAmishareas,horsedrawnbuggiesstillserveasalocaltransportationdeviceandthefaithfularenotpermittedtoownautomobiles.TheAmish’scentralreligiousconceptofDemut“humility”,clearlyreflectstheweaknessofindividualismandsocialclasssotypicaloffolkculturesandthereisacorrespondingstrengthofAmishgroupidentity.RarelydotheAmishmarryoutsidetheirsect.Thereligion,avarietyoftheMennonitefaith,providestheprincipalmechanismformaintainingorder.Bycontrastapopularcultureisalargeheterogeneousgroupoftenhighlyindividualisticandapronouncedmanyspecializedprofessions.Secularinstitutionsofcontrolsuchasthepoliceandarmytaketheplaceofreligionandfamilyinmaintainingorder,andamoney-basedeconomyprevails.Becauseofthesecontrasts,“popular”maybeviewedasclearlydifferentfrom“folk”.Thepopularisreplacingthefolkinindustrializedcountriesandinmanydevelopingnations.Folk-madeobjectsgivewaytotheirpopularequivalent,usuallybecausethepopularitemismorequicklyorcheaplyproduced,iseasierortimesavingtouseorleadsmoreprestigetotheowner.民間文化民間文化是小型的、孤立的、緊密的、保守的、近乎自給自足的群體，具有同樣的習(xí)俗、同樣的人種和強(qiáng)有力的家庭或部族結(jié)構(gòu)以及高度發(fā)展的宗教儀式。秩序由宗教或家庭的約束來維持，成員間的關(guān)系非常緊密，傳統(tǒng)至高無上，很少有變動(dòng)且變動(dòng)緩慢。勞動(dòng)專業(yè)分工相對(duì)較少。每個(gè)人都要做各類活計(jì)，盡管男女兩性分工不同。絕大多數(shù)物品是手工制造的，經(jīng)濟(jì)一般為自給自足型。個(gè)人主義和社會(huì)階層在民間文化群體中的發(fā)展十分薄弱。在象美國(guó)和加拿大這樣的工業(yè)化國(guó)家里，一成不變的民間文化群體已不復(fù)存在了。在當(dāng)代美洲的英語區(qū)，與民間文化最相似的群體也許算是Amish。Amish是美國(guó)的德裔農(nóng)耕部落，他們基本上拒絕接受工業(yè)時(shí)代的大多數(shù)產(chǎn)品和節(jié)省勞力的設(shè)施。在Amish地區(qū)，輕便馬車仍是當(dāng)?shù)氐慕煌üぞ撸磐絺儾辉试S擁有汽車。Amish宗教中的核心觀念Demut即謙卑典型地反映了在民間文化群中個(gè)人主義和階級(jí)的不發(fā)達(dá)。而與此同時(shí)，Amish對(duì)群體的認(rèn)同性卻十分強(qiáng)。Amish人很少和他們宗派以外的人通婚。其宗教，作為Mennonite信仰的一種，提供了維護(hù)秩序的主要機(jī)制。相反，大眾文化是包含不同種族的大群體，通常高度個(gè)性化而且不斷在變化。人際關(guān)系冷漠，勞動(dòng)分工明確，由此產(chǎn)生了許多專門的職業(yè)。世俗的控制機(jī)構(gòu)，比如警察和軍隊(duì)，取代了宗教和家庭來維持秩序，而且實(shí)行的是貨幣經(jīng)濟(jì)。由于存在著這些差異，"大眾的"與"民間的"可謂大相徑庭。在工業(yè)化國(guó)家以及許多發(fā)展中國(guó)家里，大眾文化正在取代民間文化。民間制造的物品正讓位于大眾化產(chǎn)品，這通常是因?yàn)榇蟊娀奈锲分圃炱饋砀?、更便宜，用起來更容易、更方便或者是能給其所有者帶來更多的威望。