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，themeanannualtemperatureofaregionreferringmainlytothenorthernhemisphere
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-windssuchashebelieveshastakenplaceatvariousperiodsinthepastwouldsufficetosoaltertheequatorialcurrentwhichnowfeedstheGulfStreamthatitsmainbulkwouldbedeflectedsouthwardinsteadofnorthward，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；itstruesignificancewasonlyunderstoodafterProfessorFerrelhadindependentlyrediscovereditjustasDaltonrediscoveredHadley’sforgottenlawofthetrade-windsandappliedittothemotionofwindcurrents。
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-centrewithitsattendantlocalwhirl
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；andwiththestillmoreintenseheatmuchgreaterthanthatofboilingwater，asIfoundbyexperimentofthemetallicchipsseparatedfromitbytheborer。
\"Takingacannonabrasssix-pounder，castsolid，andrough，asitcamefromthefoundry，andfixingithorizontallyinamachineusedforboring，andatthesametimefinishingtheoutsideofthecannonbyturning，Icauseditsextremitytobecutoff；andbyturningdownthemetalinthatpart，asolidcylinderwasformed，73/4inchesindiameterand98/10incheslong；
which，whenfinished，remainedjoinedtotherestofthemetalthatwhich，properlyspeaking，constitutedthecannonbyasmallcylindricalneck，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；whenthisboxwhichwasoccasionallyclosedabovebyawoodencoverorlidmovingonhingeswasputintoitsplace——
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，andiftherereallyisanysuchthingasmanybelievedasanigneousfluid，orasomethingcalledcaloric。
\"Fromwhencecamethisheatwhichwascontinuallygivenoffinthismanner，intheforegoingexperiments？\"
asksRumford。\"Wasitfurnishedbythesmallparticlesofmetaldetachedfromthelargersolidmassesontheirbeingrubbedtogether？This，aswehavealreadyseen，couldnotpossiblyhavebeenthecase。
\"Wasitfurnishedbytheair？Thiscouldnothavebeenthecase；for，inthreeoftheexperiments，themachinerybeingkeptimmersedinwater，theaccessoftheairoftheatmospherewascompletelyprevented。
\"Wasitfurnishedbythewaterwhichsurroundedthemachinery？Thatthiscouldnothavebeenthecaseisevident:first，becausethiswaterwascontinuallyRECEIVINGheatfromthemachinery，andcouldnot，atthesametime，beGIVINGTOandRECEIVINGHEATFROMthesamebody；and，secondly，becausetherewasnochemicaldecompositionofanypartofthiswater。Hadanysuchdecompositiontakenplacewhich，indeed，couldnotreasonablyhavebeenexpected，oneofitscomponentelasticfluidsmostprobablyhydrogenmust，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。HisviewsregardinglightincludingtheoriginalpapersfromthePhilosophicalTransactionsoftheRoyalSocietywereagaingivenpublicityinfullinhiscelebratedvolumeonnaturalphilosophy，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。TheinitialfactingalvanismwasthediscoveryofLuigiGalvani1737-1798，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。