Asyetnoonehaddiscoveredthatelectricitycouldbestored，orgeneratedinanywayotherthanbysomefrictiondevice。Butverysoontwoexperimenters，DeanvonKleist，ofCamin，Pomerania，andPietervanMusschenbroek，thefamousteacherofLeyden，apparentlyindependently，madethediscoveryofwhathasbeenknowneversinceastheLeydenjar。AndalthoughMusschenbroekissometimescreditedwithbeingthediscoverer，therecanbenodoubtthatVonKleist’sdiscoveryantedatedhisbyafewmonthsatleast。
VonKleistfoundthatbyadevicemadeofanarrow-neckedbottlecontainingalcoholormercury，intowhichanironnailwasinserted，bewasabletoretainthechargeofelectricity，afterelectrifyingthisapparatuswiththefrictionalmachine。Hemadealsoasimilardevice，morecloselyresemblingthemodernLeydenjar，fromathermometertubepartlyfilledwithwaterandawiretippedwithaballoflead。Withthesedeviceshefoundthathecouldretainthechargeofelectricityforseveralhours，andcouldproducetheusualelectricalmanifestations，eventoignitingspirits，quiteaswellaswiththefrictionalmachine。
TheseexperimentswerefirstmadeinOctober，1745，andafteramonthoffurtherexperimenting，VonKleistsentthefollowingaccountofthemtoseveraloftheleadingscientists，amongothers，Dr。Lieberkuhn，inBerlin，andDr。Kruger，ofHalle。
\"Whenanail，orapieceofthickbrasswire，isputintoasmallapothecary’sphialandelectrified，remarkableeffectsfollow；
butthephialmustbeverydry，orwarm。IcommonlyrubitoverbeforehandwithafingeronwhichIputsomepoundedchalk。Ifalittlemercuryorafewdropsofspiritofwinebeputintoit，theexperimentsucceedsbetter。Assoonasthisphialandnailareremovedfromtheelectrifying-glass，ortheprimeconductor，towhichithasbeenexposed，istakenaway，itthrowsoutapencilofflamesolongthat，withthisburningmachineinmyhand，Ihavetakenabovesixtystepsinwalkingaboutmyroom。
Whenitiselectrifiedstrongly，Icantakeitintoanotherroomandtherefirespiritsofwinewithit。IfwhileitiselectrifyingIputmyfinger，orapieceofgoldwhichIholdinmyhand，tothenail，Ireceiveashockwhichstunsmyarmsandshoulders。
\"Atintube，oraman，placeduponelectrics，iselectrifiedmuchstrongerbythismeansthaninthecommonway。WhenIpresentthisphialandnailtoatintube，whichIhave，fifteenfeetlong，nothingbutexperiencecanmakeapersonbelievehowstronglyitiselectrified。Iampersuaded，\"headds，\"thatinthismannerMr。Bosewouldnothavetakenasecondelectricalkiss。Twothinglasseshavebeenbrokenbytheshockofit。Itappearstomeveryextraordinary，thatwhenthisphialandnailareincontactwitheitherconductingornon-conductingmatter，thestrongshockdoesnotfollow。Ihavecementedittowood，metal，glass，sealing-wax，etc。，whenIhaveelectrifiedwithoutanygreateffect。Thehumanbody，therefore，mustcontributesomethingtoit。ThisopinionisconfirmedbymyobservingthatunlessIholdthephialinmyhandIcannotfirespiritsofwinewithit。\"[2]
ButitseemsthatnoneofthemenwhosawthisaccountwereabletorepeattheexperimentandproducetheeffectsclaimedbyVonKleist，andprobablyforthisreasonthediscoveryoftheobscurePomeranianwasforatimelostsightof。
Musschenbroek’sdiscoverywasmadewithinashorttimeafterVonKleist’s——infact，onlyamatterofabouttwomonthslater。Butthedifferenceinthereputationsofthetwodiscoverersinsuredaverydifferentreceptionfortheirdiscoveries。MusschenbroekwasoneoftheforemostteachersofEurope，andsowidelyknownthatthegreatuniversitiesviedwitheachother，andkingswerebidding，forhisservices。Naturally，anydiscoverymadebysuchafamouspersonwouldsoonbeheraldedfromoneendofEuropetotheother。AndsowhenthisprofessorofLeydenmadehisdiscovery，theapparatuscametobecalledthe\"Leydenjar，\"forwantofabettername。TherecanbelittledoubtthatMusschenbroekmadehisdiscoveryentirelyindependentlyofanyknowledgeofVonKleist’s，or，forthatmatter，withouteverhavingheardofthePomeranian，andhisactionsinthematterareentirelyhonorable。
Hisdiscoverywastheresultofanaccident。Whileexperimentingtodeterminethestrengthofelectricityhesuspendedagun-barrel，whichhechargedwithelectricityfromarevolvingglassglobe。Fromtheendofthegun-barreloppositetheglobewasabrasswire，whichextendedintoaglassjarpartlyfilledwithwater。Musschenbroekheldinonehandthisjar，whilewiththeotherheattemptedtodrawsparksfromthebarrel。Suddenlyhereceivedashockinthehandholdingthejar，that\"shookhimlikeastrokeoflightning，\"andforamomentmadehimbelievethat\"hewasdonefor。\"Continuinghisexperiments，nevertheless，hefoundthatifthejarwereplacedonapieceofmetalonthetable，ashockwouldbereceivedbytouchingthispieceofmetalwithonehandandtouchingthewirewiththeother——thatis，apathwasmadefortheelectricaldischargethroughthebody。ThiswaspracticallythesameexperimentasmadebyVonKleistwithhisbottleandnail，butcarriedonestepfarther，asitshowedthatthe\"jar\"neednotnecessarilybeheldinthehand，asbelievedbyVonKleist。Furtherexperiments，continuedbymanyphilosophersatthetime，revealedwhatVonKleisthadalreadypointedout，thattheelectrifiedjarremainedchargedforsometime。
SoonafterthisDanielGralath，wishingtoobtainstrongerdischargesthancouldbehadfromasingleLeydenjar，conceivedtheideaofcombiningseveraljars，thusforthefirsttimegroupingthegeneratorsina\"battery\"whichproducedadischargestrongenoughtokillbirdsandsmallanimals。Healsoattemptedtomeasurethestrengthofthedischarges，butsoongaveitupindespair，andthesolutionofthisproblemwasleftforlatenineteenth-centuryscientists。
TheadventoftheLeydenjar，whichmadeitpossibletoproducestrongelectricaldischargesfromasmallandcomparativelysimpledevice，wasfollowedbymorespectaculardemonstrationsofvariouskindsalloverEurope。Theseexhibitionsarousedtheinterestofthekingsandnoblemen，sothatelectricitynolongerremaineda\"playthingofthephilosophers\"alone，butofkingsaswell。Afavoritedemonstrationwasthatofsendingtheelectricaldischargethroughlonglinesofsoldierslinkedtogetherbypiecesofwire，thedischargecausingthemto\"springintotheairsimultaneously\"inamostastonishingmanner。AcertainmonkinParispreparedamostelaborateseriesofdemonstrationsfortheamusementoftheking，amongotherthingslinkingtogetheranentireregimentofninehundredmen，causingthemtoperformsimultaneousspringsandcontortionsinamannermostamusingtotheroyalguests。Butnotalltheexperimentsbeingmadewereofapurelyspectacularcharacter，althoughmostofthemaccomplishedlittleexceptinanegativeway。ThefamousAbbeNollet，forexample，combinedusefulexperimentswithspectaculardemonstrations，thuskeepinguppopularinterestwhileaidingthecauseofscientificelectricity。