新東方優(yōu)美背誦短文50篇Unit46:VisionHumanvisionlikethatofotherprimateshasevolvedinanarborealenvironment.Inthedensecomplexworldofatropicalforest,itismoreimportanttoseewellthattodevelopanacutesenseofsmell.Inthecourseofevolutionmembersoftheprimatelinehaveacquiredlargeeyeswhilethesnouthasshrunktogivetheeyeanunimpededview.Ofmammalsonlyhumansandsomeprimatesenjoycolorvision.Theredflagisblacktothebull.Horsesliveinamonochromeworld.lightvisibletohumaneyeshoweveroccupiesonlyaverynarrowbandinthewholeelectromagneticspectrum.Ultravioletraysareinvisibletohumansthoughantsandhoneybeesaresensitivetothem.Humansthoughantsandhoneybeesaresensitivetothem.Humanshavenodirectperceptionofinfraredraysunliketherattlesnakewhichhasreceptorstunedintowavelengthslongerthanmicron.Theworldwouldlookeerilydifferentifhumaneyesweresensitivetoinfraredradiation.Theninsteadofthedarknessofnight,wewouldbeabletomoveeasilyinastrangeshadowlessworldwhereobjectsglowedwithvaryingdegreesofintensity.Buthumaneyesexcelinotherways.Theyareinfactremarkablydiscerningincolorgradation.Thecolorsensitivityofnormalhumanvisionisrarelysurpassedevenbysophisticatedtechnicaldevices.視覺人類的視覺，和其它靈長(zhǎng)目動(dòng)物的一樣，是在叢林環(huán)境中進(jìn)化出來的。在稠密、復(fù)雜的熱帶叢林里，好的視覺比靈敏的嗅覺更加重要。在進(jìn)化過程中，靈長(zhǎng)目動(dòng)物的眼睛變大，同時(shí)鼻子變小以使視野不受阻礙。在哺乳類動(dòng)物中，只有人和一些靈長(zhǎng)目動(dòng)物能夠分辨顏色。紅旗在公?？磥硎呛谏模R則生活在一個(gè)單色的世界里。然而，人眼可見的光在整個(gè)光譜中只占一個(gè)非常狹窄的頻段。人是看不到紫外線的，盡管螞蟻和蜜蜂可以感覺到。與響尾蛇不同，人也不能直接感受到紅外線。響尾蛇的感覺器可以感受波長(zhǎng)超過微米的光線。如果人能感受到紅外線的話，這世界看上去將十分不同，而且恐怖。到那時(shí)，將與夜的黑暗相反，我們能輕易地在一個(gè)奇異的沒有陰影的世界里走動(dòng)。任何物體都強(qiáng)弱不等地閃著光。然而，人眼在其它方面有優(yōu)越之處。事實(shí)上，人眼對(duì)顏色梯度具有非凡的分辨能力。普通人類的視覺感受色彩的靈敏程度，甚至連精密的技術(shù)裝備都很難超越。新東方優(yōu)美背誦短文50篇Unit45:TheSourceofEnergesourceofEnergyAsummaryofthephysicalandchemicalnatureoflifemustbegin,notontheEarth,butintheSun;infact,attheSun’sverycenter.ItisherethatistobefoundthesourceoftheenergythattheSunconstantlypoursoutintospaceaslightandheat.ThisenergyislibratedatthecenteroftheSunasbillionsuponbillionsofnucleiofhydrogenatomscollidewitheachotherandfusetogethertoformnucleiofhelium,andindoingso,releasesomeoftheenergythatisstoredinthenucleiofatoms.TheoutputoflightandheatoftheSunrequiresthatsome600milliontonsofhydrogenbeconvertedintoheliumintheSuneverysecond.ThistheSunhasbeendoingforseveralthousandsofmillionsofyear.ThenuclearenergyisreleasedattheSun’scenterashigh-energygammaradiation,aformofelectromagneticradiationlikelightandradiowaves,onlyofverymuchshorterwavelength.ThisgammaradiationisabsorbedbyatomsinsidetheSuntobereemittedatslightlylongerwavelengths.Thisradiation,initsturnisabsorbedandreemitted.Astheenergyfiltersthroughthelayersofthesolarinterior,itpassesthroughtheX-raypartofthespectrumeventuallybecominglight.Atthisstage,ithasreachedwhatwecallthesolarsurface,andcanescapeintospacewithoutbeingabsorbedfurtherbysolaratoms.AverysmallfractionoftheSun’slightandheatisemittedinsuchdirectionsthatafterpassingunhinderedthroughinterplanetaryspace,ithitstheEarth.