第19章

TheverynextyearafterDr。Wells’spaperwaspublishedthereappearedinFrancethethirdvolumeoftheMemoiresdePhysiqueetdeChimiedelaSocieted’Arcueil，andanewepochinmeteorologywasinaugurated。

Thesocietyinquestionwasnumericallyaninconsequentialband，listingonlyadozenmembers；buteverynamewasafamousone:Arago，Berard，Berthollet，Biot，Chaptal，DeCandolle，Dulong，Gay-Lussac，Humboldt，Laplace，Poisson，andThenard——rarespiritseveryone。Littledangerthatthememoirsofsuchabandwouldberelegatedtothedustyshelveswheremostproceedingsofsocietiesbelong——nomilk-for-babesfarewouldbeservedtosuchacompany。

Theparticularpaperwhichhereinterestsusclosesthisthirdandlastvolumeofmemoirs。Itisentitled"DesLignesIsothermesetdelaDistributiondelaChaleursurleGlobe。"TheauthorisAlexanderHumboldt。

Needlesstosay，thetopicishandledinamasterlymanner。Thedistributionofheatonthesurfaceoftheglobe，onthemountain-sides，intheinterioroftheearth；thecausesthatregulatesuchdistribution；theclimaticresults——thesearethetopicsdiscussed。Butwhatgivesepochalcharactertothepaperistheintroductionofthoseisothermallinescirclingtheearthinirregularcourse，joiningtogetherplaceshavingthesamemeanannualtemperature，andthuslayingthefoundationforascienceofcomparativeclimatology。

Itistruetheattempttostudyclimatescomparativelywasnotnew。Mairanhadattempteditinthosepapersinwhichhedevelopedhisbizarreideasastocentralemanationsofheat。Eulerhadbroughthisprofoundmathematicalgeniustobearonthetopic，evolvingthe"extraordinaryconclusionthatundertheequatoratmidnightthecoldoughttobemorerigorousthanatthepolesinwinter。"AndinparticularRichardKirwan，theEnglishchemist，hadcombinedthemathematicalandtheempiricalmethodsandcalculatedtemperaturesforalllatitudes。ButHumboldtdiffersfromallthesepredecessorsinthathegraspstheideathatthebasisofallsuchcomputationsshouldbenottheory，butfact。Hedrewhisisothermallinesnotwheresomeoccultcalculationwouldlocatethemonanidealglobe，butwherepracticaltestswiththethermometerlocatethemonourglobeasitis。London，forexample，liesinthesamelatitudeasthesouthernextremityofHudsonBay；buttheisothermofLondon，asHumboldtoutlinesit，passesthroughCincinnati。

Ofcoursesuchdeviationsofclimaticconditionsbetweenplacesinthesamelatitudehadlongbeenknown。

AsHumboldthimselfobserves，theearliestsettlersofAmericawereastonishedtofindthemselvessubjectedtorigorsofclimateforwhichtheirEuropeanexperiencehadnotatallpreparedthem。Moreover，sagacioustravellers，inparticularCook’scompaniononhissecondvoyage，youngGeorgeForster，hadnotedasageneralprinciplethatthewesternbordersofcontinentsintemperateregionsarealwayswarmerthancorrespondinglatitudesoftheireasternborders；andofcoursethegeneraltruthoftemperaturesbeingmilderinthevicinityoftheseathanintheinteriorofcontinentshadlongbeenfamiliar。ButHumboldt’sisothermallinesforthefirsttimegavetangibilitytotheseideas，andmadepracticableatrulyscientificstudyofcomparativeclimatology。

Instudyingtheselines，particularlyaselaboratedbyfurtherobservations，itbecameclearthattheyarebynomeanshaphazardinarrangement，butaredependentupongeographicalconditionswhichinmostcasesarenotdifficulttodetermine。Humboldthimselfpointedoutveryclearlythemaincausesthattendtoproducedeviationsfromtheaverage——or，asDovelateroncalledit，thenormal——temperatureofanygivenlatitude。Forexample，themeanannualtemperatureofaregion(referringmainlytothenorthernhemisphere)

israisedbytheproximityofawesterncoast；

byadividedconfigurationofthecontinentintopeninsulas；

bytheexistenceofopenseastothenorthorofradiatingcontinentalsurfacestothesouth；bymountainrangestoshieldfromcoldwinds；bytheinfrequencyofswampstobecomecongealed；bytheabsenceofwoodsinadry，sandysoil；andbytheserenityofskyinthesummermonthsandthevicinityofanoceancurrentbringingwaterwhichisofahighertemperaturethanthatofthesurroundingsea。

Conditionsoppositetothesetend，ofcourse，correspondinglytolowerthetemperature。Inaword，Humboldtsaystheclimaticdistributionofheatdependsontherelativedistributionoflandandsea，andonthe"hypsometricalconfigurationofthecontinents"；

andheurgesthat"greatmeteorologicalphenomenacannotbecomprehendedwhenconsideredindependentlyofgeognosticrelations"——atruthwhich，likemostothergeneralprinciples，seemssimpleenoughonceitispointedout。

Withthatbroadsweepofimaginationwhichcharacterizedhim，Humboldtspeaksoftheatmosphereasthe"aerialocean，inthelowerstrataandontheshoalsofwhichwelive，"andhestudiestheatmosphericphenomenaalwaysinrelationtothoseofthatotheroceanofwater。Ineachoftheseoceanstherearevastpermanentcurrents，flowingalwaysindeterminatedirections，whichenormouslymodifytheclimaticconditionsofeveryzone。Theoceanofairisavastmaelstrom，boilingupalwaysundertheinfluenceofthesun’sheatattheequator，andflowingasanuppercurrenttowardseitherpole，whileanundercurrentfromthepoles，whichbecomesthetrade-winds，flowstowardstheequatortosupplyitsplace。

Butthesuperheatedequatorialair，becomingchilled，descendstothesurfaceintemperatelatitudes，andcontinuesitspolewardjourneyastheanti-trade-winds。

Thetrade-windsaredeflectedtowardsthewest，becauseinapproachingtheequatortheyconstantlypassoversurfacesoftheearthhavingagreaterandgreatervelocityofrotation，andso，asitwere，tendtolagbehind——

anexplanationwhichHadleypointedoutin1735，butwhichwasnotaccepteduntilDaltonindependentlyworkeditoutandpromulgateditin1793。

Fortheoppositereason，theanti-tradesaredeflectedtowardstheeast；henceitisthatthewestern，bordersofcontinentsintemperatezonesarebathedinmoistsea-breezes，whiletheireasternborderslackthiscold-

dispellinginfluence。

Intheoceanofwaterthemaincurrentsrunasmoresharplycircumscribedstreams——veritableriversinthesea。OfthesethebestknownandmostsharplycircumscribedisthefamiliarGulfStream，whichhasitsorigininanequatorialcurrent，impelledwestwardbytrade-winds，whichisdeflectednorthwardinthemainatCapeSt。Roque，enteringtheCaribbeanSeaandGulfofMexico，toemergefinallythroughtheStraitofFlorida，andjourneyoffacrosstheAtlantictowarmtheshoresofEurope。

Such，atleast，istheGulfStreamasHumboldtunderstoodit。Sincehistime，however，oceancurrentsingeneral，andthisoneinparticular，havebeenthesubjectofnoendofcontroversy，itbeinghotlydisputedwhethereithercausesoreffectsoftheGulfStreamarejustwhatHumboldt，incommonwithothersofhistime，conceivedthemtobe。AboutthemiddleofthecenturyLieutenantM。F。Maury，thedistinguishedAmericanhydrographerandmeteorologist，advocatedatheoryofgravitationasthechiefcauseofthecurrents，claimingthatdifferenceindensity，duetodifferenceintemperatureandsaltness，wouldsufficientlyaccountfortheoceaniccirculation。ThistheorygainedgreatpopularitythroughthewidecirculationofMaury’sPhysicalGeographyoftheSea，whichissaidtohavepassedthroughmoreeditionsthananyotherscientificbookoftheperiod；butitwasablyandvigorouslycombatedbyDr。JamesCroll，theScottishgeologist，inhisClimateandTime，andlatterlytheoldtheorythatoceancurrentsareduetothetrade-windshasagaincomeintofavor。Indeed，veryrecentlyamodelhasbeenconstructed，withtheaidofwhichitissaidtohavebeendemonstratedthatprevailingwindsinthedirectionoftheactualtrade-windswouldproducesuchacurrentastheGulfStream。