WILLIAMWATSON
Naturally，thenewdiscoveriesmadenecessaryanewnomenclature，newwordsandelectricaltermsbeingconstantlyemployedbythevariouswritersofthatday。AmongthesewriterswastheEnglishscientistWilliamWatson，whowasnotonlyamostprolificwriterbutatirelessinvestigator。Manyofthewordscoinedbyhimarenowobsolete，butoneatleast，\"circuit，\"stillremainsinuse。
In1746，aFrenchscientist，LouisGuillaumeleMonnier，badmadeacircuitincludingmetalandwaterbylayingachainhalf-wayaroundtheedgeofapond，amanateitherendholdingit。Oneofthesemendippedhisfreehandinthewater，theotherpresentingaLeydenjartoarodsuspendedonacorkfloatonthewater，bothmenreceivingashocksimultaneously。Watson，ayearlater，attemptedthesameexperimentonalargerscale。HelaidawireabouttwelvehundredfeetlongacrossWestminsterBridgeovertheThames，bringingtheendstothewater’sedgeontheoppositebanks，amanatoneendholdingthewireandtouchingthewater。
AsecondmanontheoppositesideheldthewireandaLeydenjar；
andathirdtouchedthejarwithonehand，whilewiththeotherhegraspedawirethatextendedintotheriver。Inthiswaytheynotonlyreceivedtheshock，butfiredalcoholasreadilyacrossthestreamascouldbedoneinthelaboratory。InthisexperimentWatsondiscoveredthesuperiorityofwireoverchainasaconductor，rightlyascribingthissuperioritytothecontinuityofthemetal。
Watsoncontinuedmakingsimilarexperimentsoverlongerwatercourses，someofthemaslongaseightthousandfeet，andwhileengagedinmakingoneofthesehemadethediscoverysoessentialtolaterinventions，thattheearthcouldbeusedaspartofthecircuitinthesamemannerasbodiesofwater。
Lengtheninghiswireshecontinuedhisexperimentsuntilacircuitoffourmileswasmade，andstilltheelectricityseemedtotraversethecourseinstantaneously，andwithapparentlyundiminishedforce，iftheinsulationwasperfect。
BENJAMINFRANKLIN
Watson’swritingsnowcarriedthefieldofactivediscoveryacrosstheAtlantic，andforthefirsttimeanAmericanscientistappeared——ascientistwhonotonlyrivalled，butexcelled，hisEuropeancontemporaries。BenjaminFranklin，ofPhiladelphia，comingintopossessionofsomeofWatson’sbooks，becamesointerestedintheexperimentsdescribedinthemthathebeganatonceexperimentingwithelectricity。InWatson’sbookweregivendirectionsformakingvariousexperiments，andtheseassistedFranklininrepeatingtheoldexperiments，andeventuallyaddingnewones。AssociatedwithFranklin，andequallyinterestedandenthusiastic，ifnotequallysuccessfulinmakingdiscoveries，werethreeothermen，ThomasHopkinson，PhilipSing，andEbenezerKinnersley。Thesemenworkedtogetherconstantly，althoughitappearstohavebeenFranklinwhomadeindependentlytheimportantdiscoveries，andformulatedthefamousFrankliniantheory。
Workingsteadily，andkeepingconstantlyintouchwiththeprogressoftheEuropeaninvestigators，Franklinsoonmadesomeexperimentswhichhethoughtdemonstratedsomehithertounknownphasesofelectricalmanifestation。Thiswastheeffectofpointedbodies\"inDRAWINGOFFandTHROWINGOFFtheelectricalfire。\"Inhisdescriptionofthisphenomenon，Franklinwrites:
\"Placeanironshotofthreeorfourinchesdiameteronthemouthofaclean，dry，glassbottle。Byafinesilkenthreadfromtheceilingrightoverthemouthofthebottle，suspendasmallcorkball，aboutthebignessofamarble；thethreadofsuchalengththatthecorkballmayrestagainstthesideoftheshot。
Electrifytheshot，andtheballwillberepelledtothedistanceoffourorfiveinches，moreorless，accordingtothequantityofelectricity。Wheninthisstate，ifyoupresenttotheshotthepointofalong，slendershaft-bodkin，atsixoreightinchesdistance，therepellencyisinstantlydestroyed，andthecorkfliestotheshot。Abluntbodymustbebroughtwithinaninch，anddrawaspark，toproducethesameeffect。
\"ToprovethattheelectricalfireisDRAWNOFFbythepoint，ifyoutakethebladeofthebodkinoutofthewoodenhandleandfixitinastickofsealing-wax，andthenpresentitatthedistanceaforesaid，orifyoubringitverynear，nosucheffectfollows；
butslidingonefingeralongthewaxtillyoutouchtheblade，andtheballfliestotheshotimmediately。Ifyoupresentthepointinthedarkyouwillsee，sometimesatafootdistance，andmore，alightgatheruponitlikethatofafire-flyorglow-worm；thelesssharpthepoint，theneareryoumustbringittoobservethelight；andatwhateverdistanceyouseethelight，youmaydrawofftheelectricalfireanddestroytherepellency。
Ifacorkballsosuspendedberepelledbythetube，andapointbepresentedquicktoit，thoughataconsiderabledistance，’tissurprisingtoseehowsuddenlyitfliesbacktothetube。Pointsofwoodwilldoaswellasthoseofiron，providedthewoodisnotdry；forperfectlydrywoodwillnomoreconductelectricitythansealing-wax。
\"ToshowthatpointswillTHROWOFFaswellasDRAWOFFtheelectricalfire，layalong，sharpneedleupontheshot，andyoucannotelectrifytheshotsoastomakeitrepelthecorkball。
Orfixaneedletotheendofasuspendedgun-barrelorironrod，soastopointbeyonditlikealittlebayonet，andwhileitremainsthere，thegun-barrelorrodcannot，byapplyingthetubetotheotherend，beelectrifiedsoastogiveaspark，thefirecontinuallyrunningoutsilentlyatthepoint。Inthedarkyoumayseeitmakethesameappearanceasitdoesinthecasebeforementioned。\"[3]
VonGuericke，Hauksbee，andGrayhadnoticedthatpointedbodiesattractedelectricityinapeculiarmanner，butthisdemonstrationofthe\"drawingoff\"of\"electricalfire\"wasoriginalwithFranklin。Originalalsowasthetheorythathenowsuggested，whichhadatleastthemeritofbeingthinkableevenbynon-philosophicalminds。Itassumesthatelectricityislikeafluid，thatwillflowalongconductorsandaccumulateinproperreceptacles，verymuchasordinaryfluidsdo。Thisconceptionisprobablyentirelyincorrect，butneverthelessitislikelytoremainapopularone，atleastoutsideofscientificcircles，oruntilsomethingequallytangibleissubstituted。
FRANKLIN’STHEORYOFELECTRICITY
AccordingtoFranklin’stheory，electricityexistsinallbodiesasa\"commonstock，\"andtendstoseekandremaininastateofequilibrium，justasfluidsnaturallytendtoseekalevel。Butitmay，nevertheless，beraisedorlowered，andthisequilibriumbethusdisturbed。IfabodyhasmoreelectricitythanitsnormalamountitissaidtobePOSITIVELYelectrified；butifithasless，itisNEGATIVELYelectrified。Anover-electrifiedor\"plus\"
bodytendstogiveitssurplusstocktoabodycontainingthenormalamount；whilethe\"minus\"orunder-electrifiedbodywilldrawelectricityfromonecontainingthenormalamount。