能量的來源概說生命的物理和化學(xué)特性必須始于太陽--確切地說，是太陽的核心，而非地球。能量來自太陽的核心。在這里，太陽不停地以光和熱的形式向空間傾瀉出能量。數(shù)十億計(jì)的氫原子核在太陽的核心碰撞并且聚變生成氦。在此過程中一部分原本儲(chǔ)存于原子核中的能量被釋放出來。太陽所產(chǎn)生的光和熱需要每秒將六億噸氫轉(zhuǎn)化為氦。這樣的轉(zhuǎn)化在太陽中已經(jīng)持續(xù)幾十億年了。核能在太陽的核心被釋放為高能的伽馬射線。這是一種電磁射線，就象光波和無線電波一樣，只是波長(zhǎng)要短得多。這種伽瑪射線被太陽內(nèi)的原子所吸收，然后重新釋放為波長(zhǎng)稍長(zhǎng)一些的光波。這新的射線再次被吸收，而后釋放。在能量由太陽內(nèi)部一層層滲透出來的過程中，它經(jīng)過了光譜中X射線部分，最后變成了光。在此階段，能量到達(dá)我們所稱的太陽表層，并且離散到空間而不再被太陽原子所吸收。只有很小一部分太陽的光和熱由此方向釋放出來，并且未被阻擋，穿越星空，來到地球。新東方優(yōu)美背誦短文50篇Unit44:ObtainingFreshwaterfromicebergsFreshwaterfromicebergsTheconceptofobtainingfreshwaterfromicebergsthataretowedtopopulatedareasandaridregionsoftheworldwasoncetreatedasajokemoreappropriatetocartoonsthanreallife.Butnowitisbeingconsideredquiteseriouslybymanynations,especiallysincescientistshavewarnedthatthehumanracewilloutgrowitsfreshwatersupplyfasterthanitrunsoutoffood.Glaciersareapossiblesourceoffreshwaterthathasbeenoverlookeduntilrecently.Three-quartersoftheEarth’sfreshwatersupplyisstilltiedupinglacialice,areservoirofuntappedfreshwatersoimmensethatitcouldsustainalltheriversoftheworldfor1,000years.Floatingontheoceanseveryyearare7,659trillionmetrictonsoficeencasedin10000icebergsthatbreakawayfromthepolaricecaps,morethanninetypercentofthemfromAntarctica.Hugeglaciersthatstretchovertheshallowcontinentalshelfgivebirthtoicebergsthroughouttheyear.Icebergsarenotlikeseaice,whichisformedwhentheseaitselffreezes,rather,theyareformedentirelyonland,breakingoffwhenglaciersspreadoverthesea.Astheydriftawayfromthepolarregion,icebergssometimesmovemysteriouslyinadirectionoppositetothewind,pulledbysubsurfacecurrents.Becausetheymeltmoreslowlythansmallerpiecesofice,icebergshavebeenknowntodriftasfarnorthas35degreessouthoftheequatorintheAtlanticOcean.Tocorralthemandsteerthemtopartsoftheworldwheretheyareneededwouldnotbetoodifficult.Thedifficultyarisesinothertechnicalmatters,suchasthepreventionofrapidmeltinginwarmerclimatesandthefunnelingoffreshwatertoshoreingreatvolume.Buteveniftheicebergslosthalfoftheirvolumeintowing,thewatertheycouldprovidewouldbefarcheaperthanthatproducedbydesalinization,orremovingsaltfromwater.