Meantime，however，itisbynomeanssurethatgravitationdoesnotenterintothecasetotheextentofproducinganinsensiblegeneraloceaniccirculation，independentoftheGulfStreamandsimilarmarkedcurrents，andsimilarinitslargeroutlinestothepolar-

equatorialcirculationoftheair。TheideaofsuchoceaniccirculationwasfirstsuggestedindetailbyProfessorLenz，ofSt。Petersburg，in1845，butitwasnotgenerallyrecognizeduntilDr。Carpenterindependentlyhitupontheideamorethantwentyyearslater。Theplausibilityoftheconceptionisobvious；

yettheallegedfactofsuchcirculationhasbeenhotlydisputed，andthequestionisstillsubjudice。

Butwhetherornotsuchgeneralcirculationofoceanwatertakesplace，itisbeyonddisputethattherecognizedcurrentscarryanenormousquantityofheatfromthetropicstowardsthepoles。Dr。Croll，whohasperhapsgivenmoreattentiontothephysicsofthesubjectthanalmostanyotherperson，computesthattheGulfStreamconveystotheNorthAtlanticone-

fourthasmuchheatasthatbodyreceivesdirectlyfromthesun，andhearguesthatwereitnotforthetransportationofheatbythisandsimilarPacificcurrents，onlyanarrowtropicalregionoftheglobewouldbewarmenoughforhabitationbytheexistingfaunas。

Dr。Crollarguesthataslightchangeintherelativevaluesofnorthernandsoutherntrade-winds(suchashebelieveshastakenplaceatvariousperiodsinthepast)wouldsufficetosoaltertheequatorialcurrentwhichnowfeedstheGulfStreamthatitsmainbulkwouldbedeflectedsouthwardinsteadofnorthward，bytheangleofCapeSt。Roque。ThustheGulfStreamwouldbenippedinthebud，and，accordingtoDr。

Croll’sestimates，theresultswouldbedisastrousforthenorthernhemisphere。Theanti-trades，whichnowarewarmedbytheGulfStream，wouldthenblowascoldwindsacrosstheshoresofwesternEurope，andinallprobabilityaglacialepochwouldsupervenethroughoutthenorthernhemisphere。

Thesameconsequences，sofarasEuropeisconcernedatleast，wouldapparentlyensueweretheIsthmusofPanamatosettleintothesea，allowingtheCaribbeancurrenttopassintothePacific。Butthegeologisttellsusthatthisisthmusroseatacomparativelyrecentgeologicalperiod，thoughitishintedthattherehadbeensometimepreviouslyatemporarylandconnectionbetweenthetwocontinents。Arewetoinfer，then，thatthetwoAmericasintheirunionsanddisunionshavejuggledwiththeclimateoftheotherhemisphere？Apparentlyso，iftheestimatesmadeoftheinfluenceoftheGulfStreambetenable。ItisafarcryfromPanamatoRussia。YetitseemswithinthepossibilitiesthatthemeteorologistmaylearnfromthegeologistofCentralAmericasomethingthatwillenablehimtoexplaintothepaleontologistofEuropehowitchancedthatatonetimethemammothandrhinocerosroamedacrossnorthernSiberia，whileatanothertimethereindeerandmusk-oxbrowsedalongtheshoresoftheMediterranean。

Possibilities，Isaid，notprobabilities。YeteventhefaintglimmerofsoalluringapossibilitybringshometoonewithvividnessthetruthofHumboldt’sperspicuousobservationthatmeteorologycanbeproperlycomprehendedonlywhenstudiedinconnectionwiththecompanionsciences。Therearenoisolatedphenomenainnature。

CYCLONESANDANTI-CYCLONES

Yet，afterall，itisnottobedeniedthatthechiefconcernofthemeteorologistmustbewiththatothermedium，the"oceanofair，ontheshoalsofwhichwelive。"Forwhatevermaybeaccomplishedbywatercurrentsinthewayofconveyingheat，itisthewindcurrentsthateffectthefinaldistributionofthatheat。

AsDr。Crollhasurged，thewatersoftheGulfStreamdonotwarmtheshoresofEuropebydirectcontact，butbywarmingtheanti-trade-winds，whichsubsequentlyblowacrossthecontinent。Andeverywheretheheataccumulatedbywaterbecomeseffectualinmodifyingclimate，notsomuchbydirectradiationasbydiffusionthroughthemediumoftheair。

Thisveryobviousimportanceofaerialcurrentsledtotheirpracticalstudylongbeforemeteorologyhadanytitletotherankofscience，andDalton’sexplanationofthetrade-windshadlaidthefoundationforascienceofwinddynamicsbeforethebeginningofthenineteenthcentury。Butnosubstantialfurtheradvanceinthisdirectionwaseffecteduntilabout1827，whenHeinrichW。Dove，ofKonigsberg，afterwardstobeknownasperhapstheforemostmeteorologistofhisgeneration，includedthewindsamongthesubjectsofhiselaboratestatisticalstudiesinclimatology。

Doveclassifiedthewindsaspermanent，periodical，andvariable。Hisgreatdiscoverywasthatallwinds，ofwhatevercharacter，andnotmerelythepermanentwinds，comeundertheinfluenceoftheearth’srotationinsuchawayastobedeflectedfromtheircourse，andhencetotakeonagyratorymotion——that，inshort，alllocalwindsareminoreddiesinthegreatpolar-equatorialwhirl，andtendtoreproduceinminiaturethecharacterofthatvastmaelstrom。Forthefirsttime，then，temporaryorvariablewindswereseentoliewithintheprovinceoflaw。

Agenerationlater，ProfessorWilliamFerrel，theAmericanmeteorologist，whohadbeenledtotakeupthesubjectbyaperusalofMaury’sdiscourseonoceanwinds，formulatedageneralmathematicallaw，totheeffectthatanybodymovinginarightlinealongthesurfaceoftheearthinanydirectiontendstohaveitscoursedeflected，owingtotheearth’srotation，totherighthandinthenorthernandtothelefthandinthesouthernhemisphere。Thislawhadindeedbeenstatedasearlyas1835bytheFrenchphysicistPoisson，butnoonethenthoughtofitasotherthanamathematicalcuriosity；itstruesignificancewasonlyunderstoodafterProfessorFerrelhadindependentlyrediscoveredit(justasDaltonrediscoveredHadley’sforgottenlawofthetrade-winds)andappliedittothemotionofwindcurrents。

Thenitbecameclearthathereisakeytothephenomenaofatmosphericcirculation，fromthegreatpolar-equatorialmaelstromwhichmanifestsitselfinthetrade-windstothemostcircumscribedrifflewhichisannouncedasalocalstorm。Andthemorethephenomenawerestudied，themorestrikingseemedtheparallelbetweenthegreatermaelstromandtheselessereddies。Justastheentireatmosphericmassofeachhemisphereisseen，whenviewedasawhole，tobecarriedinagreatwhirlaboutthepoleofthathemisphere，sothelocaldisturbanceswithinthisgreattidearefoundalwaystotaketheformofwhirlsaboutalocalstorm-centre——whichstorm-centre，meantime，iscarriedalonginthemajorcurrent，asoneoftenseesalittlewhirlpoolinthewatersweptalongwiththemaincurrentofthestream。Sometimes，indeed，thelocaleddy，caughtasitwereinanancillarycurrentofthegreatpolarstream，isdeflectedfromitsnormalcourseandmayseemtotravelagainstthestream；butsuchdeviationsaredeparturesfromtherule。Inthegreatmajorityofcases，forexample，inthenorthtemperatezone，astorm-centre(withitsattendantlocalwhirl)

travelstothenortheast，alongthemaincurrentoftheanti-trade-wind，ofwhichitisapart；andthoughexceptionallyitscoursemaybetothesoutheastinstead，italmostneverdepartssowidelyfromthemainchannelastoprogresstothewestward。ThusitisthatstormssweepingovertheUnitedStatescanbeannounced，asarule，attheseaboardinadvanceoftheircomingbytelegraphiccommunicationfromtheinterior，whilesimilarstormscometoEuropeofftheoceanunannounced。Hencethemorepracticalavailabilityoftheforecastsofweatherbureausintheformercountry。