Workingalonglinessuggestedbythistheory，Franklinattemptedtoshowthatelectricityisnotcreatedbyfriction，butsimplycollectedfromitsdiversifiedstate，therubbedglassglobeattractingacertainquantityof\"electricalfire，\"buteverreadytogiveituptoanybodythathasless。HeexplainedthechargedLeydenjarbyshowingthattheinnercoatingoftin-foilreceivedmorethantheordinaryquantityofelectricity，andinconsequenceisPOSITIVELYelectrified，whiletheoutercoating，havingtheordinaryquantityofelectricitydiminished，iselectrifiedNEGATIVELY。
ThesestudiesoftheLeydenjar，andthestudiesofpiecesofglasscoatedwithsheetmetal，ledFranklintoinventhisbattery，constructedofelevenlargeglassplatescoatedwithsheetsoflead。Withthismachine，afterovercomingsomedefects，hewasabletoproduceelectricalmanifestationsofgreatforce——aforcethat\"knewnobounds，\"ashedeclared\"exceptinthematterofexpenseandoflabor\"，andwhichcouldbemadetoexceed\"thegreatestknoweffectsofcommonlightning。\"
ThisreferencetolightningwouldseemtoshowFranklin’sbelief，evenatthattime，thatlightningiselectricity。Manyeminentobservers，suchasHauksbee，Wall，Gray，andNollet，hadnoticedtheresemblancebetweenelectricsparksandlightning，butnoneofthesehadmorethansurmisedthatthetwomightbeidentical。
In1746，thesurgeon，JohnFreke，alsoassertedhisbeliefinthisidentity。Winkler，shortlyafterthistime，expressedthesamebelief，and，assumingthattheywerethesame，declaredthat\"thereisnoproofthattheyareofdifferentnatures\"；andstillhedidnotprovethattheywerethesamenature。
FRANKLININVENTSTHELIGHTNING-ROD
EvenbeforeFranklinprovedconclusivelythenatureoflightning，hisexperimentsindrawingofftheelectricchargewithpointsledtosomepracticalsuggestionswhichresultedintheinventionofthelightning-rod。IntheletterofJuly，1750，whichhewroteonthesubject，hegavecarefulinstructionsastothewayinwhichtheserodsmightbeconstructed。InpartFranklinwrote:
\"Maynottheknowledgeofthispowerofpointsbeofusetomankindinpreservinghouses，churches，ships，etc。，fromthestrokeoflightningbydirectingustofixonthehighestpartsoftheedificesuprightrodsofironmadesharpasaneedle，andgilttopreventrusting，andfromthefootoftheserodsawiredowntheoutsideofthebuildingintothegrounds，ordownroundoneoftheshroudsofashipanddownhersidetillitreachesthewater？Wouldnotthesepointedrodsprobablydrawtheelectricalfiresilentlyoutofacloudbeforeitcamenighenoughtostrike，andtherebysecureusfromthatmostsuddenandterriblemischief？
\"Todeterminethisquestion，whetherthecloudsthatcontainthelightningareelectrifiedornot，Iproposeanexperimenttobetriedwhereitmaybedoneconveniently。Onthetopofsomehightowerorsteeple，placeakindofsentry-box，bigenoughtocontainamanandanelectricalstand。Fromthemiddleofthestandletanironrodriseandpass，bendingoutofthedoor，andthenuprighttwentyorthirtyfeet，pointedverysharpattheend。Iftheelectricalstandbekeptcleananddry，amanstandingonitwhensuchcloudsarepassinglowmightbeelectrifiedandaffordsparks，theroddrawingfiretohimfromacloud。IfanydangertothemanbeapprehendedthoughIthinktherewouldbenone，lethimstandonthefloorofhisboxandnowandthenbringneartotherodtheloopofawirethathasoneendfastenedtotheleads，heholdingitbyawaxhandle；sothesparks，iftherodiselectrified，willstrikefromtherodtothewireandnoteffecthim。\"[4]
Notsatisfiedwithalltheevidencethathehadcollectedpointingtotheidentityoflightningandelectricity，headdsonemorestrikingandverysuggestivepieceofevidence。
Lightningwasknownsometimestostrikepersonsblindwithoutkillingthem。Inexperimentingonpigeonsandpulletswithhiselectricalmachine，Franklinfoundthatafowl，whennotkilledoutright，wassometimesrenderedblind。ThereportoftheseexperimentswereincorporatedinthisfamousletterofthePhiladelphiaphilosopher。
TheattitudeoftheRoyalSocietytowardsthisclearlystatedletter，withitsusefulsuggestions，mustalwaysremainasablotontherecordofthisusuallyveryreceptiveandliberal-mindedbody。Farfrompublishingitorreceivingitatall，theyderidedthewholematterastoovisionaryfordiscussionbythesociety。
Howwasitpossiblethatanygreatscientificdiscoverycouldbemadebyaself-educatedcolonialnewspapereditor，whoknewnothingofEuropeanscienceexceptbyhearsay，whenallthegreatscientificmindsofEuropehadfailedtomakethediscovery？Howindeed！AndyetitwouldseemthatifanyoftheinfluentialmembersofthelearnedsocietyhadtakenthetroubletoreadoverFranklin’sclearlystatedletter，theycouldhardlyhavefailedtoseethathissuggestionswereworthyofconsideration。Butatallevents，whethertheydidordidnotmatterslittle。Thefactremainsthattheyrefusedtoconsiderthepaperseriouslyatthetime；andlateron，whenitstruevaluebecameknown，wereobligedtoacknowledgetheirerrorbyatardyreportonthealreadywell-knowndocument。
ButifEnglishscientistswerecoldintheirreceptionofFranklin’stheoryandsuggestions，theFrenchscientistswerenot。Buffon，perceivingatoncetheimportanceofsomeofFranklin’sexperiments，tookstepstohavethefamouslettertranslatedintoFrench，andsoonnotonlythesavants，butmembersofthecourtandthekinghimselfwereintenselyinterested。Twoscientists，DeLorandD’Alibard，undertooktotestthetruthofFranklin’ssuggestionsastopointedrods\"drawingofflightning。\"InagardennearParis，thelattererectedapointedironrodfiftyfeethighandaninchindiameter。Asnothunder-cloudsappearedforseveraldays，aguardwasstationed，armedwithaninsulatedbrasswire，whowasdirectedtotesttheironrodswithitincaseastormcameonduringD’Alibard’sabsence。Thestormdidcomeon，andtheguard，notwaitingforhisemployer’sarrival，seizedthewireandtouchedtherod。Instantlytherewasareport。Sparksflewandtheguardreceivedsuchashockthathethoughthistimehadcome。Believingfromhisoutcrythathewasmortallyhurt，hisfriendsrushedforaspiritualadviser，whocamerunningthroughrainandhailtoadministerthelastrites；butwhenhefoundtheguardstillaliveanduninjured，heturnedhisvisittoaccountbytestingtherodhimselfseveraltimes，andlaterwritingareportofhisexperimentstoM。d’Alibard。ThisscientistatoncereportedtheaffairtotheFrenchAcademy，remarkingthat\"Franklin’sideawasnolongeraconjecture，butareality。\"
FRANKLINPROVESTHATLIGHTNINGISELECTRICITY
Europe，hithertosomewhatscepticalofFranklin’sviews，wasbythistimeconvincedoftheidentityoflightningandelectricity。