從冰山中獲取淡水把冰山拖到世界上人口稠密的地區(qū)和干旱地帶，再?gòu)闹蝎@取淡水，這個(gè)想法曾一度被認(rèn)為是一個(gè)笑話，更適合于卡通畫，而非現(xiàn)實(shí)生活。然而現(xiàn)在，許多國(guó)家正相當(dāng)認(rèn)真地考慮這件事情，特別是在科學(xué)家們發(fā)出警告之后?？茖W(xué)家們認(rèn)為人類將在耗盡糧食之前首先耗盡淡水資源。冰川是一個(gè)直到最近以前一直被忽視的可能的淡水源。全球四分之三的淡水還鎖在冰川的冰塊中。冰川就是一個(gè)蓄水池，其中未開發(fā)的淡水量是如此巨大，足夠支持全世界的江河1000年。每年有7，659萬億公噸冰漂流在海洋中。它們包含在10，000座從極地冰帽中斷裂出來的冰山中。這些冰山的90%以上來自南極。一年四季里，覆蓋在淺層大陸架上的巨大冰川生成了眾多冰山。冰山和海水的冰不同，后者是海水自身結(jié)冰形成的，而冰山則完全是在陸地上形成的。當(dāng)冰川伸展到海水中時(shí)，冰山就斷裂下來。當(dāng)漂離極地地區(qū)時(shí)，冰山有時(shí)會(huì)在底層洋流的推動(dòng)下頗為神秘地逆風(fēng)移動(dòng)。由于冰山比小塊的冰融化要慢，因此有的冰山在大西洋中向北飄到了赤道以南35°的地方。把冰山蓄攔起來并拖到世界上需要它們的地方將不會(huì)太困難。有困難的是其它的技術(shù)事宜。比如，如何防止冰山在較暖的氣候中迅速融化以及如何把大量的淡水收集到岸上去。但是，即便在拖的過程中冰山失去了一半體積，這樣做也遠(yuǎn)比從海水中脫鹽取得淡水便宜。新東方優(yōu)美背誦短文50篇Unit43:StatisticsThereweretwowidelydivergentinfluencesontheearlydevelopmentofstatisticalmethods.Statisticshadamotherwhowasdedicatedtokeepingorderlyrecordsofgovernmentunits(statesandstatisticscomefromthesameLatinrootstatus)andagentlemanlygamblingfatherwhoreliedonmathematicstoincreasehisskillatplayingtheoddsingamesofchance.Theinfluenceofthemotherontheoffspring,statistics,isrepresentedbycounting,measuring,describing,tabulating,ordering,andthetakingofcensuses—allofwhichledtomoderndescriptivestatistics.Fromtheinfluenceofthefathercamemoderninferentialstatistics,whichisbasedsquarelyontheoriesofprobability.Describingcollectionsinvolvestabulating,depictinganddescribingcollectionsofdata.Thesedatamaybequantitativesuchasmeasuresofheight,intelligenceorgradelevelvariablesthatarecharacterizedbyanunderlyingcontinuumorthedatamayrepresentqualitativevariables,suchassex,collegemajororpersonalitytype.Largemassesofdatamustgenerallyundergoaprocessofsummarizationorreductionbeforetheyarecomprehensible.Descriptivestatisticsisatoolfordescribingorsummarizingorreducingtocomprehensibleformthepropertiesofanotherwiseunwieldymassofdata.Inferentialstatisticsisaformalizedbodyofmethodsforsolvinganotherclassofproblemsthatpresentgreatofproblemscharacteristicallyinvolvesattemptstomakepredictionsusingasampleofobservations.Forexample,aschoolsuperintendentwishestodeterminetheproportionofchildreninalargeschoolsystemwhocometoschoolwithoutbreakfast,havebeenvaccinatedforflu,orwhatever.Havingalittleknowledgeofstatistics,thesuperintendentwouldknowthatitisunnecessaryandinefficienttoquestioneachchild:theproportionforthesampleofasfewas100children.Thus,thepurposeofinferentialstatisticsistopredictorestimatecharacteristicsofapopulationfromaknowledgeofthecharacteristicsofonlyasampleofthepopulation.