第18章

ThebowlderswhosepresenceonthecragsoftheJuratheoldGerinanaccountedforinamannersotheatricalhadlongbeenasourceofcontentionamonggeologists。TheyarefoundnotmerelyontheJura，butonnumberlessothermountainsinallnorth-temperatelatitudes，andoftenfaroutintheopencountry，asmanyafarmerwhohasbrokenhisploughagainstthemmighttestify。Theearlygeologistsaccountedforthem，asfornearlyeverythingelse，withtheirsupposititiousDeluge。BrongniartandCuvierandBucklandandtheircontemporariesappearedtohavenodifficultyinconceivingthatmassesofgraniteweighinghundredsoftonshadbeensweptbythiscurrentscoresorhundredsofmilesfromtheirsource。But，ofcourse，theuniformitarianfaithpermittednosuchexplanation，norcoulditcountenancetheprojectionidea；soLyellwasboundtofindsomeothermeansoftransportationforthepuzzlingerratics。

Theonlyavailablemediumwasice，but，fortunately，thisoneseemedquitesufficient。Icebergs，saidLyell，areobservedtocarryallmannerofdebris，anddeposititinthesea-bottoms。Presentlandsurfaceshaveoftenbeensubmergedbeneaththesea。Duringthelatestofthesesubmergencesicebergsdepositedthebowldersnowscatteredhereandthereovertheland。Nothingcouldbesimplerormoreclearlyuniformitarian。Andeventhecatastrophists，thoughtheymetLyellamicablyonalmostnoothertheoreticalground，wereinclinedtoadmittheplausibilityofhistheoryoferratics。

Indeed，ofallLyell’snonconformistdoctrines，thisseemedtheonemostlikelytomeetwithgeneralacceptance。

Yet，evenasthisicebergtheoryloomedlargeandlargerbeforethegeologicalworld，observationsweremakinginadifferentfieldthatweredestinedtoshowitsfallacy。Asearlyas1815asharp-eyedchamois-

hunteroftheAlps，Perraudinbyname，hadnotedtheexistenceoftheerratics，and，unlikemostofhiscompanionhunters，hadpuzzledhisheadastohowthebowldersgotwherehesawthem。Heknewnothingofsubmergedcontinentsoroficebergs，stilllessofupheavingmountains；andthoughhedoubtlesshadheardoftheFlood，hehadnoexperienceofheavyrocksfloatinglikecorksinwater。Moreover，hehadneverobservedstonesrollinguphillandperchingthemselvesonmountain-tops，andhewasagoodenoughuniformitarian(thoughhewouldhavebeenpuzzledindeedhadanyonetoldhimso)todisbelievethatstonesinpasttimeshaddisportedthemselvesdifferentlyinthisregardfromstonesofthepresent。Yettherethestonesare。Howdidtheygetthere？

Themountaineerthoughtthathecouldanswerthatquestion。Hesawabouthimthosegiganticserpent-

likestreamsoficecalledglaciers，"fromtheirfarfountainsslowrollingon，"carryingwiththemblocksofgraniteandotherdebristoformmorainedeposits。

Iftheseglaciershadoncebeenmuchmoreextensivethantheynoware，theymighthavecarriedthebowldersandleftthemwherewefindthem。Ontheotherhand，noothernaturalagencywithinthesphereofthechamois-hunter’sknowledgecouldhaveaccomplishedthis，ergotheglaciersmustoncehavebeenmoreextensive。Perraudinwouldprobablyhavesaidthatcommon-sensedrovehimtothisconclusion；butbethatasitmay，hehadconceivedoneofthefewtrulyoriginalandnovelideasofwhichthenineteenthcenturycanboast。

Perraudinannouncedhisideatothegreatestscientistinhislittleworld——JeandeCharpentier，directoroftheminesatBex，askilledgeologistwhohadbeenafellow-pupilofVonBuchandVonHumboldtunderWernerattheFreibergSchoolofMines。Charpentierlaughedatthemountaineer’sgrotesqueidea，andthoughtnomoreaboutit。AndtenyearselapsedbeforePerraudincouldfindanyonewhotreatedhisnotionwithgreaterrespect。ThenhefoundalistenerinM。Venetz，acivilengineer，whoreadapaperonthenovelglacialtheorybeforealocalsocietyin1823。

ThisbroughtthematteroncemoretotheattentionofDeCharpentier，whonowfeltthattheremightbesomethinginitworthinvestigation。

AsurveyofthefieldinthelightofthenewtheorysoonconvincedCharpentierthatthechamois-hunterhadallalongbeenright。HebecameanenthusiasticsupporteroftheideathattheAlpshadoncebeenimbeddedinamassofice，andin1836hebroughtthenotiontotheattentionofLouisAgassiz，whowasspendingthesummerintheAlps。Agassizwasscepticalatfirst，butsoonbecameaconvert。

In1840AgassizpublishedapaperinwhichtheresultsofhisAlpinestudieswereelaborated。

"Letusconsider，"hesays，"thosemoreconsiderablechangestowhichglaciersaresubject，orrather，theimmenseextentwhichtheyhadintheprehistoricperiod。Thisformerimmenseextension，greaterthananythattraditionhaspreserved，isproved，inthecaseofnearlyeveryvalleyintheAlps，byfactswhicharebothmanyandwellestablished。Thestudyofthesefactsiseveneasyifthestudentislookingoutforthem，andifhewillseizetheleastindicationoftheirpresence；and，ifitwerealongtimebeforetheywereobservedandconnectedwithglacialaction，itisbecausetheevidencesareoftenisolatedandoccuratplacesmoreorlessremovedfromtheglacierwhichoriginatedthem。Ifitbetruethatitistheprerogativeofthescientificobservertogroupinthefieldofhismentalvisionthosefactswhichappeartobewithoutconnectiontothevulgarherd，itis，aboveall，insuchacaseasthisthatheiscalledupontodoso。Ihaveoftencomparedthesefeebleeffects，producedbytheglacialactionofformerages，withtheappearanceofthemarkingsuponalithographicstone，preparedforthepurposeofpreservation，anduponwhichonecannotseethelinesofthedraughtsman’sworkunlessitisknownbeforehandwhereandhowtosearchforthem。

"Thefactoftheformerexistenceofglacierswhichhavenowdisappearedisprovedbythesurvivalofthevariousphenomenawhichalwaysaccompanythem，andwhichcontinuetoexistevenaftertheicehasmelted。Thesephenomenaareasfollows:

"1。Moraines——Thedispositionandcompositionofmorainesenablethemtobealwaysrecognized，evenwhentheyarenolongeradjacenttoaglaciernorimmediatelysurrounditslowerextremities。Imayremarkthatlateralandterminalmorainesaloneenableustorecognizewithcertaintythelimitsofglacialextension，becausetheycanbeeasilydistinguishedfromthedikesandirregularlydistributedstonescarrieddownbytheAlpinetorrents，Thelateralmorainesdepositeduponthesidesofvalleysarerarelyaffectedbythelargertorrents，buttheyare，however，oftencutbythesmallstreamswhichfalldownthesideofamountain，andwhich，byinterferingwiththeircontinuity，makethemsomuchmoredifficulttorecognize。

"2。ThePerchedBowlders——Itoftenhappensthatglaciersencounterprojectingpointsofrock，thesidesofwhichbecomerounded，andaroundwhichfunnel-

likecavitiesareformedwithmoreorlessprofundity。

Whenglaciersdiminishandretire，theblockswhichhavefallenintothesefunnelsoftenremainpercheduponthetopoftheprojectingrockypointwithinit，insuchastateofequilibriumthatanyideaofacurrentofwaterasthecauseoftheirtransportationiscompletelyinadmissibleonaccountoftheirposition。Whensuchpointsofrockprojectabovethesurfaceoftheglacierorappearasamoreconsiderableisletinthemidstofitsmass(suchasisthecaseintheJardinoftheMerdeGlace，aboveMontavert)，suchprojectionsbecomesurroundedonallsidesbystoneswhichultimatelyformasortofcrownaroundthesummitwhenevertheglaciersdecreaseorretirecompletely。Watercurrentsneverproduceanythinglikethis；but，onthecontrary，wheneverastreambreaksitselfagainstaprojectingrock，thestoneswhichitcarriesdownareturnedasideandformamoreorlessregulartrail。

Never，undersuchcircumstances，canthestonesremaineitheratthetoporatthesidesoftherock，for，ifsuchathingwerepossible，therapidityofthecurrentwouldbeacceleratedbytheincreasedresistance，andthemovingbowlderswouldbecarriedbeyondtheobstructionbeforetheywerefinallydeposited。

"3。Thepolishedandstriatedrocks，suchashavebeendescribedinChapterXIV。，affordyetfurtherevidenceofthepresenceofaglacier；for，ashasbeensaidalready，neitheracurrentnortheactionofwavesuponanextensivebeachproducessucheffects。Thegeneraldirectionofthechannelsandfurrowsindicatesthedirectionofthegeneralmovementoftheglacier，andthestreakswhichvarymoreorlessfromthisdirectionareproducedbythelocaleffectsofoscillationandretreat，asweshallpresentlysee。

"4。TheLapiaz，orLapiz，whichtheinhabitantsofGermanSwitzerlandcallKarrenfelder，cannotalwaysbedistinguishedfromerosions，because，bothproducedastheyarebywater，theydonotdifferintheirexteriorcharacteristics，butonlyintheirpositions。

Erosionsduetotorrentsarealwaysfoundinplacesmoreorlessdepressed，andneveroccuruponlargeinclinedsurfaces。TheLapiaz，onthecontrary，arefrequentlyfoundupontheprojectingpartsofthesidesofvalleysinplaceswhereitisnotpossibletosupposethatwaterhaseverformedacurrent。Somegeologists，intheirembarrassmenttoexplainthesephenomena，havesupposedthattheywereduetotheinfiltrationofacidulatedwater，butthishypothesisispurelygratuitous。