ItwasnowFranklin’sturntobesceptical。Tohimthefactthatarod，onehundredfeethigh，becameelectrifiedduringastormdidnotnecessarilyprovethatthestorm-cloudswereelectrified。
Arodofthatlengthwasnotreallyprojectedintothecloud，forevenaverylowthunder-cloudwasmorethanahundredfeetabovetheground。Irrefutableproofcouldonlybehad，ashesawit，by\"extracting\"thelightningwithsomethingactuallysentupintothestorm-cloud；andtoaccomplishthisFranklinmadehissilkkite，withwhichhefinallydemonstratedtohisownandtheworld’ssatisfactionthathistheorywascorrect。
Takinghiskiteoutintoanopencommonontheapproachofathunder-storm，heflewitwellupintothethreateningclouds，andthen，touching，thesuspendedkeywithhisknuckle，receivedtheelectricspark；andalittlelaterhechargedaLeydenjarfromtheelectricitydrawnfromthecloudswithhiskite。
Inabriefbutdirectletter，hesentanaccountofhiskiteandhisexperimenttoEngland:
\"Makeasmallcrossoftwolightstripsofcedar，\"hewrote，\"thearmssolongastoreachtothefourcornersofalarge，thin，silkhandkerchiefwhenextended；tiethecornersofthehandkerchieftotheextremitiesofthecrosssoyouhavethebodyofakite；whichbeingproperlyaccommodatedwithatail，loop，andstring，willriseintheairlikethosemadeofpaper；butthisbeingofsilkisfittertobearthewindandwetofathunder-gustwithouttearing。Tothetopoftheuprightstickofthecrossistobefixedaverysharp-pointedwire，risingafootormoreabovethewood。Totheendofthetwine，nextthehand，istobetiedasilkribbon；wherethesilkandtwinejoinakeymaybefastened。Thiskiteistoberaisedwhenathunder-gustappearstobecomingon，andthepersonwhoholdsthestringmuststandwithinadoororwindoworundersomecover，sothatthesilkribbonmaynotbewet；andcaremustbetakenthatthetwinedoesnottouchtheframeofthedoororwindow。Assoonasanyofthethunder-cloudscomeoverthekite，thepointedwirewilldrawtheelectricfirefromthem，andthekite，withallthetwine，willbeelectrifiedandtheloosefilamentswillstandouteverywhereandbeattractedbytheapproachingfinger，andwhentherainhaswetthekiteandtwinesothatitcanconducttheelectricfirefreely，youwillfinditstreamoutplentifullyfromthekeyontheapproachofyourknuckle，andwiththiskeythephialmaybecharged；andfromelectricfirethusobtainedspiritsmaybekindledandallotherelectricexperimentsperformedwhichareusuallydonebythehelpofarubbedglassglobeortube，andtherebythesamenessoftheelectricmatterwiththatoflightningcompletelydemonstrated。\"[5]
InexperimentingwithlightningandFranklin’spointedrodsinEurope，severalscientistsreceivedsevereshocks，inonecasewithafatalresult。ProfessorRichman，ofSt。Petersburg，whileexperimentingduringathunder-storm，withanironrodwhichhehaderectedonhishouse，receivedashockthatkilledhiminstantly。
About1733，aswehaveseen，Dufayhaddemonstratedthatthereweretwoapparentlydifferentkindsofelectricity；onecalledVITREOUSbecauseproducedbyrubbingglass，andtheotherRESINOUSbecauseproducedbyrubbedresinousbodies。Dufaysupposedthatthesetwoapparentlydifferentelectricitiescouldonlybeproducedbytheirrespectivesubstances；buttwentyyearslater，JohnCanton1715-1772，anEnglishman，demonstratedthatundercertainconditionsbothmightbeproducedbyrubbingthesamesubstance。Canton’sexperiment，madeuponaglasstubewitharoughenedsurface，provedthatifthesurfaceofthetubewererubbedwithoiledsilk，vitreousorpositiveelectricitywasproduced，butifrubbedwithflannel，resinouselectricitywasproduced。Hediscoveredstillfurtherthatbothkindscouldbeexcitedonthesametubesimultaneouslywithasinglerubber。Todemonstratethisheusedatube，one-halfofwhichhadaroughenedtheotheraglazedsurface。Withasinglestrokeoftherubberhewasabletoexcitebothkindsofelectricityonthistube。Hefoundalsothatcertainsubstances，suchasglassandamber，wereelectrifiedpositivelywhentakenoutofmercury，andthisledtohisimportantdiscoverythatanamalgamofmercuryandtin，whenusedonthesurfaceoftherubber，wasveryeffectiveinexcitingglass。
XV。NATURALHISTORYTOTHETIMEOFLINNAeUS
ModernsystematicbotanyandzoologyareusuallyheldtohavetheirbeginningswithLinnaeus。ButtherewerecertainprecursorsofthefamousSwedishnaturalist，someofthemantedatinghimbymorethanacentury，whoseworkmustnotbealtogetherignored——suchmenasKonradGesner1516-1565，AndreasCaesalpinus1579-1603，FranciscoRedi1618-1676，GiovanniAlfonsoBorelli1608-1679，JohnRay1628-1705，RobertHooke1635-1703，JohnSwammerdam1637-1680，MarcelloMalpighi1628-1694，NehemiahGrew1628-1711，JosephTournefort1656-1708，RudolfJacobCamerarius1665-1721，andStephenHales1677-1761。Thelastnamedofthesewas，tobesure，acontemporaryofLinnaeushimself，butGesnerandCaesalpinusbelong，itwillbeobserved，tosoremoteanepochasthatofCopernicus。
ReferencehasbeenmadeinanearlierchaptertothemicroscopicinvestigationsofMarcelloMalpighi，who，asthererelated，wasthefirstobserverwhoactuallysawbloodcorpusclespassthroughthecapillaries。Anotherfeatofthisearliestofgreatmicroscopistswastodissectmusculartissue，andthusbecomethefatherofmicroscopicanatomy。ButMalpighididnotconfinehisobservationstoanimaltissues。Hedissectedplantsaswell，andheisalmostasfullyentitledtobecalledthefatherofvegetableanatomy，thoughherehishonorsaresharedbytheEnglishmanGrew。In1681，whileMalpighi’swork，Anatomiaplantarum，wasonitswaytotheRoyalSocietyforpublication，Grew’sAnatomyofVegetableswasinthehandsofthepublishers，makingitsappearanceafewmonthsearlierthantheworkofthegreatItalian。Grew’sbookwasepoch-markinginpointingoutthesex-differencesinplants。
RobertHookedevelopedthemicroscope，andtookthefirststepstowardsstudyingvegetableanatomy，publishingin1667，amongotherresults，thediscoveryofthecellularstructureofcork。
Hookeappliedthename\"cell\"forthefirsttimeinthisconnection。ThesediscoveriesofHooke，Malpighi，andGrew，andthediscoveryofthecirculationofthebloodbyWilliamHarveyshortlybefore，hadcalledattentiontothesimilarityofanimalandvegetablestructures。Halesmadeaseriesofinvestigationsuponanimalstodeterminetheforceofthebloodpressure；andsimilarlyhemadenumerousstaticalexperimentstodeterminethepressureoftheflowofsapinvegetables。HisVegetableStatics，publishedin1727，wasthefirstimportantworkonthesubjectofvegetablephysiology，andforthisreasonHaleshasbeencalledthefatherofthisbranchofscience。