統(tǒng)計(jì)學(xué)統(tǒng)計(jì)方法的早期發(fā)展受到兩種截然不同的影響。統(tǒng)計(jì)學(xué)有一個(gè)"母親"，她致力于井井有條地記錄政府機(jī)構(gòu)的文件(國(guó)家和統(tǒng)計(jì)學(xué)這兩個(gè)詞源于同一個(gè)拉丁語詞根，status)，還有一個(gè)有紳士般的賭博"父親"，他依靠數(shù)學(xué)來提高賭技，以便在幾率的游戲中取勝。"母親"對(duì)其子女統(tǒng)計(jì)學(xué)的影響表現(xiàn)在計(jì)數(shù)、測(cè)量、描述、制表、歸類和人口普查。所有這些導(dǎo)致了現(xiàn)代描述統(tǒng)計(jì)學(xué)的誕生。由于"父親"的影響則產(chǎn)生了完全基于概率論原理的現(xiàn)代推理統(tǒng)計(jì)學(xué)。描述統(tǒng)計(jì)學(xué)涉及對(duì)所收集數(shù)據(jù)的制表、制圖和描述。這些數(shù)據(jù)可以是數(shù)量性的數(shù)據(jù)，如高度、智商、或者是層級(jí)性的數(shù)據(jù)--具有連續(xù)性的變量--或數(shù)據(jù)也可以代表性質(zhì)變量，如性別、大學(xué)專業(yè)或性格類型等等。數(shù)量龐大的數(shù)據(jù)通常必須經(jīng)過概括或刪減的程序才能為人所理解。描述統(tǒng)計(jì)學(xué)就是這樣一個(gè)工具，它對(duì)極其龐雜的數(shù)據(jù)進(jìn)行描述、概括或刪減，使其變成能為人理解的東西。推理統(tǒng)計(jì)學(xué)是一套已定形了的方法體系，它解決的是光憑人腦極難解決的另一類問題。這類問題的顯著特點(diǎn)是試圖通過取樣調(diào)查來作出預(yù)測(cè)。例如，有一位教育督察想知道在一個(gè)龐大的學(xué)校系統(tǒng)中，不吃早飯就上學(xué)的學(xué)生、已經(jīng)做過防感冒免疫的學(xué)生，或其它任何類型的學(xué)生占多大比例。若具備一些統(tǒng)計(jì)學(xué)的知識(shí)，這位督察應(yīng)明白，詢問每個(gè)孩子是沒有必要而且沒有效率的，只要用100個(gè)孩子為樣本，他就可以相當(dāng)精確地得出這些孩子占整個(gè)學(xué)區(qū)的比例了。因此，推理統(tǒng)計(jì)學(xué)的目的就是通過了解一個(gè)群體中一些樣本的特性，從而對(duì)整個(gè)群體的特性進(jìn)行推測(cè)和估算。新東方優(yōu)美背誦短文50篇Unit42:Coal-firedpowerplantspowerplantsTheinventionoftheincandescentlightbulbbyThomasA.Edisonin1879createdademandforacheap,readilyavailablefuelwithwhichtogeneratelargeamountsofelectricpower.Coalseemedtofitthebill,anditfueledtheearliestpowerstations.(whichweresetupattheendofthenineteenthcenturybyEdisonhimself).Asmorepowerplantswereconstructedthroughoutthecountry,therelianceoncoalincreasedthroughoutthecountry,therelianceoncoalincreased.SincetheFirstWorldWar,coal-firedpowerplantshadacombinedintheUnitedStateseachyear.In1986suchplantshadacombinedgeneratingcapacityof289,000megawattsandconsumed83percentofthenearly900milliontonsofcoalminedinthecountrythatyear.Giventheuncertaintyinthefuturegrowthofthenearly900milliontonsofcoalminedinthecountrythatyear.Giventheuncertaintyinthefuturegrowthofnuclearpowerandinthesupplyofoilandnaturalgas,coal-firedpowerplantscouldwellprovideupto70percentoftheelectricpowerintheUnitedStatesbytheendofthecentury.Yet,inspiteofthefactthatcoalhaslongbeenasourceofelectricityandmayremainonformanyyears(coalrepresentsabout80percentofUnitedStatesfossil-fuelreserves),ithasactuallyneverbeenthemostdesirablefossilfuelforpowerplants.Coalcontainslessenergyperunitofweightthanweightthannaturalgasoroil;itisdifficulttotransport,anditisassociatedwithahostofenvironmentalissues,amongthemacidrain.Sincethelate1960’Changesinthetechnologicalbaseofcoal-firedpowerplantscouldrestoretheirattractiveness,however.Whereassomeofthesechangesareintendedmainlytoincreasetheproductivityofexistingplants,completelynewtechnologiesforburningcoalcleanlyarealsobeingdeveloped.火力發(fā)電廠托馬斯愛迪生1879年發(fā)明的白熾燈導(dǎo)致對(duì)便宜、易得、可生產(chǎn)大量電能的燃料的需求。煤似乎符合這個(gè)要求，并成為第一批電廠的燃料(正是愛迪生本人在19世紀(jì)末建造了第一批電廠)。全國(guó)到處興建電廠時(shí)，對(duì)煤的依賴加深了。自第一次世界大戰(zhàn)以來，美國(guó)每年約有一半的電力是以煤為燃料的電廠提供的。1986年這些電廠的總發(fā)電能力達(dá)到28，900千瓦并且消耗了當(dāng)年全國(guó)開采的九億噸煤的83%?？紤]到核能發(fā)展以及石油、天然氣供應(yīng)中的不確定因素，到本世紀(jì)末，火力發(fā)電廠仍可能為美國(guó)提供多達(dá)70%的電力。然而，盡管煤長(zhǎng)期以來一直是電力的原料之一并且可能會(huì)繼續(xù)如此(煤占美國(guó)化石燃料儲(chǔ)量的80%)，它卻不是電廠的理想燃料。煤的單位能量含量低于石油和天然氣，而且會(huì)導(dǎo)致包括酸雨在內(nèi)的一系列環(huán)境問題。從1960年以來，排放控制和垃圾處理的問題極大地削弱了燃煤電廠的魅力。由于減輕這些環(huán)境問題需要大量資金，而且建造龐大復(fù)雜的燃煤電廠的費(fèi)用不斷上漲，也使得這些電廠從經(jīng)濟(jì)角度上不具備吸引力。改變火力發(fā)電廠的基礎(chǔ)技術(shù)卻可能恢復(fù)它們的吸引力。雖然某些技術(shù)改進(jìn)是漸進(jìn)的，其目的只是提高現(xiàn)有電廠的生產(chǎn)率，但人們正在開發(fā)全新的清潔燃煤的技術(shù)。新東方優(yōu)美背誦短文50篇Unit41:AmericanBlackBearsblackbearsAmericanblackbearsappearinavarietyofcolorsdespitetheirname.Intheeasternpartoftheirrange,mostofthesebrown,red,orevenyellowcoats.Tothenorth,theblackbearisactuallygrayorwhiteincolor.Eveninthesamelitter,bothbrownandblackfurredbearsmaybeborn.BlackbearsarethesmallestofallAmericanbears,ranginginlengthfromfivetosixfeet,weighingfromthreehundredtofivehundredpoundsTheireyesandearsaresmallandtheireyesightandhearingarenotasgoodastheirsenseofsmell.Likeallbears,theblackbearistimid,clumsy,andrarelydangerous,butifattacked,mostcanclimbtreesandcovergroundatgreatspeeds.Whenangryorfrightened,itisaformidableenemy.Blackbearsfeedonleaves,herbs.Fruit,berries,insects,fish,andevenlargeranimals.Oneofthemostinterestingcharacteristicsofbears,includingtheblackbear,istheirwintersleep.Unlikesquirrels,woodchucks,andmanyotherwoodlandanimals,bearsdonotactuallyhibernate.Althoughthebeardoesnotduringthewintermoths,sustainingitselffrombodyfat,itstemperatureremainsalmostnormal,anditbreathesregularlyfourorfivetimesperminute.Mostblackbearslivealone,exceptduringmatingseason.Theyprefertoliveincaves,hollowlogs,ordensethickets.AlittleofonetofourcubsisborninJanuaryorFebruaryafteragestationperiodofsixtoninemonths,andtheyremainwiththeirmotheruntiltheyarefullygrownoraboutoneandahalfyearsold.Blackbearscanliveaslongasthirtyyearsinthewild,andevenlongeringamepreservessetasideforthem.美國(guó)黑熊美國(guó)黑熊雖然被叫做黑熊但卻有各種各樣的顏色。在它們生活區(qū)域的東部，大部分黑熊長(zhǎng)有富有光澤的黑毛，但在西部，他們則長(zhǎng)著棕色、紅色甚至是黃色的毛。在北部，黑熊其實(shí)長(zhǎng)著灰色或白色的毛。就是在一胎所生的小熊中，都可能混雜棕毛和黑毛。黑熊是所有美洲熊中最小的，5～6英尺長(zhǎng)，300～500磅重。它們的眼睛和耳朵都很小，他們的視力和聽覺不如嗅覺那樣好。像所有的熊一樣，黑熊膽小，笨拙，很少具有危險(xiǎn)性。但如果受到攻擊，大部分黑熊會(huì)以很快的速度爬上樹和奔跑。當(dāng)發(fā)怒或受驚嚇時(shí)，黑熊會(huì)成為可怕的對(duì)手。黑熊以樹葉、草、樹根、水果、漿果、昆蟲、魚，甚至更大的動(dòng)物為食。熊類，包括黑熊的最有趣的一個(gè)特點(diǎn)是他們的冬眠。與松鼠、旱獺和其它別的林地動(dòng)物不同，熊并不真正地冬眠。雖然熊在冬天的幾個(gè)月中不吃東西，靠體內(nèi)脂肪維持生命，但它們的體溫保持正常，并有規(guī)律地一分鐘呼吸4或5次。除交配季節(jié)外，大多數(shù)黑熊獨(dú)自生活。他們喜歡住在洞里、空心的大木頭里或茂密的樹叢里。經(jīng)過6到9個(gè)月的懷孕期后一胎1～4個(gè)小熊在1月或2月出生。它們同母熊住在一起，直到它們完全長(zhǎng)大，即1歲半左右。黑熊在野外可以活到長(zhǎng)達(dá)30年，在專門的保護(hù)區(qū)中甚至能活得更長(zhǎng)。新東方優(yōu)美背誦短文50篇Unit40:Cohesion-tensionTheory40Cohesion-tensionTheoryAtmosphericpressurecansupportacolumnofwaterupto10metershigh.Butplantscanmovewatermuchhigher;thesequoiatreecanpumpwatertoitsverytopmorethan100metersabovetheground.Untiltheendofthenineteenthcentury,themovementofwaterintreesandothertallplantswasamystery.Somebotanistshypothesizedthatthelivingcellsofplantsactedaspumps.Butmanyexperimentsdemonstratedthatthestemsofplantsinwhichallthecellsarekilledcanstillmovewatertoappreciableheights.Otherexplanationsforthemovementofwaterinplantshavebeenbasedonrootpressure,apushonthewaterfromtherootsatthebottomoftheplant.Butrootpressureisnotnearlygreatenoughtopushwatertothetopsoftalltrees.Furthermore,theconifers,whichareamongthetallesttrees,haveunusuallylowrootpressures.Ifwaterisnotpumpedtothetopofatalltree,andifitisnotpushedtothetopofatalltree,thenwemayask:howdoesitgetthereAccordingtothecurrentlyacceptedcohesion-tensiontheory,waterispulledthere.Thepullonarisingcolumnofwaterinaplantresultsfromtheevaporationofwateratthetopoftheplant.Aswaterislostfromthesurfaceoftheleaves,anegativepressure,ortension,iscreated.Theevaporatedwaterisreplacedbywatermovingfrominsidetheplantinunbrokencolumnsthatextendfromthetopofaplanttoitsroots.Thesameforcesthatcreatesurfacetensioninanysampleofwaterareresponsibleforthemaintenanceoftheseunbrokencolumnsofwater.Whenwaterisconfinedintubesofverysmallbore,theforcesofcohesion(theattractionbetweenwatermolecules)aresogreatthatthestrengthofacolumnofwatercompareswiththestrengthofasteelwireofthesamediameter.Thiscohesivestrengthpermitscolumnsofwatertobepulledtogreatheightswithoutbeingbroken.內(nèi)聚壓力理論大氣壓能夠支持10米高的水柱，但植物可將水送得更高。美洲紅杉就能把水泵到地面以上100多米高的樹頂。直到19世紀(jì)末，水在樹木和其它高大植物中的輸送還是一個(gè)謎。一些植物學(xué)家假定植物中的活細(xì)胞充當(dāng)了水泵的角色。但許多實(shí)驗(yàn)表明細(xì)胞都已死亡的植物莖干仍能將水輸送到相當(dāng)可觀的高度。對(duì)于植物中輸送水的其它解釋都基于根壓--植物底端的根對(duì)水的推動(dòng)。但根壓完全不足以將水推到樹頂。況且，最高樹木中的松柏只有很低的根壓。如果水不是被泵到高樹的樹頂，也不是被推到樹頂，那么我們會(huì)問：它是怎樣到達(dá)樹頂?shù)哪馗鶕?jù)目前為人們所接受的內(nèi)聚壓力的理論，水是被拉到上面去的。一株植物中作用于一個(gè)正在升高的水柱之上的拉力來自該植物頂部水的蒸發(fā)。由于水從葉子表面喪失，一個(gè)負(fù)壓力，或張力就得以產(chǎn)生。蒸發(fā)出去的水被植物里流動(dòng)的水代替。這些水形成水柱從植物頂端一直延伸到根部。在任何水樣中造成表面張力的力支持著這些不斷的水柱。當(dāng)水被限制在內(nèi)徑很小的管道中時(shí)，內(nèi)聚壓力(水分子之間的相互吸引力)是如此之大以致一支水柱的強(qiáng)度相當(dāng)于一根直徑相同的鋼絲的強(qiáng)度。這種內(nèi)聚壓力使得水柱被拉到非常高的地方而不會(huì)斷裂。新東方優(yōu)美背誦短文50篇Unit39:TheSalinityofOceanWaters39TheSalinityofOceanWatersIfthesalinityofoceanwatersisanalyzed,itisfoundtovaryonlyslightlyfromplacetoplace.Nevertheless,someofthesesmallchangesareimportant.Therearethreebasicprocessesthatcauseachangeinoceanicsalinity.Oneoftheseisthesubtractionofwaterfromtheoceanbymeansofevaporationconversionofliquidwatertowatervapor.Inthismannerthesalinityisincreased,sincethesaltsstaybehind.Ifthisiscarriedtotheextreme,ofcourse,whitecrystalsofsaltwouldbeleftbehind.Theoppositeofevaporationisprecipitation,suchasrain,bywhichwaterisaddedtotheocean.Heretheoceanisbeingdilutedsothatthesalinityisdecreased.Thismayoccurinareasofhighrainfallorincoastalregionswhereriversflowintotheocean.Thussalinitymaybeincreasedbythesubtractionofwaterbyevaporation,ordecreasedbytheadditionoffreshwaterbyprecipitationorrunoff.Normally,intropicalregionswherethesunisverystrong,theoceansalinityissomewhathigherthanitisinotherpartsoftheworldwherethereisnotasmuchevaporation.Similarly,incoastalregionswhereriversdilutethesea,salinityissomewhatlowerthaninotheroceanicareas.Athirdprocessbywhichsalinitymaybealteredisassociatedwiththeformationandmeltingofseaice.Whenseawaterisfrozen,the