"WewillnowdescribetheremainsofthesevariousphenomenaastheyarefoundintheAlpsoutsidetheactualglaciallimits，inordertoprovethatatacertainepochglaciersweremuchlargerthantheyareto-day。

"Theancientmoraines，situatedastheyareatagreatdistancefromthoseofthepresentday，arenowheresodistinctorsofrequentasinValais，whereMM。VenetzandJ。deCharpentiernoticedthemforthefirsttime；butastheirobservationsareasyetunpublished，andtheythemselvesgavemetheinformation，itwouldbeanappropriationoftheirdiscoveryifIweretodescribethemhereindetail。Iwilllimitmyselftosaythattherecanbefoundtraces，moreorlessdistinct，ofancientterminalmorainesintheformofvaulteddikesatthefootofeveryglacier，atadistanceofafewminutes’walk，aquarterofanhour，ahalf-hour，anhour，andevenofseveralleaguesfromtheirpresentextremities。Thesetracesbecomelessdistinctinproportiontotheirdistancefromtheglacier，and，sincetheyarealsooftentraversedbytorrents，theyarenotascontinuousasthemoraineswhicharenearertotheglaciers。Thefarthertheseancientmorainesareremovedfromtheterminationofaglacier，thehigheruptheyreachuponthesidesofthevalley，whichprovestousthatthethicknessoftheglaciermusthavebeengreaterwhenitssizewaslarger。Atthesametime，theirnumberindicatessomanystopping-placesintheretreatoftheglacier，orsomanyextremelimitsofitsextension——limitswhichwereneverreachedagainafterithadretired。Iinsistuponthispoint，becauseifitistruethatallthesemorainesdemonstratealargerextentoftheglacier，theyalsoprovethattheirretreatintotheirpresentboundaries，farfromhavingbeencatastrophic，wasmarkedonthecontrarybyperiodsofreposemoreorlessfrequent，whichcausedtheformationofaseriesofconcentricmoraineswhichevennowindicatetheirretrogression。

"Theremainsoflongitudinalmorainesarelessfrequent，lessdistinct，andmoredifficulttoinvestigate，because，indicatingastheydothelevelstowhichtheedgesoftheglacierreachedatdifferentepochs，itisgenerallynecessarytolookforthemabovethelineofthepathsalongtheescarpmentsofthevalleys，andhenceitisnotalwayspossibletofollowthemalongavalley。Often，also，thesidesofavalleywhichenclosedaglacieraresosteepthatitisonlyhereandtherethatthestoneshaveremainedinplace。Theyare，nevertheless，verydistinctinthelowerpartofthevalleyoftheRhone，betweenMartignyandtheLakeofGeneva，whereseveralparallelridgescanbeobserved，oneabovetheother，ataheightofonethousand，onethousandtwohundred，andevenonethousandfivehundredfeetabovetheRhone。ItisbetweenSt。MauriceandthecascadeofPissevache，closetothehamletofChaux-Fleurie，thattheyaremostaccessible，foratthisplacethesidesofthevalleyatdifferentlevelsascendinlittleterraces，uponwhichthemoraineshavebeenpreserved。TheyarealsoverydistinctabovetheBainsdeLavey，andabovethevillageofMontheyattheentranceoftheVald’Illiers，wherethesidesofthevalleyarelessinclinedthaninmanyotherplaces。

"TheperchedbowlderswhicharefoundintheAlpinevalleys，atconsiderabledistancesfromtheglaciers，occupyattimespositionssoextraordinarythattheyexciteinahighdegreethecuriosityofthosewhoseethem。Forinstance，whenoneseesanangularstonepercheduponthetopofanisolatedpyramid，orrestinginsomewayinaverysteeplocality，thefirstinquiryofthemindis，Whenandhowhavethesestonesbeenplacedinsuchpositions，wheretheleastshockwouldseemtoturnthemover？Butthisphenomenonisnotintheleastastonishingwhenitisseentooccuralsowithinthelimitsofactualglaciers，anditisrecalledbywhatcircumstancesitisoccasioned。

"ThemostcuriousexamplesofperchedstoneswhichcanbecitedarethosewhichcommandthenorthernpartofthecascadeofPissevache，closetoChaux-Fleurie，andthoseabovetheBainsdeLavey，closetothevillageofMorcles；andthose，evenmorecurious，whichIhaveseeninthevalleyofSt。NicolasandOberhasli。AtKirchet，nearMeiringen，canbeseensomeveryremarkablecrownsofbowldersaroundseveraldomesofrockwhichappeartohavebeenprojectedabovethesurfaceoftheglacierwhichsurroundedthem。SomethingverysimilarcanbeseenaroundthetopoftherockofSt。Triphon。

"TheextraordinaryphenomenonofperchedstonescouldnotescapetheobservingeyeofDeSaussure，whonoticedseveralatSaleve，ofwhichhedescribedthepositionsinthefollowingmanner:’Onesees，’

saidhe，’upontheslopeofaninclinedmeadow，twoofthesegreatbowldersofgranite，elevatedoneupontheother，abovethegrassataheightoftwoorthreefeet，uponabaseoflimestonerockonwhichbothrest。

Thisbaseisacontinuationofthehorizontalstrataofthemountain，andisevenunitedwithitvisiblyonitslowerface，beingcutperpendicularlyupontheothersides，andisnotlargerthanthestonewhichitsupports。’Butseeingthattheentiremountainiscomposedofthesamelimestone，DeSaussurenaturallyconcludedthatitwouldbeabsurdtothinkthatitwaselevatedpreciselyandonlybeneaththeblocksofgranite。But，ontheotherhand，sincehedidnotknowthemannerinwhichtheseperchedstonesaredepositedinourdaysbyglacialaction，hehadrecoursetoanotherexplanation:Hesupposesthattherockwaswornawayarounditsbasebythecontinualerosionofwaterandair，whiletheportionoftherockwhichservedasthebaseforthegranitehadbeenprotectedbyit。Thisexplanation，althoughveryingenious，couldnolongerbeadmittedaftertheresearchesofM。EliedeBeaumonthadprovedthattheactionofatmosphericagencieswasnotbyagooddealsodestructiveaswastheretoforesupposed。DeSaussurespeaksalsoofadetachedbowlder，situatedupontheoppositesideoftheTete-Noire，’whichis，’hesays，’ofsogreatasizethatoneistemptedtobelievethatitwasformedintheplaceitoccupies；anditiscalledBarmerusse，becauseitiswornawaybeneathintheformofacavewhichcanaffordaccommodationformorethanthirtypersonsatatime。"[4]

Buttheimplicationsofthetheoryofglaciersextend，soAgassizhascometobelieve，farbeyondtheAlps。IftheAlpshadbeencoveredwithanicesheet，sohadmanyotherregionsofthenorthernhemisphere。

Castingabroadforevidencesofglacialaction，Agassizfoundthemeverywhereintheformoftransportederratics，scratchedandpolishedoutcroppingrocks，andmoraine-likedeposits。Finally，hebecameconvincedthattheicesheetthatcoveredtheAlpshadspreadoverthewholeofthehigherlatitudesofthenorthernhemisphere，forminganicecapovertheglobe。

Thusthecommon-senseinductionofthechamois-

hunterblossomedinthemindofAgassizintotheconceptionofauniversaliceage。

In1837Agassizhadintroducedhistheorytotheworld，inapaperreadatNeuchatel，andthreeyearslaterhepublishedhisfamousEtudessurlesGlaciers，fromwhichwehavejustquoted。Neverdidideamakeamoreprofounddisturbanceinthescientificworld。

VonBuchtreateditwithalternateridicule，contempt，andrage；Murchisonopposeditwithcustomaryvigor；

evenLyell，whosemostremarkablementalendowmentwasanunfailingreceptivenesstonewtruths，couldnotatoncediscardhisicebergtheoryinfavorofthenewclaimant。Dr。Buckland，however，afterAgassizhadshownhimevidenceofformerglacialactioninhisownScotland，becameaconvert——themorereadily，perhaps，asitseemedtohimtoopposetheuniformitarianidea。Graduallyothersfellinline，andaftertheusualimbitteredcontroversyandtheinevitablefullgenerationofprobation，theideaofaniceagetookitsplaceamongtheacceptedtenetsofgeology。Allmannerofmootpointsstilldemandedattention——thecauseoftheiceage，theexactextentoftheicesheet，theprecisemannerinwhichitproduceditseffects，andtheexactnatureoftheseeffects；andnotallofthesehaveevenyetbeendetermined。But，detailsaside，theiceagenowhasfullrecognitionfromgeologistsasanhistoricalperiod。Theremayhavebeenmanyiceages，asDr。Crollcontends；therewassurelyone；andtheconceptionofsuchaperiodisoneoftheveryfewideasofourcenturythatnopreviouscenturyhadevensomuchasfaintlyadumbrated。

THEGEOLOGICALAGES

But，forthatmatter，theentiresubjectofhistoricalgeologyisonethathadbutthebarestbeginningbeforeourcentury。Untilthepaleontologistfoundoutthekeytotheearth’schronology，noone——notevenHutton——

couldhaveanydefiniteideaastothetruestoryoftheearth’spast。TheonlyconspicuousattempttoclassifythestratawasthatmadebyWerner，whodividedtherocksintothreesystems，basedontheirsupposedorderofdeposition，andcalledprimary，transition，andsecondary。

ThoughWerner’sobservationswereconfinedtothesmallprovinceofSaxony，hedidnothesitatetoaffirmthatallovertheworldthesuccessionofstratawouldbefoundthesameasthere，theconcentriclayers，accordingtothisconception，beingarrangedabouttheearthwiththeregularityoflayersonanonion。ButinthisWernerwasasmistakenasinhistheoreticalexplanationoftheoriginofthe"primary"rocks。Itrequiredbutlittleobservationtoshowthattheexactsuccessionofstrataisneverpreciselythesameinanywidelyseparatedregions。Nevertheless，therewasagermoftruthinWerner’ssystem。Itcontainedtheidea，howeverfaultilyinterpreted，ofachronologicalsuccessionofstrata；anditfurnishedaworkingoutlinefortheobserverswhoweretomakeoutthetruestoryofgeologicaldevelopment。ButthecorrectinterpretationoftheobservedfactscouldonlybemadeaftertheHuttonianviewastotheoriginofstratahadgainedcompleteacceptance。