Inbotany，aswellasinzoology，theclassificationsofLinnaeusofcoursesupplantedallprecedingclassifications，fortheobviousreasonthattheyweremuchmoresatisfactory；buthisworkwasaculminationofmanysimilarandmoreorlesssatisfactoryattemptsofhispredecessors。Abouttheyear1670
Dr。RobertMorison1620-1683，ofAberdeen，publishedaclassificationofplants，hissystemtakingintoaccountthewoodyorherbaceousstructure，aswellastheflowersandfruit。
ThisclassificationwassupplantedtwelveyearslaterbytheclassificationofRay，whoarrangedallknownvegetablesintothirty-threeclasses，thebasisofthisclassificationbeingthefruit。AfewyearslaterRivinus，aprofessorofbotanyintheUniversityofLeipzig，madestillanotherclassification，determiningthedistinguishingcharacterchieflyfromtheflower，andCamerariusandTournefortalsomadeelaborateclassifications。OntheContinentTournefort’sclassificationwasthemostpopularuntilthetimeofLinnaeus，hissystematicarrangementincludingabouteightthousandspeciesofplants，arrangedchieflyaccordingtotheformofthecorolla。
Mostoftheseearlyworkersgaveattentiontobothvegetableandanimalkingdoms。Theywerecallednaturalists，andthefieldoftheirinvestigationswasspokenofas\"naturalhistory。\"Thespecializationofknowledgehadnotreachedthatlaterstageinwhichbotanist，zoologist，andphysiologistfelttheirlaborstobesharplydivided。Suchadivisionwasbecomingmoreandmorenecessaryasthefieldofknowledgeextended；butitdidnotbecomeimperativeuntillongafterthetimeofLinnaeus。Thatnaturalisthimself，asweshallsee，wasequallydistinguishedasbotanistandaszoologist。Hisgreattaskoforganizingknowledgewasappliedtotheentirerangeoflivingthings。
CarolusLinnaeuswasborninthetownofRashult，inSweden，onMay13，1707。Asachildheshowedgreataptitudeinlearningbotanicalnames，andrememberingfactsaboutvariousplantsastoldhimbyhisfather。Hiseagernessforknowledgedidnotextendtotheordinaryprimarystudies，however，and，asidefromthesingleexceptionofthestudyofphysiology，heprovedhimselfanindifferentpupil。Hisbackwardnesswasasoretrialtohisfather，whowasdesirousthathissonshouldentertheministry；butastheyoungLinnaeusshowednolikingforthatcalling，andashehadacquittedhimselfwellinhisstudyofphysiology，hisfatheratlastdecidedtoallowhimtotakeupthestudyofmedicine。Hereatlastwasafieldmoretothelikingoftheboy，whosoonviedwiththebestofhisfellow-studentsforfirsthonors。Meanwhilehekeptsteadilyatworkinhisstudyofnaturalhistory，acquiringconsiderableknowledgeofornithology，entomology，andbotany，andaddingcontinuallytohiscollectionofbotanicalspecimens。In1729hisbotanicalknowledgewasbroughttotheattentionofOlafRudbeck，professorofbotanyintheUniversityofUpsala，byashortpaperonthesexesofplantswhichLinnaeushadprepared。Rudbeckwassoimpressedbysomeoftheideasexpressedinthispaperthatheappointedtheauthorashisassistantthefollowingyear。
ThiswasthebeginningofLinnaes’scareerasabotanist。Theacademicgardenswerethusthrownopentohim，andhefoundtimeathisdisposalforpursuinghisstudiesbetweenlecturehoursandintheevenings。ItwasatthistimethathebeganthepreparationofhisworktheSystemanaturae，thefirstofhisgreatworks，containingacomprehensivesketchofthewholefieldofnaturalhistory。Whenthisworkwaspublished，theclearnessoftheviewsexpressedandthesystematicarrangementofthevariousclassificationsexcitedgreatastonishmentandadmiration，andplacedLinaeusatonceintheforemostrankofnaturalists。Thisworkwasfollowedshortlybyotherpublications，mostlyonbotanicalsubjects，inwhich，amongotherthings，heworkedoutindetailhisfamous\"system。\"
Thissystemisfoundedonthesexesofplants，andisusuallyreferredtoasan\"artificialmethod\"ofclassificationbecauseittakesintoaccountonlyafewmarkedcharactersofplants，withoutunitingthembymoregeneralnaturalaffinities。Atthepresenttimeitisconsideredonlyasastepping-stonetothe\"natural\"system；butatthetimeofitspromulgationitwasepoch-markinginitsdirectnessandsimplicity，andthereforesuperiority，overanyexistingsystems。
OneofthegreatreformseffectedbyLinnaeuswasinthematterofscientificterminology。Technicaltermsareabsolutelynecessarytoscientificprogress，andparticularlysoinbotany，whereobscurity，ambiguity，orprolixityindescriptionsarefatallymisleading。Linnaeus’sworkcontainssomethinglikeathousandterms，whosemeaningsandusesarecarefullyexplained。
Suchanarrayseemsatfirstglancearbitraryandunnecessary，butthefactthatithasremainedinuseforsomethingliketwocenturiesisindisputableevidenceofitspracticality。Thedescriptivelanguageofbotany，asemployedbyLinnaeus，stillstandsasamodelforallothersubjects。
Closelyalliedtobotanicalterminologyisthesubjectofbotanicalnomenclature。TheoldmethodofusinganumberofLatinwordstodescribeeachdifferentplantisobviouslytoocumbersome，andseveralattemptshadbeenmadepriortothetimeofLinnaeustosubstitutesimplermethods。Linnaeushimselfmadeseveralunsatisfactoryattemptsbeforehefinallyhituponhissystemof\"trivialnames，\"whichwasdevelopedinhisSpeciesplantarum，andwhich，withsome，minoralterations，remainsinusetothisday。Theessenceofthesystemistheintroductionofbinomialnomenclature——thatistosay，theuseoftwonamesandnomoretodesignateanysinglespeciesofanimalorplant。Theprincipleisquitethesameasthataccordingtowhichinmodernsocietyamanhastwonames，letussay，JohnDoe，theonedesignatinghisfamily，theotherbeingindividual。Similarlyeachspeciesofanimalorplant，accordingtotheLinnaeeansystem，receivedaspecificor\"trivial\"name；whilevariousspecies，associatedaccordingtotheirseemingnaturalaffinitiesintogroupscalledgenera，weregiventhesamegenericname。ThusthegenericnamegivenallmembersofthecattribebeingFelis，thenameFelisleodesignatesthelion；Felispardus，theleopard；Felisdomestica，thehousecat，andsoon。Thisseemsperfectlysimpleandnaturalnow，buttounderstandhowgreatareformthebinomialnomenclatureintroducedwehavebuttoconsulttheworkofLinnaeus’spredecessors。Asingleillustrationwillsuffice。Thereis，forexample，akindofgrass，inreferringtowhichthenaturalistanteriortoLinnaeus，ifhewouldbeabsolutelyunambiguous，wasobligedtousethefollowingdescriptiveformula:GramenXerampelino，Miliacea，praetenuisramosaquesparsapanicula，siveXerampelinocongener，arvense，aestivum；gramenminutissimosemine。LinnaeusgavetothisplantthenamePoabulbosa——anamethatsufficed，accordingtothenewsystem，todistinguishthisfromeveryotherspeciesofvegetable。Itdoesnotrequireanyspecialknowledgetoappreciatetheadvantageofsuchasimplification。