Buttheselocalwhirls，itmustbeunderstood，arelocalonlyinaverygeneralsenseoftheword，inasmuchasasingleonemaybemorethanathousandmilesindiameter，andasmalloneistwoorthreehundredmilesacross。Butquitewithoutregardtothesizeofthewhirl，theaircomposingitconductsitselfalwaysinoneoftwoways。Itneverwhirlsinconcentriccircles；italwayseitherrushesintowardsthecentreinadescendingspiral，inwhichcaseitiscalledacyclone，oritspreadsoutfromthecentreinawideningspiral，inwhichcaseitiscalledananti-cyclone。Thewordcycloneisassociatedinpopularphraseologywithaterrificstorm，butithasnosuchrestrictionintechnicalusage。Agentlezephyrflowingtowardsa"storm-

centre"isjustasmuchacyclonetothemeteorologistasisthewhirlconstitutingaWest-Indianhurricane。

Indeed，itisnotproperlythewinditselfthatiscalledthecycloneineithercase，buttheentiresystemofwhirls——includingthestorm-centreitself，wheretheremaybenowindatall。

What，then，isthisstorm-centre？Merelyanareaoflowbarometricpressure——anareawheretheairhasbecomelighterthantheairofsurroundingregions。

Underinfluenceofgravitationtheairseeksitsleveljustaswaterdoes；sotheheavyaircomesflowinginfromallsidestowardsthelow-pressurearea，whichthusbecomesa"storm-centre。"Buttheinrushingcurrentsnevercomestraighttotheirmark。InaccordancewithFerrel’slaw，theyaredeflectedtotheright，andtheresult，aswillreadilybeseen，mustbeavortexcurrent，whichwhirlsalwaysinonedirection——namely，fromlefttoright，orinthedirectionoppositetothatofthehandsofawatchheldwithitsfaceupward。Thevelocityofthecycloniccurrentswilldependlargelyuponthedifferenceinbarometricpressurebetweenthestorm-centreandtheconfinesofthecyclonesystem。

Andthevelocityofthecurrentswilldeterminetosomeextentthedegreeofdeflection，andhencetheexactpathofthedescendingspiralinwhichthewindapproachesthecentre。Butineverycaseandineverypartofthecyclonesystemitistrue，asBuysBallot’sfamousrulefirstpointedout，thatapersonstandingwithhisbacktothewindhasthestorm-centreathisleft。

Theprimarycauseofthelowbarometricpressurewhichmarksthestorm-centreandestablishesthecycloneisexpansionoftheairthroughexcessoftemperature。

Theheatedair，risingintocoldupperregions，hasaportionofitsvaporcondensedintoclouds，andnowanewdynamicfactorisadded，foreachparticleofvapor，incondensing，givesupitsmodicumoflatentheat。Eachpoundofvaporthusliberates，accordingtoProfessorTyndall’sestimate，enoughheattomeltfivepoundsofcastiron；sotheamountgivenoutwherelargemassesofcloudareformingmustenormouslyaddtotheconvectioncurrentsoftheair，andhencetothestorm-developingpoweroftheformingcyclone。Indeed，oneschoolofmeteorologists，ofwhomProfessorEspywastheleader，hasheldthat，withoutsuchaddedincrementofenergyconstantlyaugmentingthedynamiceffects，nostormcouldlongcontinueinviolentaction。Anditisdoubtedwhetheranystormcouldeverattain，muchlesscontinue，theterrificforceofthatmostdreadedofwindsoftemperatezones，thetornado——astormwhichobeysallthelawsofcyclones，butdiffersfromordinarycyclonesinhavingavortexcoreonlyafewfeetoryardsindiameter——

withouttheaidofthosegreatmassesofcondensingvaporwhichalwaysaccompanyitintheformofstorm-

clouds。

Theanti-cyclonesimplyreversestheconditionsofthecyclone。Itscentreisanareaofhighpressure，andtheairrushesoutfromitinalldirectionstowardssurroundingregionsoflowpressure。Asbefore，allpartsofthecurrentwillbedeflectedtowardstheright，andtheresult，clearly，isawhirloppositeindirectiontothatofthecyclone。Butherethereisatendencytodissipationratherthantoconcentrationofenergy，hence，consideredasastorm-generator，theanti-

cycloneisofrelativeinsignificance。

Inparticulartheprofessionalmeteorologistwhoconductsa"weatherbureau"——as，forexample，thechiefoftheUnitedStatessignal-servicestationinNewYork——issopreoccupiedwiththeobservationofthisphenomenonthatcyclone-huntingmightbesaidtobehischiefpursuit。Itisforthispurpose，inthemain，thatgovernmentweatherbureausorsignal-

servicedepartmentshavebeenestablishedallovertheworld。Theirchiefworkistofollowupcyclones，withtheaidoftelegraphicreports，mappingtheircourseandrecordingtheattendantmeteorologicalconditions。

Theirso-calledpredictionsorforecastsareessentiallypredications，gaininglocallytheeffectofpredictionsbecausethetelegraphoutstripsthewind。

Atonlyoneplaceontheglobehasitbeenpossibleasyetforthemeteorologisttomakelong-timeforecastsmeritingthetitleofpredictions。ThisisinthemiddleGangesValleyofnorthernIndia。Inthiscountrytheclimaticconditionsarelargelydependentupontheperiodicalwindscalledmonsoons，whichblowsteadilylandwardfromApriltoOctober，andseawardfromOctobertoApril。Thesummermonsoonsbringtheall-essentialrains；iftheyaredelayedorrestrictedinextent，therewillbedroughtandconsequentfamine。

AndsuchrestrictionofthemonsoonislikelytoresultwhentherehasbeenanunusuallydeeporverylatesnowfallontheHimalayas，becauseoftheloweringofspringtemperaturebythemeltingsnow。Thushereitispossible，byobservingthesnowfallinthemountains，topredictwithsomemeasureofsuccesstheaveragerainfallofthefollowingsummer。Thedroughtof1896，withtheconsequentfamineandplaguethatdevastatedIndiathefollowingwinter，wasthuspredictedsomemonthsinadvance。

Thisisthegreatestpresenttriumphofpracticalmeteorology。

Nothinglikeitisyetpossibleanywhereintemperatezones。Butnoonecansaywhatmaynotbepossibleintimestocome，whenthedatanowbeinggatheredallovertheworldshallatlastbeco-ordinated，classified，andmadethebasisofbroadinductions。

Meteorologyispre-eminentlyascienceofthefuture。

VI

MODERNTHEORIESOFHEATANDLIGHT

THEeighteenth-centuryphilosophermadegreatstridesinhisstudiesofthephysicalpropertiesofmatterandtheapplicationofthesepropertiesinmechanics，asthesteam-engine，theballoon，theoptictelegraph，thespinning-jenny，thecotton-gin，thechronometer，theperfectedcompass，theLeydenjar，thelightning-rod，andahostofminorinventionstestify。

Inaspeculativewayhehadthoughtoutmoreorlesstenableconceptionsastotheultimatenatureofmatter，aswitnessthetheoriesofLeibnitzandBoscovichandDavy，towhichwemayrecur。Buthehadnotasyetconceivedthenotionofadistinctionbetweenmatterandenergy，whichissofundamentaltothephysicsofalaterepoch。Hedidnotspeakofheat，light，electricity，asformsofenergyor"force"；heconceivedthemassubtileformsofmatter——ashighlyattenuatedyettangiblefluids，subjecttogravitationandchemicalattraction；thoughhehadlearnedtomeasurenoneofthembutheatwithaccuracy，andthisonehecouldtestonlywithinnarrowlimitsuntillateinthecentury，whenJosiahWedgwood，thefamouspotter，taughthimtogaugethehighesttemperatureswiththeclaypyrometer。

Hespokeofthematterofheatasbeingthemostuniversallydistributedfluidinnature；asenteringinsomedegreeintothecompositionofnearlyallothersubstances；

asbeingsometimesliquid，sometimescondensedorsolid，andashavingweightthatcouldbedetectedwiththebalance。FollowingNewton，hespokeoflightasa"corpuscularemanation"orfluid，composedofshiningparticleswhichpossiblyaretransmutableintoparticlesofheat，andwhichenterintochemicalcombinationwiththeparticlesofotherformsofmatter。Electricityheconsideredastillmoresubtilekindofmatter-perhapsanattenuatedformoflight。Magnetism，"vitalfluid，"andbysomeevena"gravicfluid，"andafluidofsoundwereplacedinthesamescale；and，takentogether，allthesesupposedsubtileformsofmatterwereclassedas"imponderables。"