WhenWilliamSmith，havingfoundthetruekeytothisstory，attemptedtoapplyit，theterritorywithwhichhehadtodealchancedtobeonewherethesurfacerocksareofthatlaterserieswhichWernertermedsecondary。Hemadenumeroussubdivisionswithinthissystem，basedmainlyonthefossils。Meantimeitwasfoundthat，judgedbythefossils，thestratathatBrongniartandCuvierstudiednearPariswereofastillmorerecentperiod(presumedatfirsttobeduetothelatestdeluge)，whichcametobespokenofastertiary。

Itwasinthesebeds，someofwhichseemedtohavebeenformedinfresh-waterlakes，thatmanyofthestrangemammalswhichCuvierfirstdescribedwerefound。

Butthe"transition"rocks，underlyingthe"secondary"

systemthatSmithstudied，werestillpracticallyunexploredwhen，alonginthethirties，theyweretakeninhandbyRoderickImpeyMurchison，thereformedfox-hunterandex-captain，whohadturnedgeologisttosuchnotableadvantage，andAdamSedgwick，thebrilliantWoodwardianprofessoratCambridge。

Workingtogether，thesetwofriendsclassifiedthetransitionrocksintochronologicalgroups，sincefamiliartoeveryoneinthelargeroutlinesastheSiluriansystem(ageofinvertebrates)andtheDevoniansystem(ageoffishes)——namesderivedrespectivelyfromthecountryoftheancientSilures，inWalesandDevonshire，England。Itwassubsequentlydiscoveredthatthesesystemsofstrata，whichcropoutfrombeneathnewerrocksinrestrictedareasinBritain，arespreadoutintobroad，undisturbedsheetsoverthousandsofmilesincontinentalEuropeandinAmerica。LateronMurchisonstudiedtheminRussia，anddescribedthem，conjointlywithVerneuilandVonKerserling，inaponderousandclassicalwork。InAmericatheywerestudiedbyHall，Newberry，Whitney，Dana，Whitfield，andotherpioneergeologists，whoallbutanticipatedtheirEnglishcontemporaries。

TherocksthatareofstillolderformationthanthosestudiedbyMurchisonandSedgwick(correspondinginlocationtothe"primary"rocksofWerner’sconception)

arethesurfacefeatureofvastareasinCanada，andwerefirstprominentlystudiedtherebyWilliamI。

Logan，oftheCanadianGovernmentSurvey，asearlyas1846，andlateronbySirWilliamDawson。Theserocks——comprisingtheLaurentiansystem——wereformerlysupposedtorepresentpartsoftheoriginalcrustoftheearth，formedonfirstcoolingfromamoltenstate；buttheyarenowmoregenerallyregardedasonce-stratifieddepositsmetamorphosedbytheactionofheat。

Whether"primitive"ormetamorphic，however，theseCanadianrocks，andanalogousonesbeneaththefossiliferousstrataofothercountries，aretheoldestportionsoftheearth’scrustofwhichgeologyhasanypresentknowledge。Mountainsofthisformation，astheAdirondacksandtheStormKingrange，overlookingtheHudsonnearWestPoint，arethepatriarchsoftheirkind，besidewhichAlleghaniesandSierraNevadasarerecentupstarts，andRockies，Alps，andAndesaremereparvenusofyesterday。

TheLaurentianrockswereatfirstspokenofasrepresenting"Azoic"time；butin1846Dawsonfoundaformationdeepintheirmidstwhichwasbelievedtobethefossilrelicofaverylowformoflife，andafterthatitbecamecustomarytospeakofthesystemas"Eozoic。"

StillmorerecentlythetitleofDawson’ssupposedfossiltorankassuchhasbeenquestioned，andDana’ssuggestionthattheearlyrocksbetermedmerelyArchmanhasmetwithgeneralfavor。MurchisonandSedgwick’sSilurian，Devonian，andCarboniferousgroups(theagesofinvertebrates，offishes，andofcoalplants，respectively)aretogetherspokenofasrepresentingPaleozoictime。WilliamSmith’ssystemofstrata，nextabovethese，oncecalled"secondary，"representsMesozoictime，ortheageofreptiles。Stillhigher，ormorerecent，areCuvierandBrongniart’stertiaryrocks，representingtheageofmammals。Lastly，themostrecentformations，datingback，however，toaperiodfarenoughfromrecentinanybutageologicalsense，areclassedasquaternary，representingtheageofman。

Itmustnotbesupposed，however，thatthesuccessive"ages"ofthegeologistareshutofffromoneanotherinanysucharbitrarywayasthisverbalclassificationmightseemtosuggest。Inpointoffact，these"ages"

havenobetterwarrantforexistencethanhavethe"centuries"andthe"weeks"ofevery-daycomputation。

Theyareconvenient，andtheymayevenstandforlocaldivisionsinthestrata，buttheyareboundedbynoactualgapsinthesweepofterrestrialevents。

Moreover，itmustbeunderstoodthatthe"ages"ofdifferentcontinents，thoughdescribedunderthesamename，arenotnecessarilyofexactcontemporaneity。

ThereisnosuretestavailablebywhichitcouldbeshownthattheDevonianage，forinstance，asoutlinedinthestrataofEurope，didnotbeginmillionsofyearsearlierorlaterthantheperiodwhoserecordsaresaidtorepresenttheDevonianageinAmerica。Inattemptingtodecidesuchdetailsasthis，mineralogicaldatafailusutterly。Eveninrocksofadjoiningregionsidentityofstructureisnoproofofcontemporaneousorigin；fortheveritablesubstanceoftherockofoneageisgrounduptobuildtherocksofsubsequentages。

Furthermore，inseaswhereconditionschangebutlittlethesameformofrockmaybemadeageafterage。Itisbelievedthatchalk-bedsstillforminginsomeofourpresentseasmayformonecontinuousmassdatingbacktoearliestgeologicages。Ontheotherhand，rocksdifferentincharactermaybeformedatthesametimeinregionsnotfarapart——sayasandstonealongshore，acorallimestonefartherseaward，andachalk-bedbeyond。

Thiscontinuousstratum，brokenintheprocessofupheaval，mightseemtherecordofthreedifferentepochs。

Paleontology，ofcourse，suppliesfarbetterchronologicaltests，buteventhesehavetheirlimitations。

Therehasbeennotimesincerocksnowinexistencewereformed，ifever，whentheearthhadauniformclimateandasingleundiversifiedfaunaoveritsentirelandsurface，astheearlypaleontologistssupposed。

Speakingbroadly，thesamegeneralstageshaveattendedtheevolutionoforganicformseverywhere，butthereisnothingtoshowthatequalperiodsoftimewitnessedcorrespondingchangesindiverseregions，butquitethecontrary。Tocitebutasingleillustration，themarsupialorder，whichisthedominantmammaliantypeofthelivingfaunaofAustraliato-day，existedinEuropeanddiedoutthereinthetertiaryage。HenceafuturegeologistmightthinktheAustraliaofto-daycontemporaneouswithaperiodinEuropewhichinrealityantedateditbyperhapsmillionsofyears。

Allthesepuzzlingfeaturesunitetorenderthesubjectofhistoricalgeologyanythingbutthesimplematterthefathersofthescienceesteemedit。Noonewouldnowattempttotracetheexactsequenceofformationofallthemountainsoftheglobe，asEliedeBeaumontdidahalf-centuryago。Evenwithinthelimitsofasinglecontinent，thegeologistmustproceedwithmuchcautioninattemptingtochronicletheorderinwhichitsvariouspartsrosefromthematrixofthesea。Thekeytothisstoryisfoundintheidentificationofthestratathatarethesurfacefeatureineachterritory。IfDevonianrocksareatthesurfaceinanygivenregion，forexample，itwouldappearthatthisregionbecamealandsurfaceintheDevonianage，orjustafterwards。Butamoment’sconsiderationshowsthatthereisanelementofuncertaintyaboutthis，duetothesteadydenudationthatalllandsurfacesundergo。

TheDevonianrocksmaylieatthesurfacesimplybecausethethousandsoffeetofcarboniferousstratathatoncelayabovethemhavebeenwornaway。Allthatthecautiousgeologistdareassert，therefore，isthattheregioninquestiondidnotbecomepermanentlandsurfaceearlierthantheDevonianage。

Buttoknoweventhisismuch——sufficient，indeed，toestablishthechronologicalorderofelevation，ifnotitsexactperiod，forallpartsofanycontinentthathavebeengeologicallyexplored——understandingalwaysthattheremustbenoscruplingaboutalatitudeofafewmillionsorperhapstensofmillionsofyearshereandthere。

Regardingourowncontinent，forexample，welearnthroughtheresearchesofamultitudeofworkersthatintheearlydayitwasamerearchipelago。Itschiefisland——thebackboneofthefuturecontinent——wasagreatV-shapedareasurroundingwhatisnowHudsonBay，anareabuilttip，perhaps，throughdenudationofayetmoreancientpolarcontinent，whoseexistenceisonlyconjectured。TothesoutheastanislandthatisnowtheAdirondackMountains，andanotherthatisnowtheJerseyHighlandsroseabovethewasteofwaters，andfartothesouthstretchedprobablyalineofislandsnowrepresentedbytheBlueRidgeMountains。

FarofftothewestwardanotherlineofislandsforeshadowedourpresentPacificborder。Afewminorislandsintheinteriorcompletedthearchipelago。