WhilevisitingParisin1738Linnaeusmetandbotanizedwiththetwobotanistswhose\"naturalmethod\"ofclassificationwaslatertosupplanthisown\"artificialsystem。\"ThesewereBernardandAntoineLaurentdeJussieu。Theeffortsofthesetwoscientistsweredirectedtowardsobtainingasystemwhichshouldaimatclearness，simplicity，andprecision，andatthesametimebegovernedbythenaturalaffinitiesofplants。Thenaturalsystem，asfinallypropoundedbythem，isbasedonthenumberofcotyledons，thestructureoftheseed，andtheinsertionofthestamens。Succeedingwritersonbotanyhavemadevariousmodificationsofthissystem，butneverthelessitstandsasthefoundation-stoneofmodernbotanicalclassification。
TheworkofJohannesHevelius——HalleyandHevelius——Halley’sobservationofthetransitofMercury，andhismethodofdeterminingtheparallaxoftheplanets——Halley’sobservationofmeteors——Hisinabilitytoexplainthesebodies——TheimportantworkofJamesBradley——Lacaille’smeasurementofthearcofthemeridian——Thedeterminationofthequestionastotheexactshapeoftheearth——D’Alembertandhisinfluenceuponscience——
Delambre’sHistoryofAstronomy——TheastronomicalworkofEuler。
CHAPTERII。THEPROGRESSOFMODERNASTRONOMY
TheworkofWilliamHerschel——HisdiscoveryofUranus——Hisdiscoverythatthestarsaresuns——Hisconceptionoftheuniverse——Hisdeductionthatgravitationhascausedthegroupingoftheheavenlybodies——Thenebula，hypothesis，——ImmanuelKant’sconceptionoftheformationoftheworld——DefectsinKant’sconception——Laplace’sfinalsolutionoftheproblem——Hisexplanationindetail——ChangeinthementalattitudeoftheworldsinceBruno——Asteroidsandsatellites——DiscoveriesofOlbers1——ThemathematicalcalculationsofAdamsandLeverrier——ThediscoveryoftheinnerringofSaturn——ClerkMaxwell’spaperonthestabilityofSaturn’srings——Helmholtz’sconceptionoftheactionoftidalfriction——ProfessorG。H。Darwin’sestimateoftheconsequencesoftidalaction——Cometsandmeteors——Bredichin’scometarytheory——Thefinalsolutionofthestructureofcomets——Newcomb’sestimateoftheamountofcometarydustsweptupdailybytheearth——Thefixedstars——JohnHerschel’sstudiesofdoublestars——Fraunhofer’sperfectionoftherefractingtelescope——Bessel’smeasurementoftheparallaxofastar，——Henderson’smeasurements——KirchhoffandBunsen’sperfectionofthespectroscope——Wonderfulrevelationsofthespectroscope——LordKelvin’sestimateofthetimethatwillberequiredfortheearthtobecomecompletelycooled——
AlvanClark’sdiscoveryofthecompanionstarofSirius——
Theadventofthephotographicfilminastronomy——Dr。
Huggins’sstudiesofnebulae——SirNormanLockyer’s\"cosmogonicguess，\"——Croll’spre-nebulartheory。
CHAPTERIII。THENEWSCIENCEOFPALEONTOLOGY
WilliamSmithandfossilshells——Hisdiscoverythatfossilrocksarearrangedinregularsystems——Smith’sinquiriestakenupbyCuvier——HisOssementsFossilescontainingthefirstdescriptionofhairyelephant——Hiscontentionthatfossilsrepresentextinctspeciesonly——Dr。Buckland’sstudiesofEnglishfossil-beds——CharlesLyellcombatscatastrophism，——Elaborationofhisideaswithreferencetotherotationofspecies——Theestablishmentofthedoctrineofuniformitarianism，——Darwin’sOriginofSpecies——Fossilman——Dr。Falconer’svisittothefossil-bedsinthevalleyoftheSomme——InvestigationsofPrestwichandSirJohnEvans——DiscoveryoftheNeanderthalskull，——Cuvier’srejectionofhumanfossils——Thefindingofprehistoriccarvingonivory——Thefossil-bedsofAmerica——ProfessorMarsh’spaperonthefossilhorsesinAmerica——TheWarrenmastodon，——TheJavafossil，PithecanthropusErectus。
CHAPTERIV。THEORIGINANDDEVELOPMENTOFMODERNGEOLOGY
JamesHuttonandthestudyoftherocks——Histheoryoftheearth——Hisbeliefinvolcaniccataclysmsinraisingandformingthecontinents——HisfamouspaperbeforetheRoyalSocietyofEdinburgh，1781——Hisconclusionsthatallstrataoftheearthhavetheiroriginatthebottomofthesea——Hisdeductionthatheatedandexpandedmattercausedtheelevationoflandabovethesea-level——Indifferenceatfirstshownthisremarkablepaper——NeptunistsversusPlutonists——
Scrope’sclassicalworkonvolcanoes——FinalacceptanceofHutton’sexplanationoftheoriginofgranites——Lyellanduniformitarianism——Observationsonthegradualelevationofthecoast-linesofSwedenandPatagonia——Observationsontheenormousamountoflanderosionconstantlytakingplace，——Agassizandtheglacialtheory——Perraudinthechamois-
hunter，andhisexplanationofperchedbowlders——DeCharpentier’sacceptanceofPerraudin’sexplanation——Agassiz’spaperonhisAlpinestudies——HisconclusionthattheAlpswereoncecoveredwithanice-sheet——Finalacceptanceoftheglacialtheory——Thegeologicalages——TheworkofMurchisonandSedgwick——FormationoftheAmericancontinents——Past，present，andfuture。
CHAPTERV。THENEWSCIENCEOFMETEOROLOGY
Biot’sinvestigationsofmeteors——TheobservationsofBrandesandBenzenbergonthevelocityoffallingstars——
ProfessorOlmstead’sobservationsonthemeteoricshowerof1833——
ConfirmationofChladni’shypothesisof1794——Theauroraborealis——Franklin’ssuggestionthatitisofelectricalorigin——Itscloseassociationwithterrestrialmagnetism——Evaporation，cloud-formation，anddew——Dalton’sdemonstrationthatwaterexistsintheairasanindependentgas——Hutton’stheoryofrain——LukeHoward’spaperonclouds——Observationsondew，byProfessorWilsonandMr。Six——Dr。Wells’sessayondew——Hisobservationsonseveralappearancesconnectedwithdew——Isothermsandoceancurrents——Humboldtandthe-scienceofcomparativeclimatology——Hisstudiesofoceancurrents——
Maury’stheorythatgravityisthecauseofoceancurrents——
Dr。CrollonClimateandTime——Cyclonesandanti-cyclones，——Dove’sstudiesinclimatology——ProfessorFerrel’smathematicallawofthedeflectionofwinds——Tyndall’sestimateoftheamountofheatgivenoffbytheliberationofapoundofvapor——Meteorologicalobservationsandweatherpredictions。
CHAPTERVI。MODERNTHEORIESOFHEATANDLIGHT
JosiahWedgwoodandtheclaypyrometer——CountRumfordandthevibratorytheoryofheat——Hisexperimentswithboringcannontodeterminethenatureofheat——Causingwatertoboilbythefrictionoftheborer——Hisfinaldeterminationthatheatisaformofmotion——ThomasYoungandthewavetheoryoflight——Hispaperonthetheoryoflightandcolors——Hisexpositionofthecolorsofthinplates——Ofthecolorsofthickplates，andofstriatedsurfaces，——AragoandFresnelchampionthewavetheory——oppositiontothetheorybyBiot——TheFrenchAcademy’stacitacceptanceofthecorrectnessofthetheorybyitsadmissionofFresnelasamember。