Thisviewofthenatureofthe"imponderables"wasinsomemeasurearetrogression，formanyseventeenth-

centuryphilosophers，notablyHookeandHuygensandBoyle，hadheldmorecorrectviews；butthematerialisticconceptionaccordedsowellwiththeeighteenth-

centurytendenciesofthoughtthatonlyhereandthereaphilosopherlikeEulercalleditinquestion，untilwellontowardsthecloseofthecentury。Currentspeechreferredtothematerialityofthe"imponderables"

unquestioningly。Studentsofmeteorology——asciencethatwasjustdawning——explainedatmosphericphenomenaonthesuppositionthatheat，theheaviestimponderable，predominatedintheloweratmosphere，andthatlight，electricity，andmagnetismprevailedinsuccessivelyhigherstrata。AndLavoisier，themostphilosophicalchemistofthecentury，retainedheatandlightonaparwithoxygen，hydrogen，iron，andtherest，inhislistofelementarysubstances。

COUNTRUMFORDANDTHEVIBRATORYTHEORYOFHEAT

ButjustatthecloseofthecenturytheconfidenceinthestatusoftheimponderableswasrudelyshakeninthemindsofphilosophersbytherevivaloftheoldideaofFraPaoloandBaconandBoyle，thatheat，atanyrate，isnotamaterialfluid，butmerelyamodeofmotionorvibrationamongtheparticlesof"ponderable"

matter。Thenewchampionoftheolddoctrineastothenatureofheatwasaverydistinguishedphilosopheranddiplomatistofthetime，who，itmaybeworthrecalling，wasanAmerican。HewasasadlyexpatriatedAmerican，itistrue，ashisname，givenalltheofficialappendages，willamplytestify；buthehadbeenbornandrearedinaMassachusettsvillagenonetheless，andheseemsalwaystohaveretainedakindlyinterestinthelandofhisnativity，eventhoughhelivedabroadintheserviceofotherpowersduringallthelateryearsofhislife，andwasknightedbyEngland，ennobledbyBavaria，andhonoredbythemostdistinguishedscientificbodiesofEurope。TheAmerican，then，whochampionedthevibratorytheoryofheat，inoppositiontoallcurrentopinion，inthisclosingeraoftheeighteenthcentury，wasLieutenant-GeneralSirBenjaminThompson，CountRumford，F。R。S。

Rumfordshowedthatheatmaybeproducedinindefinitequantitiesbyfrictionofbodiesthatdonotthemselvesloseanyappreciablematterintheprocess，andclaimedthatthisprovestheimmaterialityofheat。

Lateronheaddedforcetotheargumentbyproving，inrefutationoftheexperimentsofBowditch，thatnobodyeithergainsorlosesweightinvirtueofbeingheatedorcooled。Hethoughthehadprovedthatheatisonlyaformofmotion。

Hisexperimentforproducingindefinitequantitiesofheatbyfrictionisrecordedbyhiminhispaperentitled，"InquiryConcerningtheSourceofHeatExcitedbyFriction。"

"Beingengaged，lately，insuperintendingtheboringofcannonintheworkshopsofthemilitaryarsenalatMunich，"hesays，"Iwasstruckwiththeveryconsiderabledegreeofheatwhichabrassgunacquiresinashorttimeinbeingbored；andwiththestillmoreintenseheat(muchgreaterthanthatofboilingwater，asIfoundbyexperiment)ofthemetallicchipsseparatedfromitbytheborer。

"Takingacannon(abrasssix-pounder)，castsolid，andrough，asitcamefromthefoundry，andfixingithorizontallyinamachineusedforboring，andatthesametimefinishingtheoutsideofthecannonbyturning，Icauseditsextremitytobecutoff；andbyturningdownthemetalinthatpart，asolidcylinderwasformed，73/4inchesindiameterand98/10incheslong；

which，whenfinished，remainedjoinedtotherestofthemetal(thatwhich，properlyspeaking，constitutedthecannon)byasmallcylindricalneck，only21/5inchesindiameterand38/10incheslong。

"Thisshortcylinder，whichwassupportedinitshorizontalposition，andturnedrounditsaxisbymeansoftheneckbywhichitremainedunitedtothecannon，wasnowboredwiththehorizontalborerusedinboringcannon。

"Thiscylinderbeingdesignedfortheexpresspurposeofgeneratingheatbyfriction，byhavingabluntborerforcedagainstitssolidbottomatthesametimethatitshouldbeturnedrounditsaxisbytheforceofhorses，inorderthattheheataccumulatedinthecylindermightfromtimetotimebemeasured，asmall，roundhole0。37ofaninchonlyindiameterand4。2

inchesindepth，forthepurposeofintroducingasmallcylindricalmercurialthermometer，wasmadeinit，ononeside，inadirectionperpendiculartotheaxisofthecylinder，andendinginthemiddleofthesolidpartofthemetalwhichformedthebottomofthebore。

"Atthebeginningoftheexperiment，thetemperatureoftheairintheshade，asalsointhecylinder，wasjustsixtydegreesFahrenheit。Attheendofthirtyminutes，whenthecylinderhadmade960revolutionsaboutitsaxis，thehorsesbeingstopped，acylindricalmercurythermometer，whosebulbwas32/100ofaninchindiameterand31/4inchesinlength，wasintroducedintotheholemadetoreceiveitinthesideofthecylinder，whenthemercuryrosealmostinstantlytoonehundredandthirtydegrees。

"Inorder，byonedecisiveexperiment，todeterminewhethertheairoftheatmospherehadanypartornotinthegenerationoftheheat，Icontrivedtorepeattheexperimentundercircumstancesinwhichitwasevidentlyimpossibleforittoproduceanyeffectwhatever。

Bymeansofapistonexactlyfittedtothemouthoftheboreofthecylinder，throughthemiddleofwhichpistonthesquareironbar，totheendofwhichthebluntsteelborerwasfixed，passedinasquareholemadeperfectlyair-tight，theexcessoftheexternalair，totheinsideoftheboreofthecylinder，waseffectuallyprevented。

Ididnotfind，however，bythisexperimentthattheexclusionoftheairdiminishedinthesmallestdegreethequantityofheatexcitedbythefriction。

"Therestillremainedonedoubt，which，thoughitappearedtometobesoslightashardlytodeserveanyattention，Iwas，however，desiroustoremove。Thepistonwhichchokedthemouthoftheboreofthecylinder，inorderthatitmightbeair-tight，wasfittedintoitwithsomuchnicety，bymeansofitscollarsofleather，andpressedagainstitwithsomuchforce，that，notwithstandingitsbeingoiled，itoccasionedaconsiderabledegreeoffrictionwhenthehollowcylinderwasturnedrounditsaxis。Wasnottheheatproduced，oratleastsomepartofit，occasionedbythisfrictionofthepiston？and，astheexternalairhadfreeaccesstotheextremityofthebore，whereitcameintocontactwiththepiston，isitnotpossiblethatthisairmayhavehadsomeshareinthegenerationoftheheatproduced？

"Aquadrangularoblongdealbox，water-tight，beingprovidedwithholesorslitsinthemiddleofeachofitsends，justlargeenoughtoreceive，theonethesquareironrodtotheendofwhichthebluntsteelborerwasfastened，theotherthesmallcylindricalneckwhichjoinedthehollowcylindertothecannon；whenthisbox(whichwasoccasionallyclosedabovebyawoodencoverorlidmovingonhinges)wasputintoitsplace——

thatistosay，when，bymeansofthetwoverticalopeningorslitsinitstwoends，theboxwasfixedtothemachineryinsuchamannerthatitsbottombeingintheplaneofthehorizon，itsaxiscoincidedwiththeaxisofthehollowmetalliccylinder，itisevident，fromthedescription，thatthehollow，metalliccylinderwouldoccupythemiddleofthebox，withouttouchingitoneitherside；andthat，onpouringwaterintotheboxandfillingittothebrim，thecylinderwouldbecompletelycoveredandsurroundedoneverysidebythatfluid。And，further，astheboxwasheldfastbythestrong，squareironrodwhichpassedinasquareholeinthecentreofoneofitsends，whiletheroundorcylindricalneckwhichjoinedthehollowcylindertotheendofthecannoncouldturnroundfreelyonitsaxisintheroundholeinthecentreoftheotherendofit，itisevidentthatthemachinerycouldbeputinmotionwithouttheleastdangerofforcingtheboxoutofitsplace，throwingthewateroutofit，orderanginganypartoftheapparatus。"