Fromthisbareskeletonthecontinentgrew，partlybythedepositofsedimentfromthedenudationoftheoriginalislands(whichoncetoweredmiles，perhaps，wherenowtheyrisethousandsoffeet)，butlargelyalsobythedepositoforganicremains，especiallyintheinteriorsea，whichteemedwithlife。IntheSilurianages，invertebrates——brachiopodsandcrinoidsandcephalopods——werethedominanttypes。Butveryearly——nooneknowsjustwhen——therecamefishesofmanystrangeforms，someoftheearlyonesenclosedinturtle-likeshells。Lateryet，largespaceswithintheinteriorseahavingrisentothesurface，greatmarshesorforestsofstrangetypesofvegetationgrewanddepositedtheirremainstoformcoal-beds。Manytimesoversuchforestswereformed，onlytobedestroyedbytheoscillationsofthelandsurface。Alltold，thestrataofthisPaleozoicperiodaggregateseveralmilesinthickness，andthetimeconsumedintheirformationstandstoalllatertimeuptothepresent，accordingtoProfessorDana’sestimate，asthreetoone。

TowardsthecloseofthisPaleozoiceratheAppalachianMountainswereslowlyupheavedingreatconvolutedfolds，someofthemprobablyreachingthreeorfourmilesabovethesea-level，thoughthetoothoftimehassincegnawedthemdowntocomparativelypunylimits。ThecontinentalareasthusenlargedwerepeopledduringtheensuingMesozoictimewithmultitudesofstrangereptiles，manyofthemgiganticinsize。

Thewaters，too，stillteemingwithinvertebratesandfishes，hadtheirquotaofreptilianmonsters；andintheairwereflyingreptiles，someofwhichmeasuredtwenty-

fivefeetfromtiptotipoftheirbatlikewings。DuringthiseratheSierraNevadaMountainsrose。Neartheeasternborderoftheformingcontinentthestratawereperhapsnowtoothickandstifftobendintomountainfolds，fortheywererentintogreatfissures，lettingoutfloodsofmoltenlava，remnantsofwhicharestillinevidenceafteragesofdenudation，asthePalisadesalongtheHudson，andsuchelevationsasMountHolyokeinwesternMassachusetts。

Stillthereremainedavastinteriorsea，whichlateron，inthetertiaryage，wastobedividedbytheslowuprisingoftheland，whichonlyyesterday——thatistosay，amillion，orthreeorfiveortenmillion，yearsago——

becametheRockyMountains。Highanderecttheseyoungmountainsstandtothisday，theirsharpanglesandrockycontoursvouchingfortheiryouth，instrangecontrastwiththeshrunkenformsoftheoldAdirondacks，GreenMountains，andAppalachians，whoseloweredheadsandroundedshouldersattesttheweightofages。InthevastlakeswhichstillremainedoneithersideoftheRockyrange，tertiarystratawereslowlyformedtotheultimatedepthoftwoorthreemiles，enclosinghereandtherethosevertebrateremainswhichweretobeexposedagaintoviewbydenudationwhenthelandrosestillhigher，andthen，inourowntime，totellsowonderfulastorytothepaleontologist。

Finally，theinteriorseaswerefilled，andtheshorelinesofthecontinentassumednearlytheirpresentoutline。

Thencamethelongwinteroftheglacialepoch——perhapsofasuccessionofglacialepochs。Theicesheetextendedsouthwardtoaboutthefortiethparallel，drivingsomeanimalsbeforeit，anddestroyingthosethatwereunabletomigrate。Atitsfulness，thegreaticemasslayalmostamileindepthoverNewEngland，asattestedbythescratchedandpolishedrocksurfacesanddepositederraticsintheWhiteMountains。Suchamasspressesdownwithaweightofaboutonehundredandtwenty-fivetonstothesquarefoot，accordingtoDr。Croll’sestimate。Itcrushedandgroundeverythingbeneathitmoreorless，andinsomeregionsplanedoffhillysurfacesintoprairies。Creepingslowlyforward，itcarriedallmannerofdebriswithit。WhenitmeltedawayitsterminalmorainebuiltupthenucleusofthelandmassesnowknownasLongIslandandStatenIsland；otherofitsdepositsformedthe"drumlins"aboutBostonfamousasBunkerandBreed’shills；anditleftalong，irregularlineofridgesof"till"orbowlderclayandscatterederraticsclearacrossthecountryataboutthelatitudeofNewYorkcity。

Astheicesheetslowlyrecededitleftminormorainesallalongitscourse。Sometimesitsdepositsdammeduprivercoursesorinequalitiesinthesurface，toformthelakeswhicheverywhereaboundoverNorthernterritories。

SomeglacialistsevenholdtheviewfirstsuggestedbyRamsey，oftheBritishGeologicalSurvey，thatthegreatglacialsheetsscoopedoutthebasinsofmanylakes，includingthesystemthatfeedstheSt。

Lawrence。Atallevents，itlefttracesofitspresenceallalongthelineofitsretreat，anditsremnantsexisttothisdayasmountainglaciersandthepolaricecap。

Indeed，weliveontheborderofthelastglacialepoch，forwiththeclosingofthisperiodthelonggeologicpastmergesintothepresent。

PAST，PRESENT，ANDFUTURE

Andthepresent，nolessthanthepast，isatimeofchange。ThisisthethoughtwhichJamesHuttonconceivedmorethanacenturyago，butwhichhiscontemporariesandsuccessorsweresoveryslowtoappreciate。

Now，however，ithasbecomeaxiomatic——onecanhardlyrealizethatitwaseverdoubted。Everynewscientifictruth，saysAgassiz，mustpassthroughthreestages——first，mensayitisnottrue；thentheydeclareithostiletoreligion；finally，theyassertthateveryonehasknownitalways。Hutton’struththatnaturallawischangelessandeternalhasreachedthisfinalstage。

NowherenowcouldyoufindascientistwhowoulddisputethetruthofthattextwhichLyell，quotingfromPlayfair’sIllustrationsoftheHuttonianTheory，printedonthetitle-pageofhisPrinciples:"AmidalltherevolutionsoftheglobetheeconomyofNaturehasbeenuniform，andherlawsaretheonlythingsthathaveresistedthegeneralmovement。Theriversandtherocks，theseasandthecontinents，havebeenchangedinalltheirparts；butthelawswhichdirectthosechanges，andtherulestowhichtheyaresubject，haveremainedinvariablythesame。"

But，ontheotherhand，HuttonandPlayfair，andinparticularLyell，drewinferencesfromthisprinciplewhichthemodernphysicistcanbynomeansadmit。

Tothemitimpliedthatthechangesonthesurfaceoftheearthhavealwaysbeenthesameindegreeaswellasinkind，andmustsocontinuewhilepresentforcesholdtheirsway。Inotherwords，theythoughtoftheworldasagreatperpetual-motionmachine。Butthemodernphysicist，giventruermechanicalinsightbythedoctrinesoftheconservationandthedissipationofenergy，willhavenoneofthat。LordKelvin，inparticular，hasurgedthatintheperiodsofourearth’sinfancyandadolescenceitsdevelopmentalchangesmusthavebeen，likethoseofanyotherinfantorganism，vastlymorerapidandpronouncedthanthoseofalaterday；andtoeveryclearthinkerthistruthalsomustnowseemaxiomatic。

Whoeverthinksoftheearthasacoolingglobecanhardlydoubtthatitscrust，whenthinner，mayhaveheavedunderstrainofthemoon’stidalpull——whetherornotthatbodywasnearer——intogreatbillows，dailyrisingandfalling，likewavesofthepresentseasvastlymagnified。

UnderstressofthatsamelateralpressurefromcontractionwhichnowproducestheslowdepressionoftheJerseycoast，theslowriseofSweden，theoccasionalbelchingofaninsignificantvolcano，thejettingofageyser，orthetremblingofanearthquake，oncelargeareaswererentintwain，andvastfloodsoflavaflowedoverthousandsofsquaremilesoftheearth’ssurface，perhaps，atasinglejet；and，foraughtweknowtothecontrary，giganticmountainsmayhaveheapeduptheircontortedheadsincataclysmsasspasmodicaseventhemostardentcatastrophistoftheelderdayofgeologycouldhaveimagined。

Theatmosphereofthatearlyday，filledwithvastvolumesofcarbon，oxygen，andotherchemicalsthathavesincebeenstoredinbedsofcoal，limestone，andgranites，mayhaveworndowntherocksontheonehandandbuiltuporganicformsontheother，witharapiditythatwouldnowseemhardlyconceivable。

Andyetwhilealltheseanomalousthingswenton，thesamelawsheldswaythatnowareoperative；andatruedoctrineofuniformitarianismwouldmakenounwontedconcessioninconcedingthemall——thoughmostoftheimbitteredgeologicalcontroversiesofthemiddleofthenineteenthcenturywereduetothefailureofbothpartiestorealizethatsimplefact。

Andasofthepastandpresent，soofthefuture。Thesameforceswillcontinuetooperate；andunderoperationoftheseunchangingforceseachdaywilldifferfromeveryonethathasprecededit。Ifitbetrue，aseveryphysicistbelieves，thattheearthisacoolingglobe，then，whateveritspresentstageofrefrigeration，thetimemustcomewhenitssurfacecontourwillassumearigidityoflevelnotyetattained。Then，justassurely，theslowactionoftheelementswillcontinuetowearawaythelandsurfaces，particlebyparticle，andtransportthemtotheocean，asitdoesto-day，until，compensationnolongerbeingaffordedbytheupheavalofthecontinents，thelastfootofdrylandwillsinkforthelasttimebeneaththewater，thelastmountain-

peakmeltingaway，andourglobe，lapsinglikeanyotherorganismintoitssecondchildhood，willbeonthesurface——aspresumablyitwasbeforethefirstcontinentrose——onevast"wasteofwaters。"Aspunymanconceivestimeandthings，anawfulcyclewillhavelapsed；inthesweepofthecosmiclife，apulse-

beatwillhavethrobbed。

V。THENEWSCIENCEOFMETEOROLOGY

METEORITES

"Anastonishingmiraclehasjustoccurredinourdistrict，"

wroteM。Marais，aworthyifundistinguishedcitizenofFrance，fromhishomeatL’Aigle，underdateof"the13thFloreal，year11"——adatewhichoutsideofFrancewouldbeinterpretedasmeaningMay3，1803。This"miracle"wastheappearanceofa"fireball"

inbroaddaylight——"perhapsitwaswildfire，"

saysthenaivechronicle——which"hungoverthemeadow，"

beingseenbymanypeople，andthenexplodedwithaloudsound，scatteringthousandsofstonyfragmentsoverthesurfaceofaterritorysomemilesinextent。