CHAPTERVII。THEMODERNDEVELOPMENTOFELECTRICITYANDMAGNETISM
Galvaniandthebeginningofmodernelectricity——Theconstructionofthevoltaicpile——Nicholson’sandCarlisle’sdiscoverythatthegalvaniccurrentdecomposeswater——DecompositionofvarioussubstancesbySirHumphryDavy——Hisconstructionofanarc-light——ThedeflectionofthemagneticneedlebyelectricitydemonstratedbyOersted——Effectofthisimportantdiscovery——Amperecreatesthescienceofelectro-dynamics——JosephHenry’sstudiesofelectromagnets——MichaelFaradaybeginshisstudiesofelectromagneticinduction——HisfamouspaperbeforetheRoyalSociety，in1831，inwhichhedemonstrateselectro-magneticinduction——HisexplanationofArago’srotatingdisk——Thesearchforasatisfactorymethodofstoringelectricity——
Roentgenrays，orX-rays。
CHAPTERVIII。THECONSERVATIONOFENERGY
Faradaynarrowlymissesthediscoveryofthedoctrineofconservation——Carnot’sbeliefthatadefinitequantityofworkcanbetransformedintoadefinitequantityofheat——TheworkofJamesPrescottJoule——InvestigationsbegunbyDr。
Mayer——Mayer’spaperof1842——Hisstatementofthelawoftheconservationofenergy——MayerandHelmholtz——Joule’spaperof1843——JouleorMayer——LordKelvinandthedissipationofenergy-Thefinalunification。
CHAPTERIX。THEETHERANDPONDERABLEMATTER
JamesClerk-Maxwell’sconceptionofether——ThomasYoungand\"Luminiferousether，\"——Young’sandFresnel’sconceptionoftransverseluminiferousundulations——Faraday’sexperimentspointingtotheexistenceofether——ProfessorLodge’ssuggestionoftwoethers——LordKelvin’scalculationoftheprobabledensityofether——Thevortextheoryofatoms——Helmholtz’scalculationsinvortexmotions——ProfessorTait’sapparatusforcreatingvortexringsintheair——TheultimateconstitutionofmatterasconceivedbyBoscovich——Davy’sspeculationsastothechangesthatoccurinthesubstanceofmatteratdifferenttemperatures——Clausius’sandMaxwell’sinvestigationsofthekinetictheoryofgases——LordKelvin’sestimateofthesizeofthemolecule——
Studiesofthepotentialenergyofmolecules——Actionofgasesatlowtemperatures。
APPENDIX
AHISTORYOFSCIENCE
BOOKIII
MODERNDEVELOPMENTOFTHEPHYSICAL
SCIENCES
Withthepresentbookweenterthefieldofthedistinctivelymodern。Thereisnoprecisedateatwhichwetakeupeachofthesuccessivestories，butthemainsweepofdevelopmenthastodoineachcasewiththenineteenthcentury。Weshallseeatoncethatthisisatimebothofrapidprogressandofgreatdifferentiation。Wehaveheardalmostnothinghithertoofsuchsciencesaspaleontology，geology，andmeteorology，eachofwhichnowdemandsfullattention。
Meantime，astronomyandwhattheworkersoftheelderdaycallednaturalphilosophybecomewonderfullydiversifiedandpresentnumerousphasesthatwouldhavebeenstartlingenoughtothestar-gazersandphilosophersoftheearlierepoch。
Thus，forexample，inthefieldofastronomy，Herschelisable，thankstohisperfectedtelescope，todiscoveranewplanetandthentoreachoutintothedepthsofspaceandgainsuchknowledgeofstarsandnebulaeashithertonoonehadmorethandreamedof。
Then，inrapidsequence，awholecoterieofhithertounsuspectedminorplanetsisdiscovered，stellardistancesaremeasured，somemembersofthestarrygalaxyaretimedintheirflight，thedirectionofmovementofthesolarsystemitselfisinvestigated，thespectroscoperevealsthechemicalcompositionevenofsunsthatareunthinkablydistant，andatangibletheoryisgraspedoftheuniversalcyclewhichincludesthebirthanddeathofworlds。
Similarlythenewstudiesoftheearth’ssurfacerevealsecretsofplanetaryformationhithertoquiteinscrutable。
Itbecomesknownthatthestrataoftheearth’ssurfacehavebeenformingthroughoutuntoldages，andthatsuccessivepopulationsdifferingutterlyfromoneanotherhavepeopledtheearthindifferentgeologicalepochs。Theentirepointofviewofthoughtfulmenbecomeschangedincontemplatingthehistoryoftheworldinwhichwelive——albeitthenewestthoughtharksbacktosomeextenttothosedayswhentheinspiredthinkersofearlyGreecedreamedoutthewonderfultheorieswithwhichourearlierchaptershavemadeourreadersfamiliar。
Intheregionofnaturalphilosophyprogressisnolesspronouncedandnolessstriking。Itsufficeshere，however，bywayofanticipation，simplytonamethegreatestgeneralizationofthecenturyinphysicalscience——thedoctrineoftheconservationofenergy。
I
THESUCCESSORSOFNEWTONINASTRONOMY
HEVELIUSANDHALLEY
STRANGELYenough，thedecadeimmediatelyfollowingNewtonwasoneofcomparativebarrennessinscientificprogress，theearlyyearsoftheeighteenthcenturynotbeingasproductiveofgreatastronomersasthelateryearsoftheseventeenth，or，forthatmatter，asthelateryearsoftheeighteenthcenturyitself。Severaloftheprominentastronomersofthelaterseventeenthcenturylivedonintotheopeningyearsofthefollowingcentury，however，andtheyoungergenerationsoondevelopedacoterieofastronomers，amongwhomEuler，Lagrange，Laplace，andHerschel，asweshallsee，weretoaccomplishgreatthingsinthisfieldbeforethecenturyclosed。
Oneofthegreatseventeenth-centuryastronomers，whodiedjustbeforethecloseofthecentury，wasJohannesHevelius1611-1687，ofDantzig，whoadvancedastronomybyhisaccuratedescriptionofthefaceandthespotsofthemoon。Butheisrememberedalsoforhavingretardedprogressbyhisinfluenceinrefusingtousetelescopicsightsinhisobservations，preferringuntilhisdeaththeplainsightslongbeforediscardedbymostotherastronomers。TheadvantagesofthesetelescopesightshavebeendiscussedunderthearticletreatingofRobertHooke，butnosuchadvantageswereeverrecognizedbyHevelius。
SogreatwasHevelius’sreputationasanastronomerthathisrefusaltorecognizetheadvantageofthetelescopesightscausedmanyastronomerstohesitatebeforeacceptingthemassuperiortotheplain；andeventhefamousHalley，ofwhomweshallspeakfurtherinamoment，wassufficientlyindoubtoverthemattertopaytheagedastronomeravisittotesthisskillinusingtheold-stylesights。Sidebyside，HeveliusandHalleymadetheirobservations，HeveliuswithhisoldinstrumentandHalleywiththenew。Theresultsshowedslightlyintheyoungerman’sfavor，butnotenoughtomakeitanentirelyconvincingdemonstration。
Theexplanationofthis，however，didnotlieinthelackofsuperiorityofthetelescopicinstrument，butratherinthemarvellousskilloftheagedHevelius，whosedexterityalmostcompensatedforthedefectofhisinstrument。Whathemighthaveaccomplishedcouldhehavebeeninducedtoadoptthetelescopecanonlybesurmised。