Everythingbeingthusready，theboxwasfilledwithcoldwater，havingbeenmadewater-tightbymeansofleathercollars，andthemachineryputinmotion。

"Theresultofthisbeautifulexperiment，"saysRumford，"wasverystriking，andthepleasureitaffordedmeamplyrepaidmeforallthetroubleIhadhadincontrivingandarrangingthecomplicatedmachineryusedinmakingit。Thecylinder，revolvingattherateofthirty-twotimesinaminute，hadbeeninmotionbutashorttimewhenIperceived，byputtingmyhandintothewaterandtouchingtheoutsideofthecylinder，thatheatwasgenerated，anditwasnotlongbeforethewaterwhichsurroundedthecylinderbegantobesensiblywarm。

"AttheendofonehourIfound，byplungingathermometerintothebox，……thatitstemperaturehadbeenraisednolessthanforty-sevendegreesFahrenheit，beingnowonehundredandsevendegreesFahrenheit……Onehourandthirtyminutesafterthemachineryhadbeenputinmotiontheheatofthewaterintheboxwasonehundredandforty-twodegrees。Attheendoftwohours……itwasraisedtoonehundredandseventy-eightdegrees；andattwohoursandthirtyminutesitACTUALLYBOILED！

"Itwouldbedifficulttodescribethesurpriseandastonishmentexpressedinthecountenancesofthebystandersonseeingsolargeaquantityofcoldwaterheated，andactuallymadetoboil，withoutanyfire。

Thoughtherewas，infact，nothingthatcouldjustlybeconsideredasasurpriseinthisevent，yetIacknowledgefairlythatitaffordedmeadegreeofchildishpleasurewhich，wereIambitiousofthereputationofaGRAVEPHILOSOPHER，Ioughtmostcertainlyrathertohidethantodiscover……"

Havingthusdweltindetailontheseexperiments，Rumfordcomesnowtotheall-importantdiscussionastothesignificanceofthem——thesubjectthathadbeenthesourceofsomuchspeculationamongthephilosophers——

thequestionastowhatheatreallyis，andiftherereallyisanysuchthing(asmanybelieved)asanigneousfluid，orasomethingcalledcaloric。

"Fromwhencecamethisheatwhichwascontinuallygivenoffinthismanner，intheforegoingexperiments？"

asksRumford。"Wasitfurnishedbythesmallparticlesofmetaldetachedfromthelargersolidmassesontheirbeingrubbedtogether？This，aswehavealreadyseen，couldnotpossiblyhavebeenthecase。

"Wasitfurnishedbytheair？Thiscouldnothavebeenthecase；for，inthreeoftheexperiments，themachinerybeingkeptimmersedinwater，theaccessoftheairoftheatmospherewascompletelyprevented。

"Wasitfurnishedbythewaterwhichsurroundedthemachinery？Thatthiscouldnothavebeenthecaseisevident:first，becausethiswaterwascontinuallyRECEIVINGheatfromthemachinery，andcouldnot，atthesametime，beGIVINGTOandRECEIVINGHEATFROMthesamebody；and，secondly，becausetherewasnochemicaldecompositionofanypartofthiswater。Hadanysuchdecompositiontakenplace(which，indeed，couldnotreasonablyhavebeenexpected)，oneofitscomponentelasticfluids(mostprobablyhydrogen)must，atthesametime，havebeensetatliberty，and，inmakingitsescapeintotheatmosphere，wouldhavebeendetected；

but，thoughIfrequentlyexaminedthewatertoseeifanyair-bubblesroseupthroughit，andhadevenmadepreparationsforcatchingthemiftheyshouldappear，Icouldperceivenone；norwasthereanysignofdecompositionofanykindwhatever，orotherchemicalprocess，goingoninthewater。

"Isitpossiblethattheheatcouldhavebeensuppliedbymeansoftheironbartotheendofwhichthebluntsteelborerwasfixed？Orbythesmallneckofgun-metalbywhichthehollowcylinderwasunitedtothecannon？Thesesuppositionsseemmoreimprobableeventhaneitherofthebefore-mentioned；forheatwascontinuallygoingoff，orOUTOFTHEMACHINERY，byboththesepassagesduringthewholetimetheexperimentlasted。

"Andinreasoningonthissubjectwemustnotforgettoconsiderthatmostremarkablecircumstance，thatthesourceoftheheatgeneratedbyfrictionintheseexperimentsappearedevidentlytobeINEXHAUSTIBLE。

"ItishardlynecessarytoaddthatanythingwhichanyINSULATEDbody，orsystemofbodies，cancontinuetofurnishWITHOUTLIMITATIONcannotpossiblybeaMATERIAL

substance；anditappearstometobeextremelydifficult，ifnotquiteimpossible，toformanydistinctideaofanythingcapableofbeingexcitedandcommunicated，inthemannertheheatwasexcitedandcommunicatedintheseexperiments，exceptinMOTION。"[1]

THOMASYOUNGANDTHEWAVETHEORYOFLIGHT

Butcontemporaryjudgment，whileitlistenedrespectfullytoRumford，waslittlemindedtoaccepthisverdict。Thecherishedbeliefsofagenerationarenottobeputdownwithasingleblow。Wheremanymindshaveasimilardrift，however，thefirstblowmayprecipitateageneralconflict；andsoitwashere。YoungHumphryDavyhadduplicatedRumford’sexperiments，andreachedsimilarconclusions；andsoonothersfellintoline。Then，in1800，Dr。ThomasYoung——

"PhenomenonYoung"theycalledhimatCambridge，becausehewasreputedtoknoweverything——tookupthecudgelsforthevibratorytheoryoflight，anditbegantobeclearthatthetwo"imponderables，"heatandlight，muststandorfalltogether；butnooneasyetmadeaclaimagainstthefluidityofelectricity。

BeforewetakeupthedetailsoftheassaultmadebyYoungupontheolddoctrineofthematerialityoflight，wemustpausetoconsiderthepersonalityofYounghimself。ForitchancedthatthisQuakerphysicianwasoneofthoseprodigieswhocomebutfewtimesinacentury，andthefulllistofwhomintherecordsofhistorycouldbetoldonone’sthumbsandfingers。Hisbiographerstellusthingsabouthimthatreadlikethemostpatentfairy-tales。Asamereinfantinarmshehadbeenabletoreadfluently。BeforehisfourthbirthdaycamehehadreadtheBibletwicethrough，aswellasWatts’sHymns——poorchild！——andwhensevenoreighthehadshownapropensitytoabsorblanguagesmuchasotherchildrenabsorbnurserytattleandMotherGooserhymes。Whenhewasfourteen，ayoungladyvisitingthehouseholdofhistutorpatronizedtheprettyboybyaskingtoseeaspecimenofhispenmanship。

Theprettyboycompliedreadilyenough，andmildlyrebukedhisinterrogatorbyrapidlywritingsomesentencesforherinfourteenlanguages，includingsuchas，Arabian，Persian，andEthiopic。

Meantimelanguageshadbeenbutanincidentintheeducationofthelad。Heseemstohaveenteredeveryavailablefieldofthought——mathematics，physics，botany，literature，music，painting，languages，philosophy，archaeology，andsoontotiresomelengths——andoncehehadenteredanyfieldheseldomturnedasideuntilhehadreachedtheconfinesofthesubjectasthenknownandaddedsomethingnewfromtherecessesofhisowngenius。HewasasversatileasPriestley，asprofoundasNewtonhimself。Hehadtherangeofameredilettante，buteverywherethefullgraspofthemaster。Hetookearlyforhismottothesayingthatwhatonemanhasdone，anothermanmaydo。GrantingthattheothermanhasthebrainofaThomasYoung，itisatruemotto。

Such，then，wastheyoungQuakerwhocametoLondontofollowoutthehumdrumlifeofapractitionerofmedicineintheyear1801。ButincidentallytheyoungphysicianwasprevailedupontooccupytheinterimsofearlypracticebyfulfillingthedutiesofthechairofNaturalPhilosophyattheRoyalInstitution，whichCountRumfordhadfounded，andofwhichDavywasthenProfessorofChemistry——theinstitutionwhoseglorieshavebeenperpetuatedbysuchnamesasFaradayandTyndall，andwhichtheBritonofto-dayspeaksofasthe"PantheonofScience。"HereitwasthatThomasYoungmadethosestudieswhichhaveinsuredhimanicheinthetempleoffamenotfarremovedfromthatofIsaacNewton。

Asearlyas1793，whenhewasonlytwenty，YounghadbeguntoCommunicatepaperstotheRoyalSocietyofLondon，whichwereadjudgedworthytobeprintedinfullinthePhilosophicalTransactions；soitisnotstrangethatheshouldhavebeenaskedtodelivertheBakerianlecturebeforethatlearnedbodytheveryfirstyearafterhecametoLondon。ThelecturewasdeliveredNovember12，1801。Itssubjectwas"TheTheoryofLightandColors，"anditsreadingmarksanepochinphysicalscience；forherewasbroughtforwardforthefirsttimeconvincingproofofthatundulatorytheoryoflightwithwhicheverystudentofmodernphysicsisfamiliar——thetheorywhichholdsthatlightisnotacorporealentity，butamerepulsationinthesubstanceofanall-pervadingether，justassoundisapulsationintheair，orinliquidsorsolids。

Younghad，indeed，advocatedthistheoryatanearlierdate，butitwasnotuntil1801thathehitupontheideawhichenabledhimtobringittoanythingapproachingademonstration。Itwaswhileponderingoverthefamiliarbutpuzzlingphenomenaofcoloredringsintowhichwhitelightisbrokenwhenreflectedfromthinfilms——Newton’srings，socalled——thatanexplanationoccurredtohimwhichatonceputtheentireundulatorytheoryonanewfooting。Withthatsagacityofinsightwhichwecallgenius，hesawofasuddenthatthephenomenacouldbeexplainedbysupposingthatwhenraysoflightfallonathinglass，partoftheraysbeingreflectedfromtheuppersurface，otherrays，reflectedfromthelowersurface，mightbesoretardedintheircoursethroughtheglassthatthetwosetswouldinterferewithoneanother，theforwardpulsationofoneraycorrespondingtothebackwardpulsationofanother，thusquiteneutralizingtheeffect。

Someofthecomponentpulsationsofthelightbeingthuseffacedbymutualinterference，theremainingrayswouldnolongergivetheopticaleffectofwhitelight；hencethepuzzlingcolors。

HereisYoung’sexpositionofthesubject:

OftheColorsofThinPlates"Whenabeamoflightfallsupontworefractingsurfaces，thepartialreflectionscoincideperfectlyindirection；andinthiscasetheintervalofretardationtakenbetweenthesurfacesistotheirradiusastwicethecosineoftheangleofrefractiontotheradius。

"Letthemediumbetweenthesurfacesberarerthanthesurroundingmediums；thentheimpulsereflectedatthesecondsurface，meetingasubsequentundulationatthefirst，willrendertheparticlesoftherarermediumcapableofwhollystoppingthemotionofthedenseranddestroyingthereflection，whiletheythemselveswillbemorestronglypropelledthaniftheyhadbeenatrest，andthetransmittedlightwillbeincreased。

Sothatthecolorsbyreflectionwillbedestroyed，andthosebytransmissionrenderedmorevivid，whenthedoublethicknessorintervalsofretardationareanymultiplesofthewholebreadthoftheundulations；andatintermediatethicknessestheeffectswillbereversedaccordingtotheNewtonianobservation。

"Ifthesameproportionsbefoundtoholdgoodwithrespecttothinplatesofadensermedium，whichis，indeed，notimprobable，itwillbenecessarytoadopttheconnecteddemonstrationsofProp。IV。，but，atanyrate，ifathinplatebeinterposedbetweenararerandadensermedium，thecolorsbyreflectionandtransmissionmaybeexpectedtochangeplaces。

OftheColorsofThickPlates"Whenabeamoflightpassesthrougharefractingsurface，especiallyifimperfectlypolished，aportionofitisirregularlyscattered，andmakesthesurfacevisibleinalldirections，butmostconspicuouslyindirectionsnotfardistantfromthatofthelightitself；andifareflectingsurfacebeplacedparalleltotherefractingsurface，thisscatteredlight，aswellastheprincipalbeam，willbereflected，andtherewillbealsoanewdissipationoflight，atthereturnofthebeamthroughtherefractingsurface。Thesetwoportionsofscatteredlightwillcoincideindirection；andifthesurfacesbeofsuchaformastocollectthesimilareffects，willexhibitringsofcolors。Theintervalofretardationisherethedifferencebetweenthepathsoftheprincipalbeamandofthescatteredlightbetweenthetwosurfaces；

ofcourse，wherevertheinclinationofthescatteredlightisequaltothatofthebeam，althoughindifferentplanes，theintervalwillvanishandalltheundulationswillconspire。Atotherinclinations，theintervalwillbethedifferenceofthesecantsfromthesecantoftheinclination，orangleofrefractionoftheprincipalbeam。Fromthesecauses，allthecolorsofconcavemirrorsobservedbyNewtonandothersarenecessaryconsequences；anditappearsthattheirproduction，thoughsomewhatsimilar，isbynomeansasNewtonimagined，identicalwiththeproductionofthinplates。"[2]

Byfollowingupthisclewwithmathematicalprecision，measuringtheexactthicknessoftheplateandthespacebetweenthedifferentringsofcolor，Youngwasabletoshowmathematicallywhatmustbethelengthofpulsationforeachofthedifferentcolorsofthespectrum。Heestimatedthattheundulationsofredlight，attheextremelowerendofthevisiblespectrum，mustnumberaboutthirty-seventhousandsixhundredandfortytotheinch，andpassanygivenspotatarateoffourhundredandsixty-threemillionsofmillionsofundulationsinasecond，whiletheextremevioletnumbersfifty-ninethousandsevenhundredandfiftyundulationstotheinch，orsevenhundredandthirty-fivemillionsofmillionstothesecond。

TheColorsofStriatedSurfacesYoungsimilarlyexaminedthecolorsthatareproducedbyscratchesonasmoothsurface，inparticulartestingthelightfrom"Mr。Coventry’sexquisitemicrometers，"

whichconsistoflinesscratchedonglassatmeasuredintervals。Thesemicroscopictestsbroughtthesameresultsastheotherexperiments。Thecolorswereproducedatcertaindefiniteandmeasurableangles，andthetheoryofinterferenceofundulationsexplainedthemperfectly，while，asYoungaffirmedwithconfidence，nootherhypothesishithertoadvancedwouldexplainthematall。Herearehiswords:

"Lettherebeinagivenplanetworeflectingpointsveryneareachother，andlettheplanebesosituatedthatthereflectedimageofaluminousobjectseeninitmayappeartocoincidewiththepoints；thenitisobviousthatthelengthoftheincidentandreflectedray，takentogether，isequalwithrespecttobothpoints，consideringthemascapableofreflectinginalldirections。

Letoneofthepointsbenowdepressedbelowthegivenplane；thenthewholepathofthelightreflectedfromitwillbelengthenedbyalinewhichistothedepressionofthepointastwicethecosineofincidencetotheradius。

"If，therefore，equalundulationsofgivendimensionsbereflectedfromtwopoints，situatednearenoughtoappeartotheeyebutasone，wheneverthislineisequaltohalfthebreadthofawholeundulationthereflectionfromthedepressedpointwillsointerferewiththereflectionfromthefixedpointthattheprogressivemotionoftheonewillcoincidewiththeretrogrademotionoftheother，andtheywillbothbedestroyed；butwhenthislineisequaltothewholebreadthofanundulation，theeffectwillbedoubled，andwhentoabreadthandahalf，againdestroyed；andthusforaconsiderablenumberofalternations，andifthereflectedundulationsbeofadifferentkind，theywillbevariouslyaffected，accordingtotheirproportionstothevariouslengthofthelinewhichisthedifferencebetweenthelengthsoftheirtwopaths，andwhichmaybedenominatedtheintervalofaretardation。

"Inorderthattheeffectmaybethemoreperceptible，anumberofpairsofpointsmustbeunitedintotwoparallellines；andifseveralsuchpairsoflinesbeplacedneareachother，theywillfacilitatetheobservation。Ifoneofthelinesbemadetorevolveroundtheotherasanaxis，thedepressionbelowthegivenplanewillbeasthesineoftheinclination；andwhiletheeyeandtheluminousobjectremainfixedthedifferenceofthelengthofthepathswillvaryasthissine。

"ThebestsubjectsfortheexperimentareMr。Coventry’sexquisitemicrometers；suchofthemasconsistofparallellinesdrawnonglass，atadistanceofone-

five-hundredthofaninch，arethemostconvenient。

Eachoftheselinesappearsunderamicroscopetoconsistoftwoormorefinerlines，exactlyparallel，andatadistanceofsomewhatmorethanatwentiethmorethantheadjacentlines。Iplacedoneofthesesoastoreflectthesun’slightatanangleofforty-fivedegrees，andfixeditinsuchamannerthatwhileitrevolvedroundoneofthelinesasanaxis，Icouldmeasureitsangularmotion；Ifoundthatthelongestredcoloroccurredattheinclination101/4degrees，203/4degrees，32

degrees，and45degrees；ofwhichthesinesareasthenumbers1，2，3，and4。Atallotheranglesalso，whenthesun’slightwasreflectedfromthesurface，thecolorvanishedwiththeinclination，andwasequalatequalinclinationsoneitherside。

Thisexperimentaffordsaverystrongconfirmationofthetheory。Itisimpossibletodeduceanyexplanationofitfromanyhypothesishithertoadvanced；

andIbelieveitwouldbedifficulttoinventanyotherthatwouldaccountforit。Thereisastrikinganalogybetweenthisseparationofcolorsandtheproductionofamusicalnotebysuccessiveechoesfromequidistantironpalisades，whichIhavefoundtocorrespondprettyaccuratelywiththeknownvelocityofsoundandthedistancesofthesurfaces。

"Itisnotimprobablethatthecolorsoftheintegumentsofsomeinsects，andofsomeothernaturalbodies，exhibitingindifferentlightsthemostbeautifulversatility，maybefoundtobeofthisdescription，andnottobederivedfromthinplates。Insomecasesasinglescratchorfurrowmayproducesimilareffects，bythereflectionofitsoppositeedges。"[3]

ThisdoctrineofinterferenceofundulationswastheabsolutelynovelpartofYoung’stheory。Theall-

compassinggeniusofRobertHookehad，indeed，verynearlyapprehendeditmorethanacenturybefore，asYounghimselfpointsout，butnooneelsebadsomuchasvaguelyconceivedit；andevenwiththesagaciousHookeitwasonlyahappyguess，neverdistinctlyoutlinedinhisownmind，andutterlyignoredbyallothers。

YoungdidnotknowofHooke’sguessuntilhehimselfhadfullyformulatedthetheory，buthehastenedthentogivehispredecessorallthecreditthatcouldpossiblybeadjudgedhisduebythemostdisinterestedobserver。

ToHooke’scontemporary，Huygens，whowastheoriginatorofthegeneraldoctrineofundulationastheexplanationoflight，Youngrendersfulljusticealso。

Forhimselfheclaimsonlythemeritofhavingdemonstratedthetheorywhichtheseandafewothersofhispredecessorshadadvocatedwithoutfullproof。

ThefollowingyearDr。YoungdetailedbeforetheRoyalSocietyotherexperiments，whichthrewadditionallightonthedoctrineofinterference；andin1803

hecitedstillothers，which，heaffirmed，broughtthedoctrinetocompletedemonstration。Inapplyingthisdemonstrationtothegeneraltheoryoflight，hemadethestrikingsuggestionthat"theluminiferousetherpervadesthesubstanceofallmaterialbodieswithlittleornoresistance，asfreely，perhaps，asthewindpassesthroughagroveoftrees。"HeassertedhisbeliefalsothatthechemicalrayswhichRitterhaddiscoveredbeyondthevioletendofthevisiblespectrumarebutstillmorerapidundulationsofthesamecharacterasthosewhichproducelight。InhisearlierlecturehehadaffirmedalikeaffinitybetweenthelightraysandtheraysofradiantheatwhichHerscheldetectedbelowtheredendofthespectrum，suggestingthat"lightdiffersfromheatonlyinthefrequencyofitsundulationsorvibrations——thoseundulationswhicharewithincertainlimitswithrespecttofrequencyaffectingtheopticnerveandconstitutinglight，andthosewhichareslowerandprobablystrongerconstitutingheatonly。"Fromtheveryoutsethehadrecognizedtheaffinitybetweensoundandlight；indeed，ithadbeenthisaffinitythatledhimontoanappreciationoftheundulatorytheoryoflight。

Butwhilealltheseaffinitiesseemedsocleartothegreatco-ordinatingbrainofYoung，theymadenosuchimpressiononthemindsofhiscontemporaries。Theimmaterialityoflighthadbeensubstantiallydemonstrated，butpracticallynoonesaveitsauthoracceptedthedemonstration。Newton’sdoctrineoftheemissionofcorpuscleswastoofirmlyrootedtobereadilydislodged，andDr。Younghadtoomanyotherintereststocontinuetheassaultunceasingly。Heoccasionallywrotesomethingtouchingonhistheory，mostlypaperscontributedtotheQuarterlyReviewandsimilarperiodicals，anonymouslyorunderpseudonym，forhehadconceivedthenotionthattoogreatconspicuousnessinfieldsoutsideofmedicinewouldinjurehispracticeasaphysician。Hisviewsregardinglight(includingtheoriginalpapersfromthePhilosophicalTransactionsoftheRoyalSociety)wereagaingivenpublicityinfullinhiscelebratedvolumeonnaturalphilosophy，consistinginpartofhislecturesbeforetheRoyalInstitution，publishedin1807；buteventhentheyfailedtobringconvictiontothephilosophicworld。Indeed，theydidnotevenarouseacontroversialspirit，ashisfirstpapershaddone。

ARAGOANDFRESNELCHAMPIONTHEWAVETHEORY

Soitchancedthatwhen，in1815，ayoungFrenchmilitaryengineer，namedAugustinJeanFresnel，returningfromtheNapoleonicwars，becameinterestedinthephenomenaoflight，andmadesomeexperimentsconcerningdiffractionwhichseemedtohimtocontroverttheacceptednotionsofthematerialityoflight，hewasquiteunawarethathisexperimentshadbeenanticipatedbyaphilosopheracrosstheChannel。HecommunicatedhisexperimentsandresultstotheFrenchInstitute，supposingthemtobeabsolutelynovel。Thatbodyreferredthemtoacommittee，ofwhich，asgoodfortunewouldhaveit，thedominatingmemberwasDominiqueFrancoisArago，amanasversatileasYounghimself，andhardlylessprofound，ifperhapsnotquitesooriginal。AragoatoncerecognizedthemeritofFresnel’swork，andsoonbecameaconverttothetheory。HetoldFresnelthatYounghadanticipatedhimasregardsthegeneraltheory，butthatmuchremainedtobedone，andheofferedtoassociatehimselfwithFresnelinprosecutingtheinvestigation。

Fresnelwasnotalittledashedtolearnthathisoriginalideashadbeenworkedoutbyanotherwhilehewasalad，buthebowedgracefullytothesituationandwentaheadwithunabatedzeal。

ThechampionshipofAragoinsuredtheundulatorytheoryahearingbeforetheFrenchInstitute，butbynomeanssufficedtobringaboutitsgeneralacceptance。

Onthecontrary，abitterfeudensued，inwhichAragowasopposedbythe"JupiterOlympusoftheAcademy，"

Laplace，bytheonlylessfamousPoisson，andbytheyoungerbuthardlylessableBiot。Sobitterlyragedthefeudthatalife-longfriendshipbetweenAragoandBiotwasrupturedforever。TheoppositionmanagedtodelaythepublicationofFresnel’spapers，butAragocontinuedtofightwithhiscustomaryenthusiasmandpertinacity，andatlast，in1823，theAcademyyielded，andvotedFresnelintoitsranks，thusimplicitlyadmittingthevalueofhiswork。