Sucha"miracle"couldnothavebeenannouncedatamoreopportunetime。Forsomeyearsthescientificworldhadbeenagogoverthequestionwhethersuchaformoflightningasthatreported——appearinginaclearsky，andhurlingliteralthunderbolts——hadrealexistence。

Suchcaseshadbeenreportedoftenenough，itistrue。The"thunderbolts"themselveswereexhibitedassacredrelicsbeforemanyanaltar，andthosewhodoubtedtheirauthenticityhadbeenchidedashaving"anevilheartofunbelief。"Butscientificscepticismhadquestionedtheevidence，andlateintheeighteenthcenturyaconsensusofopinionintheFrenchAcademyhaddeclinedtoadmitthatsuchstoneshadbeen"conveyedtotheearthbylightning，"letaloneanymoremiraculousagency。

In1802，however，EdwardHowardhadreadapaperbeforetheRoyalSocietyinwhich，afterreviewingtheevidencerecentlyputforward，hehadreachedtheconclusionthatthefallofstonesfromthesky，sometimesoralwaysaccompaniedbylightning，mustbeadmittedasanactualphenomenon，howeverinexplicable。Sonow，whenthegreatstone-fallatL’Aiglewasannounced，theFrenchAcademymadehastetosendthebrilliantyoungphysicistJeanBaptisteBiottoinvestigateit，thatthemattermight，ifpossible，besetfinallyatrest。Theinvestigationwasinallrespectssuccessful，andBiot’sreporttransferredthestonyormetalliclightning-bolt——theaeroliteormeteorite——fromtherealmoftraditionandconjecturetothatofacceptedscience。

Buthowexplainthisstrangephenomenon？Atoncespeculationwasrife。Onetheorycontendedthatthestonymasseshadnotactuallyfallen，buthadbeenformedfromtheearthbytheactionofthelightning；

butthiscontentionwasearlyabandoned。Thechemistsweredisposedtobelievethattheaeroliteshadbeenformedbythecombinationofelementsfloatingintheupperatmosphere。Geologists，ontheotherhand，thoughtthemofterrestrialorigin，urgingthattheymighthavebeenthrownupbyvolcanoes。Theastronomers，asrepresentedbyOlbersandLaplace，modifiedthistheorybysuggestingthatthestonesmight，indeed，havebeencastoutbyvolcanoes，butbyvolcanoessituatednotontheearth，butonthemoon。

Andonespeculatorofthetimetookastepevenmoredaring，urgingthattheaeroliteswereneitheroftelluricnorseleniticorigin，noryetchildrenofthesun，astheoldGreekshad，manyofthem，contended，butthattheyarevisitantsfromthedepthsofcosmicspace。

ThisboldspeculatorwasthedistinguishedGermanphysicistErnstF。F。Chladni，amanofnosmallreputeinhisday。Asearlyas1794heurgedhiscosmicaltheoryofmeteorites，whentheveryexistenceofmeteoriteswasdeniedbymostscientists。Andhedidmore:hedeclaredhisbeliefthatthesefallingstoneswerereallyoneinoriginandkindwiththoseflashingmeteorsoftheupperatmospherewhicharefamiliareverywhereas"shooting-stars。"

Eachofthesecoruscatingmeteors，heaffirmed，musttelloftheignitionofabitofcosmicmatterenteringtheearth’satmosphere。Suchwanderingbitsofmattermightbethefragmentsofshatteredworlds，or，asChladnithoughtmoreprobable，merelyaggregationsof"worldstuff"neverhithertoconnectedwithanylargeplanetarymass。

Naturallyenough，souniqueaviewmetwithveryscantfavor。Astronomersatthattimesawlittletojustifyit；andthenon-scientificworldrejecteditwithfervorasbeing"atheisticandheretical，"becauseitsacceptancewouldseemtoimplythattheuniverseisnotaperfectmechanism。

SomelightwasthrownonthemootpointpresentlybytheobservationsofBrandesandBenzenberg，whichtendedtoshowthatfalling-starstravelatanactualspeedoffromfifteentoninetymilesasecond。Thisobservationtendedtodiscredittheselenitictheory，sinceanobject，inordertoacquiresuchspeedinfallingmerelyfromthemoon，musthavebeenprojectedwithaninitialvelocitynotconceivablytobegivenbyanylunarvolcanicimpulse。Moreover，therewasagrowingconvictionthattherearenoactivevolcanoesonthemoon，andotherconsiderationsofthesametenorledtothecompleteabandonmentoftheselenitictheory。

Butthetheoryoftelluricoriginofaeroliteswasbynomeanssoeasilydisposedof。Thiswasanepochwhenelectricalphenomenawereexcitingunboundedanduniversalinterest，andtherewasanotunnaturaltendencytoappealtoelectricityinexplanationofeveryobscurephenomenon；andinthiscasetheseemingsimilaritybetweenalightningflashandtheflashofanaerolitelentcolortotheexplanation。SowefindThomasForster，ameteorologistofrepute，stilladheringtotheatmospherictheoryofformationofaerolitesinhisbookpublishedin1823；and，indeed，theprevailingopinionofthetimeseemeddividedbetweenvarioustellurictheories，totheneglectofanycosmicaltheorywhatever。

Butin1833occurredaphenomenonwhichsetthematterfinallyatrest。AgreatmeteoricshoweroccurredinNovemberofthatyear，andinobservingitProfessorDenisonOlmstead，ofYale，notedthatallthestarsoftheshowerappearedtocomefromasinglecentreorvanishing-pointintheheavens，andthatthiscentreshifteditspositionwiththestars，andhencewasnottelluric。Thefullsignificanceofthisobservationwasatoncerecognizedbyastronomers；itdemonstratedbeyondallcavilthecosmicaloriginoftheshooting-stars。Someconservativemeteorologistskeptuptheargumentforthetelluricoriginforsomedecadestocome，asamatterofcourse——suchabandtrailsalwaysintherearofprogress。Buteventhesedoubtersweresilencedwhenthegreatshowerofshooting-

starsappearedagainin1866，aspredictedbyOlbersandNewton，radiatingfromthesamepointoftheheavensasbefore。

Sincethenthespectroscopehasaddeditsconfirmatoryevidenceastotheidentityofmeteoriteandshooting-star，and，moreover，haslinkedtheseatmosphericmeteorswithsuchdistantcosmicresidentsascometsandnebulae。ThusitappearsthatChladni’sdaringhypothesisof1794hasbeenmorethanverified，andthatthefragmentsofmatterdissociatedfromplanetaryconnection——whichbepostulatedandwasdeclaredatheisticforpostulating——havebeenshowntobebillionsoftimesmorenumerousthananylargercosmicbodiesofwhichwehavecognizance——sowidelydoestheexistinguniversedifferfromman’spreconceivednotionsastowhatitshouldbe。

Thusalsothe"miracle"ofthefallingstone，againstwhichthescientificscepticismofyesterdaypresented"anevilheartofunbelief，"turnsouttobethemostnaturalphenomena，inasmuchasitisrepeatedinouratmospheresomemillionsoftimeseachday。

THEAURORABOREALIS

Iffire-ballswerethoughtmiraculousandportentousindaysofyore，whatinterpretationmustneedshavebeenputuponthatvastlymorepicturesquephenomenon，theaurora？"Throughallthecity，"saystheBookofMaccabees，"forthespaceofalmostfortydays，therewereseenhorsemenrunningintheair，inclothofgold，armedwithlances，likeabandofsoldiers:andtroopsofhorsemeninarrayencounteringandrunningoneagainstanother，withshakingofshieldsandmultitudeofpikes，anddrawingofswords，andcastingofdarts，andglitteringofgoldenornamentsandharness。"

Direomensthese；andhardlylessominoustheauroraseemedtoallsucceedinggenerationsthatobserveditdownwellintotheeighteenthcentury——aswitnessthepopularexcitementinEnglandin1716overthebrilliantauroraofthatyear，whichbecamefamousthroughHalley’sdescription。

Butafter1752，whenFranklindethronedthelightning，allspectacularmeteorscametoberegardedasnaturalphenomena，theauroraamongtherest。Franklinexplainedtheaurora——whichwasseencommonlyenoughintheeighteenthcentury，thoughonlyrecordedonceintheseventeenth——asduetotheaccumulationofelectricityonthesurfaceofpolarsnows，anditsdischargetotheequatorthroughtheupperatmosphere。

ErasmusDarwinsuggestedthattheluminositymightbeduetotheignitionofhydrogen，whichwassupposedbymanyphilosopherstoformtheupperatmosphere。

Dalton，whofirstmeasuredtheheightoftheaurora，estimatingitataboutonehundredmiles，thoughtthephenomenonduetomagnetismactingonferruginousparticlesintheair，andhisexplanationwasperhapsthemostpopularoneatthebeginningofthelastcentury。

Sincethenamultitudeofobservershavestudiedtheaurora，butthescientificgrasphasfounditaselusiveinfactasitseemstocasualobservation，anditsexactnatureisasundeterminedto-dayasitwasahundredyearsago。Therehasbeennodearthoftheoriesconcerningit，however。Blot，whostudieditintheShetlandIslandsin1817，thoughtitduetoelectrifiedferruginousdust，theoriginofwhichheascribedtoIcelandicvolcanoes。Muchmorerecentlytheideaofferruginousparticleshasbeenrevived，theirpresencebeingascribednottovolcanoes，buttothemeteoritesconstantlybeingdissipatedintheupperatmosphere。

Ferruginousdust，presumablyofsuchorigin，hasbeenfoundonthepolarsnows，aswellasonthesnowsofmountain-tops，butwhetheritcouldproducethephenomenaofaurorasisatleastanopenquestion。

Othertheoristshaveexplainedtheauroraasduetotheaccumulationofelectricityoncloudsoronspiculesoficeintheupperair。Yetothersthinkitduemerelytothepassageofelectricitythroughrarefiedairitself。

HumboldtconsideredthemattersettledinyetanotherwaywhenFaradayshowed，in1831，thatmagnetismmayproduceluminouseffects。Butperhapstheprevailingtheoryofto-dayassumesthattheauroraisduetoacurrentofelectricitygeneratedattheequatorandpassingthroughupperregionsofspace，toentertheearthatthemagneticpoles——simplyreversingthecoursewhichFranklinassumed。

Thesimilarityoftheaurorallighttothatgeneratedinavacuumbulbbythepassageofelectricitylendssupporttothelong-standingsuppositionthattheauroraisofelectricalorigin，butthesubjectstillawaitscompleteelucidation。Foronceeventhatmystery-

solverthespectroscopehasbeenbaffled，forthelineitsiftsfromtheauroraisnotmatchedbythatofanyrecognizedsubstance。Alikelineisfoundinthezodiacallight，itistrue，butthisisoflittleaid，forthezodiacallight，thoughthoughtbysomeastronomerstobeduetometeorswarmsaboutthesun，isheldtobe，onthewhole，asmysteriousastheauroraitself。

Whatevertheexactnatureoftheaurora，ithaslongbeenknowntobeintimatelyassociatedwiththephenomenaofterrestrialmagnetism。Wheneverabrilliantauroraisvisible，theworldissuretobevisitedwithwhatHumboldtcalledamagneticstorm——a"storm"whichmanifestsitselftohumansensesinnowaywhatsoeverexceptbydeflectingthemagneticneedleandconjuringwiththeelectricwire。Suchmagneticstormsarecuriouslyassociatedalsowithspotsonthesun——justhownoonehasexplained，thoughthefactitselfisunquestioned。Sun-spots，too，seemdirectlylinkedwithauroras，eachofthesephenomenapassingthroughperiodsofgreatestandleastfrequencyincorrespondingcyclesofaboutelevenyears’duration。

ItwassuspectedafullcenturyagobyHerschelthatthevariationsinthenumberofsun-spotshadadirecteffectuponterrestrialweather，andheattemptedtodemonstrateitbyusingthepriceofwheatasacriterionofclimaticconditions，meantimemakingcarefulobservationofthesun-spots。Nothingverydefinitecameofhiseffortsinthisdirection，thesubjectbeingfartoocomplextobedeterminedwithoutlongperiodsofobservation。

Latterly，however，meteorologists，particularlyinthetropics，aredisposedtothinktheyfindevidenceofsomesuchconnectionbetweensun-spotsandtheweatherasHerschelsuspected。Indeed，Mr。

Meldrumdeclaresthatthereisapositivecoincidencebetweenperiodsofnumeroussun-spotsandseasonsofexcessiveraininIndia。

Thatsomesuchconnectiondoesexistseemsintrinsicallyprobable。Butthemodernmeteorologist，learningwisdomofthepast，isextremelycautiousaboutascribingcasualeffectstoastronomicalphenomena。

Hefindsithardtoforgetthatuntilrecentlyallmannerofclimaticconditionswereassociatedwithphasesofthemoon；thatnotsoverylongagoshowersoffalling-starswereconsidered"prognostic"ofcertainkindsofweather；andthatthe"equinoctialstorm"

hadbeenacceptedasaveritybyeveryone，untiltheunfeelinghandofstatisticsbanisheditfromtheearth。

Yet，ontheotherhand，itiseasilywithinthepossibilitiesthatthescienceofthefuturemayrevealassociationsbetweentheweatherandsun-spots，auroras，andterrestrialmagnetismthatasyetarehardlydreamedof。Untilsuchtime，however，thesephenomenamustfeelthemselvesverygrudginglyadmittedtotheinnercircleofmeteorology。Moreandmorethisscienceconcernsitself，inourageofconcentrationandspecialization，withweatherandclimate。

Itsvotariesnolongerconcernthemselveswithstarsorplanetsorcometsorshooting-stars——oncethoughttheveryessenceofguidestoweatherwisdom；andtheyareevenlookingaskanceatthemoon，andaskinghertoshowcausewhyshealsoshouldnotbeexcludedfromtheirdomain。Equallylittledotheycarefortheinterioroftheearth，sincetheyhavelearnedthatthecentralemanationsofheatwhichMairanimaginedasamainsourceofaerialwarmthcanclaimnosuchdistinction。Evensuchproblemsaswhythemagneticpoledoesnotcoincidewiththegeographical，andwhytheforceofterrestrialmagnetismdecreasesfromthemagneticpolestothemagneticequator，asHumboldtfirstdiscoveredthatitdoes，excitethemonlytolukewarminterest；formagnetism，theysay，isnotknowntohaveanyconnectionwhateverwithclimateorweather。

EVAPORATION，CLOUDFORMATION，ANDDEW

Thereisatleastoneformofmeteor，however，ofthosethatinterestedourforebearswhosemeteorologicalimportancetheydidnotoverestimate。Thisisthevaporofwater。Howgreatwastheinterestinthisfamiliarmeteoratthebeginningofthecenturyisattestedbythenumberoftheoriesthenextantregardingit；andtheseconflictingtheoriesbearwitnessalsotothedifficultywithwhichthefamiliarphenomenonoftheevaporationofwaterwasexplained。

Franklinhadsuggestedthatairdissolveswatermuchaswaterdissolvessalt，andthistheorywasstillpopular，thoughDeluchaddisproveditbyshowingthatwaterevaporatesevenmorerapidlyinavacuumthaninair。Deluc’sowntheory，borrowedfromearlierchemists，wasthatevaporationisthechemicalunionofparticlesofwaterwithparticlesofthesupposititiouselementheat。ErasmusDarwincombinedthetwotheories，suggestingthattheairmightholdavariablequantityofvaporinmeresolution，andinadditionapermanentmoietyinchemicalcombinationwithcaloric。

Undisturbedbytheseconflictingviews，thatstrangelyoriginalgenius，JohnDalton，afterwardstobeknownasperhapsthegreatestoftheoreticalchemists，tookthequestioninhand，andsolveditbyshowingthatwaterexistsintheairasanutterlyindependentgas。Hereachedapartialinsightintothematterin1793，whenhisfirstvolumeofmeteorologicalessayswaspublished；

butthefullelucidationoftheproblemcametohimin1801。Themeritofhisstudieswasatoncerecognized，butthetenabilityofhishypothesiswaslongandardentlydisputed。

Whilethenatureofevaporationwasindispute，asamatterofcoursethequestionofprecipitationmustbeequallyundetermined。ThemostfamoustheoryoftheperiodwasthatformulatedbyDr。HuttoninapaperreadbeforetheRoyalSocietyofEdinburgh，andpublishedinthevolumeoftransactionswhichcontainedalsothesameauthor’sepoch-makingpaperongeology。

This"theoryofrain"explainedprecipitationasduetothecoolingofacurrentofsaturatedairbycontactwithacoldercurrent，theassumptionbeingthatthesurplusageofmoisturewasprecipitatedinachemicalsense，justastheexcessofsaltdissolvedinhotwaterisprecipitatedwhenthewatercools。TheideathatthecoolingofthesaturatedaircausestheprecipitationofitsmoistureisthegermoftruththatrendersthispaperofHutton’simportant。Allcorrectlatertheoriesbuildonthisfoundation。

"Letussupposethesurfaceofthisearthwhollycoveredwithwater，"saidHutton，"andthatthesunwerestationary，beingalwaysverticalinoneplace；

then，fromthelawsofheatandrarefaction，therewouldbeformedacirculationintheatmosphere，flowingfromthedarkandcoldhemispheretotheheatedandilluminatedplace，inalldirections，towardstheplaceofthegreatestcold。

"Asthereisfortheatmosphereofthisearthaconstantcoolingcause，thisfluidbodycouldonlyarriveatacertaindegreeofheat；andthiswouldberegularlydecreasingfromthecentreofilluminationtotheoppositepointoftheglobe，mostdistantfromthelightandheat。Betweenthesetworegionsofextremeheatandcoldtherewould，ineveryplace，befoundtwostreamsofairfollowinginoppositedirections。Ifthosestreamsofair，therefore，shallbesupposedasbothsufficientlysaturatedwithhumidity，then，astheyareofdifferenttemperatures，therewouldbeformedacontinualcondensationofaqueousvapor，insomemiddleregionoftheatmosphere，bythecommixtionofpartofthosetwooppositestreams。

"Hencethereisreasontobelievethatinthissupposedcasetherewouldbeformeduponthesurfaceoftheglobethreedifferentregions——thetorridregion，thetemperate，andthefrigid。Thesethreeregionswouldcontinuestationary；andtheoperationsofeachwouldbecontinual。Inthetorridregion，nothingbutevaporationandheatwouldtakeplace；nocloudcouldbeformed，becauseinchangingthetransparencyoftheatmospheretoopacityitwouldbeheatedimmediatelybytheoperationoflight，andthusthecondensedwaterwouldbeagainevaporated。Butthispowerofthesunwouldhaveatermination；anditisthesethatwouldbegintheregionoftemperateheatandofcontinualrain。Itisnotprobablethattheregionoftemperancewouldreachfarbeyondtheregionoflight；andinthehemisphereofdarknesstherewouldbefoundaregionofextremecoldandperfectdryness。