Itisahumiliatingthoughtthatsuchcontroversiesasthismustmartheprogressofscientifictruth；butfortunatelythestoryoftheintroductionoftheundulatorytheoryhasamorepleasantside。Threemen，greatbothincharacterandinintellect，wereconcernedinpressingitsclaims——Young，Fresnel，andArago——andtherelationsofthesemenformapictureunmarredbyanyofthosepettyjealousiesthatsooftendimthelustreofgreatnames。FresnelfreelyacknowledgedYoung’sprioritysosoonashisattentionwascalledtoit；andYoungapplaudedtheworkoftheFrenchman，andaidedwithhiscounselintheapplicationoftheundulatorytheorytotheproblemsofpolarizationoflight，whichstilldemandedexplanation，andwhichFresnel’sfertilityofexperimentalresourceandprofundityofmathematicalinsightsufficedintheendtoconquer。

AfterFresnel’sadmissiontotheInstitutein1823

theoppositionweakened，andgraduallythephilosopherscametorealizethemeritsofatheorywhichYounghadvainlycalledtotheirattentionafullquarter-

centurybefore。Now，thankslargelytoArago，bothYoungandFresnelreceivedtheirfullmeedofappreciation。

FresnelwasgiventheRumfordmedaloftheRoyalSocietyofEnglandin1825，andchosenoneoftheforeignmembersofthesocietytwoyearslater，whileYounginturnwaselectedoneoftheeightforeignmembersoftheFrenchAcademy。Asafittingculminationofthechapteroffelicitiesbetweenthethreefriends，itfelltothelotofYoung，asForeignSecretaryoftheRoyalSociety，tonotifyFresnelofthehonorsshownhimbyEngland’srepresentativebodyofscientists；

whileArago，asPerpetualSecretaryoftheFrenchInstitute，conveyedtoYounginthesameyearthenotificationthathehadbeensimilarlyhonoredbythesavantsofFrance。

AfewmonthslaterFresnelwasdead，andYoungsurvivedhimonlytwoyears。Bothdiedprematurely，buttheirgreatworkwasdone，andtheworldwillrememberalwaysandlinktogetherthesetwonamesinconnectionwithatheorywhichinitsimplicationsandimportancerankslittlebelowthetheoryofuniversalgravitation。

VII。THEMODERNDEVELOPMENTOFELECTRICITYANDMAGNETISM

GALVANIANDVOLTA

ThefullimportanceofYoung’sstudiesoflightmightperhapshavegainedearlierrecognitionhaditnotchancedthat，atthetimewhentheyweremade，theattentionofthephilosophicworldwasturnedwiththefixityandfascinationofahypnoticstareuponanotherfield，whichforatimebrookednorival。

Howcouldtheold，familiarphenomenon，light，interestanyonewhenthenewagent，galvanism，wasinview？

Aswellaskonetofixattentiononastarwhileameteoriteblazesacrossthesky。

GalvanismwassocalledpreciselyastheRoentgenraywaschristenedatalaterday——asasafemeansofbeggingthequestionastothenatureofthephenomenainvolved。TheinitialfactingalvanismwasthediscoveryofLuigiGalvani(1737-1798)，aphysicianofBologna，in1791，thatbybringingmetalsincontactwiththenervesofafrog’slegviolentmuscularcontractionsareproduced。Asthissimplelittleexperimentledeventuallytothediscoveryofgalvanicelectricityandtheinventionofthegalvanicbattery，itmayberegardedasthebeginningofmodernelectricity。

ThestoryistoldthatGalvaniwasledtohisdiscoverywhilepreparingfrogs’legstomakeabrothforhisinvalidwife。Asthestoryruns，hehadremovedtheskinsfromseveralfrogs’legs，when，happeningtotouchtheexposedmuscleswithascalpelwhichhadlainincloseproximitytoanelectricalmachine，violentmuscularactionwasproduced。Impressedwiththisphenomenon，hebeganaseriesofexperimentswhichfinallyresultedinhisgreatdiscovery。Butbethisstoryauthenticornot，itiscertainthatGalvaniexperimentedforseveralyearsuponfrogs’legssuspendeduponwiresandhooks，untilhefinallyconstructedhisarcoftwodifferentmetals，which，whenarrangedsothatonewasplacedincontactwithanerveandtheotherwithamuscle，producedviolentcontractions。

Thesetwopiecesofmetalformthebasicprincipleofthemoderngalvanicbattery，andleddirectlytoAlessandroVolta’sinventionofhis"voltaicpile，"theimmediateancestorofthemoderngalvanicbattery。

Volta’sexperimentswerecarriedonatthesametimeasthoseofGalvani，andhisinventionofhispilefollowedcloseuponGalvani’sdiscoveryofthenewformofelectricity。Fromthesefactsthenewformofelectricitywassometimescalled"galvanic"andsometimes"voltaic"electricity，butinrecentyearstheterm"galvanism"and"galvaniccurrent"havealmostentirelysupplantedtheuseofthetermvoltaic。

ItwasVoltawhomadethereportofGalvani’swonderfuldiscoverytotheRoyalSocietyofLondon，readonJanuary31，1793。InthisletterhedescribesGalvani’sexperimentsindetailandreferstotheminglowingtermsofpraise。Hecallsitoneofthe"mostbeautifulandimportantdiscoveries，"andregardeditasthegermorfoundationuponwhichotherdiscoveriesweretobemade。Thepredictionprovedentirelycorrect，Voltahimselfbeingthechiefdiscoverer。

WorkingalonglinessuggestedbyGalvani’sdiscovery，Voltaconstructedanapparatusmadeupofanumberofdisksoftwodifferentkindsofmetal，suchastinandsilver，arrangedalternately，apieceofsomemoist，poroussubstance，likepaperorfelt，beinginterposedbetweeneachpairofdisks。Withthis"pile，"

asitwascalled，electricitywasgenerated，andbylinkingtogetherseveralsuchpilesanelectricbatterycouldbeformed。

Thisinventiontooktheworldbystorm。NothingliketheenthusiasmitcreatedinthephilosophicworldhadbeenknownsincetheinventionoftheLeydenjar，morethanhalfacenturybefore。WithinafewweeksafterVolta’sannouncement，batteriesmadeaccordingtohisplanwerebeingexperimentedwithineveryimportantlaboratoryinEurope。

Asthecenturyclosed，halfthephilosophicworldwasspeculatingastowhether"galvanicinfluence"

wereanewimponderable，oronlyaformofelectricity；

andtheotherhalfwaseagerlyseekingtodiscoverwhatnewmarvelsthebatterymightreveal。Theleastimaginativemancouldseethatherewasaninventionthatwouldbeepoch-making，butthemostvisionarydreamercouldnotevenvaguelyadumbratetherealmeasureofitsimportance。

Itwasevidentatoncethatalmostanyformofgalvanicbattery，despiteimperfections，wasamoresatisfactoryinstrumentforgeneratingelectricitythanthefrictionalmachinehithertoinuse，theadvantagelyinginthefactthatthecurrentfromthegalvanicbatterycouldbecontrolledpracticallyatwill，andthattheapparatusitselfwasinexpensiveandrequiredcomparativelylittleattention。Theseadvantagesweresoonmadeapparentbythepracticalapplicationoftheelectriccurrentinseveralfields。

ItwillberecalledthatdespitetheenergeticendeavorsofsuchphilosophersasWatson，Franklin，Galvani，andmanyothers，thefieldofpracticalapplicationofelectricitywasverylimitedatthecloseoftheeighteenthcentury。Thelightning-rodhadcomeintogeneraluse，tobesure，anditsvalueasaninventioncanhardlybeoverestimated。Butwhileitwastheresultofextensiveelectricaldiscoveries，andisamostpracticalinstrument，itcanhardlybecalledonethatputselectricitytopracticaluse，butsimplyactsasameansofwardingofftheevileffectsofanaturalmanifestationofelectricity。Theinvention，however，hadalltheeffectsofamechanismwhichturnedelectricitytopracticalaccount。Butwiththeadventofthenewkindofelectricitytheageofpracticalapplicationbegan。