"Letusnowsupposetheearthasturningonitsaxisintheequinoctialsituation。Thetorridregionwouldthusbechangedintoazone，inwhichtherewouldbenightandday；consequently，herewouldbemuchtemperance，comparedwiththetorridregionnowconsidered；andhereperhapstherewouldbeformedperiodicalcondensationandevaporationofhumidity，correspondingtotheseasonsofnightandday。Astemperancewouldthusbeintroducedintotheregionoftorridextremity，sowouldtheeffectofthischangebefeltoveralltheglobe，everypartofwhichwouldnowbeilluminated，consequentlyheatedinsomedegree。

Thuswewouldhavealineofgreatheatandevaporation，graduatingeachwayintoapointofgreatcoldandcongelation。Betweenthesetwoextremesofheatandcoldtherewouldbefoundineachhemispherearegionofmuchtemperance，inrelationtoheat，butofmuchhumidityintheatmosphere，perhapsofcontinualrainandcondensation。

"Thesuppositionnowformedmustappearextremelyunfitformakingthisglobeahabitableworldineverypart；buthavingthusseentheeffectofnightanddayintemperatingtheeffectsofheatandcoldineveryplace，wearenowpreparedtocontemplatetheeffectsofsupposingthisglobetorevolvearoundthesunwithacertaininclinationofitsaxis。Bythisbeautifulcontrivance，thatcomparativelyuninhabitedglobeisnowdividedintotwohemispheres，eachofwhichisthusprovidedwithasummerandawinterseason。Butourpresentviewislimitedtotheevaporationandcondensationofhumidity；and，inthiscontrivanceoftheseasons，theremustappearanampleprovisionforthosealternateoperationsineverypart；

forastheplaceoftheverticalsunismovedalternatelyfromonetropictotheother，heatandcold，theoriginalcausesofevaporationandcondensation，mustbecarriedoveralltheglobe，producingeitherannualseasonsofrainordiurnalseasonsofcondensationandevaporation，orboththeseseasons，moreorless——thatis，insomedegree。

"Theoriginalcauseofmotionintheatmosphereistheinfluenceofthesunheatingthesurfaceoftheearthexposedtothatluminary。Wehavenotsupposedthatsurfacetohavebeenofoneuniformshapeandsimilarsubstance；fromwhenceithasfollowedthattheannualpropersofthesun，perhapsalsothediurnalpropers，wouldproducearegularcondensationofrainincertainregions，andtheevaporationofhumidityinothers；andthiswouldhavearegularprogressincertaindeterminedseasons，andwouldnotvary。Butnothingcanbemoredistantfromthissupposition，thatisthenaturalconstitutionoftheearth；fortheglobeiscomposedofseaandland，innoregularshapeormixture，whilethesurfaceofthelandisalsoirregularwithrespecttoitselevationsanddepressions，andvariouswithregardtothehumidityanddrynessofthatpartwhichisexposedtoheatasthecauseofevaporation。Henceasourceofthemostvaluablemotionsinthefluidatmospherewithaqueousvapor，moreorless，sofarasothernaturaloperationswilladmit；andhenceasourceofthemostirregularcommixtureoftheseveralpartsofthiselasticfluid，whethersaturatedornotwithaqueousvapor。

"Accordingtothetheory，nothingisrequiredfortheproductionofrainbesidesthemixtureofportionsoftheatmospherewithhumidity，andofmixingthepartsthatareindifferentdegreesofheat。Butwehaveseenthecausesofsaturatingeveryportionoftheatmospherewithhumidityandofmixingthepartswhichareindifferentdegreesofheat。Consequently，overallthesurfaceoftheglobethereshouldhappenoccasionallyrainandevaporation，moreorless；andalso，ineveryplace，thosevicissitudesshouldbeobservedtotakeplacewithsometendencytoregularity，which，however，maybesodisturbedastobehardlydistinguishableuponmanyoccasions。Variablewindsandvariablerainsshouldbefoundinproportionaseachplaceissituatedinanirregularmixtureoflandandwater；whereasregularwindsshouldbefoundinproportiontotheuniformityofthesurface；

andregularrainsinproportiontotheregularchangesofthosewindsbywhichthemixtureoftheatmospherenecessarytotherainmaybeproduced。Butasitwillbeacknowledgedthatthisisthecaseinalmostallthisearthwhererainappearsaccordingtotheconditionsherespecified，thetheoryisfoundtobethusinconformitywithnature，andnaturalappearancesarethusexplainedbythetheory。"[1]

ThenextambitiousattempttoexplainthephenomenaofaqueousmeteorswasmadebyLukeHoward，inhisremarkablepaperonclouds，publishedinthePhilosophicalMagazinein1803——thepaperinwhichthenamescirrus，cumulus，stratus，etc。，afterwardssouniversallyadopted，werefirstproposed。InthispaperHowardacknowledgeshisindebtednesstoDaltonforthetheoryofevaporation；yethestillclingstotheideathatthevapor，thoughindependentoftheair，iscombinedwithparticlesofcaloric。Heholdsthatcloudsarecomposedofvaporthathaspreviouslyrisenfromtheearth，combatingtheopinionsofthosewhobelievethattheyareformedbytheunionofhydrogenandoxygenexistingindependentlyintheair；thoughheagreeswiththesetheoriststhatelectricityhasenteredlargelyintothemodusoperandiofcloudformation。HeopposestheopinionofDelucandDeSaussurethatcloudsarecomposedofparticlesofwaterintheformofhollowvesicles(miniatureballoons，inshort，perhapsfilledwithhydrogen)，whichuntenableopinionwasarevivalofthetheoryastotheformationofallvaporwhichDr。Halleyhadadvocatedearlyintheeighteenthcentury。

OfparticularinterestareHoward’sviewsastotheformationofdew，whichheexplainsascausedbytheparticlesofcaloricforsakingthevaportoenterthecoolbody，leavingthewateronthesurface。Thiscomesasnearthetruth，perhaps，ascouldbeexpectedwhiletheoldideaastothematerialityofheatheldsway。Howardbelieved，however，thatdewisusuallyformedintheairatsomeheight，andthatitsettlestothesurface，opposingtheopinion，whichhadgainedvogueinFranceandinAmerica(whereNoahWebsterprominentlyadvocatedit)，thatdewascendsfromtheearth。

Thecompletesolutionoftheproblemofdewformation——

whichreallyinvolvedalsotheentirequestionofprecipitationofwateryvaporinanyform——wasmadebyDr。W。C。Wells，amanofAmericanbirth，whoselife，however，afterboyhood，wasspentinScotland(whereasayoungmanheenjoyedthefriendshipofDavidHume)andinLondon。Inspired，nodoubt，bytheresearchesofMack，Hutton，andtheirconfreresofthatEdinburghschool，Wellsmadeobservationsonevaporationandprecipitationasearlyas1784，butotherthingsclaimedhisattention；andthoughheassertsthatthesubjectwasofteninhismind，hedidnottakeitupagaininearnestuntilabout1812。

MeantimetheobservationsonheatofRumfordandDavyandLesliehadclearedthewayforaproperinterpretationofthefacts——aboutthefactsthemselvestherehadlongbeenpracticalunanimityofopinion。

Dr。Black，withhislatent-heatobservations，hadreallygiventheclewtoallsubsequentdiscussionsofthesubjectofprecipitationofvapor；andfromthistimeonithadbeenknownthatheatistakenupwhenwaterevaporates，andgivenoutagainwhenitcondenses。

Dr。Darwinhadshownin1788，inapaperbeforetheRoyalSociety，thatairgivesoffheatoncontractingandtakesituponexpanding；andDalton，inhisessayof1793，hadexplainedthisphenomenonasduetothecondensationandvaporizationofthewatercontainedintheair。

ButsomecuriousandpuzzlingobservationswhichProfessorPatrickWilson，professorofastronomyintheUniversityofGlasgow，hadcommunicatedtotheRoyalSocietyofEdinburghin1784，andsomesimilaronesmadebyMr。Six，ofCanterbury，afewyearslater，hadremainedunexplained。Boththesegentlemenobservedthattheairiscoolerwheredewisformingthantheairafewfeethigher，andtheyinferredthatthedewinforminghadtakenupheat，inapparentviolationofestablishedphysicalprinciples。

ItremainedforWells，inhismemorablepaperof1816，toshowthattheseobservershadsimplyplacedthecartbeforethehorse。Hemadeitclearthattheairisnotcoolerbecausethedewisformed，butthatthedewisformedbecausetheairiscooler——havingbecomesothroughradiationofheatfromthesolidsonwhichthedewforms。Thedewitself，informing，givesoutitslatentheat，andsotendstoequalizethetemperature。

Wells’spaperissoadmirableanillustrationofthelucidpresentationofclearlyconceivedexperimentsandlogicalconclusionsthatweshoulddoitinjusticenottopresentitentire。Theauthor’smentionoftheobservationsofSixandWilsongivesaddedvaluetohisownpresentation。

Dr。Wells’sEssayonDew"Iwasledintheautumnof1784，bytheeventofarudeexperiment，tothinkitprobablethattheformationofdewisattendedwiththeproductionofcold。

In1788，apaperonhoar-frost，byMr。PatrickWilson，ofGlasgow，waspublishedinthefirstvolumeoftheTransactionsoftheRoyalSocietyofEdinburgh，bywhichitappearedthatthisopinionbadbeenentertainedbythatgentlemanbeforeithadoccurredtomyself。Inthecourseofthesameyear，Mr。Six，ofCanterbury，mentionedinapapercommunicatedtotheRoyalSocietythatonclearanddewynightshealwaysfoundthemercurylowerinathermometerlaiduponthegroundinameadowinhisneighborhoodthanitwasinasimilarthermometersuspendedintheairsixfeetabovetheformer；andthatupononenightthedifferenceamountedtofivedegreesofFahrenheit’sscale。Mr。Six，however，didnotsuppose，agreeablytotheopinionofMr。Wilsonandmyself，thatthecoldwasoccasionedbytheformationofdew，butimaginedthatitproceededpartlyfromthelowtemperatureoftheair，throughwhichthedew，alreadyformedintheatmosphere，haddescended，andpartlyfromtheevaporationofmoisturefromtheground，onwhichhisthermometerhadbeenplaced。TheconjectureofMr。

WilsonandtheobservationsofMr。Six，togetherwithmanyfactswhichIafterwardslearnedinthecourseofreading，strengthenedmyopinion；butImadenoattempt，beforetheautumnof1811，toascertainbyexperimentifitwerejust，thoughithadinthemeantimealmostdailyoccurredtomythoughts。Happening，inthatseason，tobeinthatcountryinaclearandcalmnight，Ilaidathermometerupongrasswetwithdew，andsuspendedasecondintheair，twofeetabovetheother。Anhourafterwardsthethermometeronthegrasswasfoundtobeeightdegreeslower，byFahrenheit’sdivision，thantheoneintheair。Similarresultshavingbeenobtainedfromseveralsimilarexperiments，madeduringthesameautumn，Ideterminedinthenextspringtoprosecutethesubjectwithsomedegreeofsteadiness，andwiththatviewwentfrequentlytothehouseofoneofmyfriendswholivesinSurrey。

AttheendoftwomonthsIfanciedthatIhadcollectedinformationworthyofbeingpublished；but，fortunately，whilepreparinganaccountofitImetbyaccidentwithasmallposthumousworkbyMr。Six，printedatCanterburyin1794，inwhicharerelateddifferencesobservedondewynightsbetweenthermometersplacedupongrassandothersintheairthataremuchgreaterthanthosementionedinthepaperpresentedbyhimtotheRoyalSocietyin1788。Inthiswork，too，thecoldofthegrassisattributed，inagreementwiththeopinionofMr。Wilson，altogethertothedewdepositeduponit。Thevalueofmyownobservationsappearingtomenowmuchdiminished，thoughtheyembracedmanypointsleftuntouchedbyMr。Six，Igaveupmyintentionsofmakingthemknown。Shortlyafter，however，uponconsideringthesubjectmoreclosely，IbegantosuspectthatMr。Wilson，Mr。Six，andmyselfhadallcommittedanerrorregardingthecoldwhichaccompaniesdewasaneffectoftheformationofthatfluid。Ithereforeresumedmyexperiments，andhavingbymeansofthem，Ithink，notonlyestablishedthejustnessofmysuspicions，butascertainedtherealcausebothofdewandofseveralothernaturalappearanceswhichhavehithertoreceivednosufficientexplanation，Iventurenowtosubmittotheconsiderationofthelearnedanaccountofsomeofmylabors，withoutregardtotheorderoftimeinwhichtheywereperformed，andofvariousconclusionswhichmaybedrawnfromthem，mixedwithfactsandopinionsalreadypublishedbyothers:

"Therearevariousoccurrencesinnaturewhichseemtomestrictlyalliedtodew，thoughtheirrelationtoitbenotalwaysatfirstsightperceivable。Thestatementandexplanationofseveralofthesewillformtheconcludingpartofthepresentessay。

"1。Iobservedonemorning，inwinter，thattheinsidesofthepanesofglassinthewindowsofmybedchamberwereallofthemmoist，butthatthosewhichhadbeencoveredbyaninsideshutterduringthenightweremuchmoresothantheotherswhichhadbeenuncovered。Supposingthatthisdiversityofappearancedependeduponadifferenceoftemperature，I

appliedthenakedbulbsoftwodelicatethermometerstoacoveredanduncoveredpane；onwhichIfoundthattheformerwasthreedegreescolderthanthelatter。Theairofthechamber，thoughnofirewaskeptinit，wasatthistimeelevenandone-halfdegreeswarmerthanthatwithout。Similarexperimentsweremadeonmanyothermornings，theresultsofwhichwerethatthewarmthoftheinternalairexceededthatoftheexternalfromeighttoeighteendegrees，thetemperatureofthecoveredpaneswouldbefromonetofivedegreeslessthantheuncovered；thatthecoveredweresometimesdewed，whiletheuncoveredweredry；thatatothertimesbothwerefreefrommoisture；thattheoutsidesofthecoveredanduncoveredpaneshadsimilardifferenceswithrespecttoheat，thoughnotsogreatasthoseoftheinnersurfaces；andthatnovariationinthequantityofthesedifferenceswasoccasionedbytheweather’sbeingcloudyorfair，providedtheheatoftheinternalairexceededthatoftheexternalequallyinbothofthosestatesoftheatmosphere。

"Theremotereasonofthesedifferencesdidnotimmediatelypresentitself。Isoon，however，sawthattheclosedshuttershieldedtheglasswhichitcoveredfromtheheatthatwasradiatedtothewindowsbythewallsandfurnitureoftheroom，andthuskeptitnearertothetemperatureoftheexternalairthanthosepartscouldbewhich，frombeinguncovered，receivedtheheatemittedtothembythebodiesjustmentioned。

"Inmakingtheseexperiments，Iseldomobservedtheinsideofanypanetobemorethanalittledamped，thoughitmightbefromeighttotwelvedegreescolderthanthegeneralmassoftheairintheroom；while，intheopenair，Ihadoftenfoundagreatdewtoformonsubstancesonlythreeorfourdegreescolderthantheatmosphere。Thisatfirstsurprisedme；butthecausenowseemsplain。Theairofthechamberhadoncebeenaportionoftheexternalatmosphere，andhadafterwardsbeenheated，whenitcouldreceivelittleaccessoriestoitsoriginalmoisture。Itconstantlyrequiredbeingcooledconsiderablybeforeitwasevenbroughtbacktoitsformernearnesstorepletionwithwater；whereasthewholeexternalairiscommonly，atnight，nearlyrepletewithmoisture，andthereforereadilyprecipitatesdewonbodiesonlyalittlecolderthanitself。

"Whentheairofaroomiswarmerthantheexternalatmosphere，theeffectofanoutsideshutteronthetemperatureoftheglassofthewindowwillbedirectlyoppositetowhathasjustbeenstated；sinceitmustpreventtheradiation，intotheatmosphere，oftheheatofthechambertransmittedthroughtheglass。

"2。CountRumfordappearstohaverightlyconjecturedthattheinhabitantsofcertainhotcountries，whosleepatnightsonthetopsoftheirhouses，arecooledduringthisexposurebytheradiationoftheirheattothesky；or，accordingtohismannerofexpression，byreceivingfrigorificraysfromtheheavens。

Anotherfactofthiskindseemstobethegreaterchillwhichweoftenexperienceuponpassingatnightfromthecoverofahouseintotheairthanmighthavebeenexpectedfromthecoldoftheexternalatmosphere。

Thecause，indeed，issaidtobethequicknessoftransitionfromonesituationtoanother。Butifthiswerethewholereason，anequalchillwouldbefeltintheday，whenthedifference，inpointofheat，betweentheinternalandexternalairwasthesameasatnight，whichisnotthecase。Besides，ifIcantrustmyownobservation，thefeelingofcoldfromthiscauseismoreremarkableinaclearthaninacloudynight，andinthecountrythanintowns。Thefollowingappearstobethemannerinwhichthesethingsarechieflytobeexplained:

"Duringthedayourbodieswhileintheopenair，althoughnotimmediatelyexposedtothesun’srays，areyetconstantlyderivingheatfromthembymeansofthereflectionoftheatmosphere。Thisheat，thoughitproduceslittlechangeonthetemperatureoftheairwhichittraverses，affordsussomecompensationfortheheatwhichweradiatetotheheavens。Atnight，also，iftheskybeovercast，somecompensationwillbemadetous，bothinthetownandinthecountry，thoughinalessdegreethanduringtheday，asthecloudswillremittowardstheearthnoinconsiderablequantityofheat。Butonaclearnight，inanopenpartofthecountry，nothingalmostcanbereturnedtousfromaboveinplaceoftheheatwhichweradiateupward。

Intowns，however，somecompensationwillbeaffordedevenontheclearestnightsfortheheatwhichweloseintheopenairbythatwhichisradiatedtousfromthesunroundbuildings。

Toourlossofheatbyradiationattimesthatwederivelittlecompensationfromtheradiationofotherbodiesisprobablytobeattributedagreatpartofthehurtfuleffectsofthenightair。Descartessaysthatthesearenotowingtodew，aswasthecommonopinionofhiscontemporaries，buttothedescentofcertainnoxiousvaporswhichhavebeenexhaledfromtheearthduringtheheatoftheday，andareafterwardscondensedbythecoldofaserenenight。Theeffectsinquestioncertainlycannotbeoccasionedbydew，sincethatfluiddoesnotformuponahealthyhumanbodyintemperateclimates；buttheymay，notwithstanding，arisefromthesamecausethatproducesdewonthosesubstanceswhichdonot，likethehumanbody，possessthepowerofgeneratingheatforthesupplyofwhattheylosebyradiationoranyothermeans。"[2]

Thisexplanationmadeitplainwhydewformsonaclearnight，whentherearenocloudstoreflecttheradiantheat。CombinedwithDalton’stheorythatvaporisanindependentgas，limitedinquantityinanygivenspacebythetemperatureofthatspace，itsolvedtheproblemoftheformationofclouds，rain，snow，andhoar-frost。ThusthispaperofWells’sclosedtheepochofspeculationregardingthisfieldofmeteorology，asHutton’spaperof1784hadopenedit。ThefactthatthevolumecontainingHutton’spapercontainedalsohisepoch-makingpaperongeologyfindscuriouslyaduplicationinthefactthatWells’svolumecontainedalsohisessayonAlbinism，inwhichthedoctrineofnaturalselectionwasforthefirsttimeformulated，asCharlesDarwinfreelyadmittedafterhisowneffortshadmadethedoctrinefamous。