Halleyhimselfwasbynomeansatyroinmattersastronomicalatthattime。Astheonlysonofawealthysoap-boilerlivingnearLondon，hehadbeengivenaliberaleducation，andevenbeforeleavingcollegemadesuchnovelscientificobservationsasthatofthechangeinthevariationofthecompass。Atnineteenyearsofagehediscoveredanewmethodofdeterminingtheelementsoftheplanetaryorbitswhichwasadistinctimprovementovertheold。TheyearfollowinghesailedfortheIslandofSt，Helenatomakeobservationsoftheheavensinthesouthernhemisphere。
ItwaswhileinSt。HelenathatHalleymadehisfamousobservationofthetransitofMercuryoverthesun’sdisk，thisobservationbeingconnected，indirectlyatleast，withhisdiscoveryofamethodofdeterminingtheparallaxoftheplanets。Byparallaxismeanttheapparentchangeinthepositionofanobject，duereallytoachangeinthepositionoftheobserver。
Thus，ifweimaginetwoastronomersmakingobservationsofthesunfromoppositesidesoftheearthatthesametime，itisobviousthattotheseobserversthesunwillappeartobeattwodifferentpointsinthesky。Halftheanglemeasuringthisdifferencewouldbeknownasthesun’sparallax。Thiswoulddepend，then，uponthedistanceoftheearthfromthesunandthelengthoftheearth’sradius。
Sincetheactuallengthofthisradiushasbeendetermined，theparallaxofanyheavenlybodyenablestheastronomertodetermineitsexactdistance。
Theparallaxescanbedeterminedequallywell，however，iftwoobserversareseparatedbyexactlyknowndistances，severalhundredsorthousandsofmilesapart。
InthecaseofatransitofVenusacrossthesun’sdisk，forexample，anobserveratNewYorknotestheimageoftheplanetmovingacrossthesun’sdisk，andnotesalsotheexacttimeofthisobservation。InthesamemanneranobserveratLondonmakessimilarobservations。
KnowingthedistancebetweenNewYorkandLondon，andthedifferenttimeofthepassage，itisthuspossibletocalculatethedifferenceoftheparallaxesofthesunandaplanetcrossingitsdisk。Theideaofthusdeterminingtheparallaxoftheplanetsoriginated，oratleastwasdeveloped，byHalley，andfromthisphenomenonhethoughtitpossibletoconcludethedimensionsofalltheplanetaryorbits。Asweshallseefurtheron，hisviewswerefoundtobecorrectbylaterastronomers。
In1721HalleysucceededFlamsteedasastronomerroyalattheGreenwichObservatory。Althoughsixty-
fouryearsofageatthattimehisactivityinastronomycontinuedunabatedforanotherscoreofyears。AtGreenwichheundertooksometediousobservationsofthemoon，andduringthoseobservationswasfirsttodetecttheaccelerationofmeanmotion。Hewasunabletoexplainthis，however，anditremainedforLaplaceintheclosingyearsofthecenturytodoso，asweshallseelater。
Halley’sbook，theSynopsisAstronomiaeCometicae，isoneofthemostvaluableadditionstoastronomicalliteraturesincethetimeofKepler。Hewasfirsttoattemptthecalculationoftheorbitofacomet，havingrevivedtheancientopinionthatcometsbelongtothesolarsystem，movingineccentricorbitsroundthesun，andhiscalculationoftheorbitofthecometof1682ledhimtopredictcorrectlythereturnofthatcometin1758。Halley’sStudyofMeteors。
Likeotherastronomersofhistimebewasgreatlypuzzledoverthewell-knownphenomenaofshooting-
stars，ormeteors，makingmanyobservationshimself，andexaminingcarefullytheobservationsofotherastronomers。In1714hegavehisviewsastotheoriginandcompositionofthesemysteriousvisitorsintheearth’satmosphere。Asthissubjectwillbeagainreferredtoinalaterchapter，Halley’sviews，representingthemostadvancedviewsofhisage，areofinterest。
\"Thetheoryoftheairseemethatpresent，\"hesays，\"tobeperfectlywellunderstood，andthedifferingdensitiesthereofatallaltitudes；forsupposingthesameairtooccupyspacesreciprocallyproportionaltothequantityofthesuperiororincumbentair，Ihaveelsewhereprovedthatatfortymileshightheairisrarerthanatthesurfaceoftheearthatthreethousandtimes；andthattheutmostheightoftheatmosphere，whichreflectslightintheCrepusculum，isnotfullyforty-fivemiles，notwithstandingwhich’tisstillmanifestthatsomesortofvapors，andthoseinnosmallquantity，arisenearlytothatheight。AninstanceofthismaybegiveninthegreatlightthesocietyhadanaccountofvideTransact。Sep。，1676
fromDr。Wallis，whichwasseeninverydistantcountiesalmostoverallthesouthpartofEngland。Ofwhichthoughthedoctorcouldnotgetsoparticulararelationaswasrequisitetodeterminetheheightthereof，yetfromthedistantplacesitwasseenin，itcouldnotbutbeverymanymileshigh。
\"Solikewisethatmeteorwhichwasseenin1708，onthe31stofJuly，betweennineandteno’clockatnight，wasevidentlybetweenfortyandfiftymilesperpendicularlyhigh，andasnearasIcangather，overSherenessandthebuoyontheNore。ForitwasseenatLondonmovinghorizontallyfromeastbynorthtoeastbysouthatleastfiftydegreeshigh，andatRedgrove，inSuffolk，ontheYarmouthroad，abouttwentymilesfromtheeastcoastofEngland，andatleastfortymilestotheeastwardofLondon，itappearedalittletothewestwardofthesouth，supposesouthbywest，andwasseenaboutthirtydegreeshigh，slidingobliquelydownward。Iwasshowninbothplacesthesituationthereof，whichwasasdescribed，butcouldwishsomepersonskilledinastronomicalmattersbadseenit，thatwemightpronounceconcerningitsheightwithmorecertainty。Yet，asitis，wemaysecurelyconcludethatitwasnotmanymoremileswesterlythanRedgrove，which，asIsaidbefore，isaboutfortymilesmoreeasterlythanLondon。Supposeit，therefore，whereperpendicular，tohavebeenthirty-fivemileseastfromLondon，andbythealtitudeitappearedatinLondon——
viz。，fiftydegrees，itstangentwillbeforty-twomiles，fortheheightofthemeteorabovethesurfaceoftheearth；whichalsoisratheroftheleast，becausethealtitudeoftheplaceshownmeisrathermorethanlessthanfiftydegrees；andthelikemaybeconcludedfromthealtitudeitappearedinatRedgrove，nearseventymilesdistant。Thoughatthisverygreatdistance，itappearedtomovewithanincrediblevelocity，darting，inaveryfewsecondsoftime，forabouttwelvedegreesofagreatcirclefromnorthtosouth，beingverybrightatitsfirstappearance；anditdiedawayattheeastofitscourse，leavingforsometimeapalewhitenessintheplace，withsomeremainsofitinthetrackwhereithadgone；butnohissingsoundasitpassed，orbounceofanexplosionwereheard。
\"Itmaydeservethehonorablesociety’sthoughts，howsogreataquantityofvaporshouldberaisedtothetopoftheatmosphere，andtherecollected，soasuponitsascensionorotherwiseillumination，togivealighttoacircleofaboveonehundredmilesdiameter，notmuchinferiortothelightofthemoon；
soasonemightseetotakeapinfromthegroundintheotherwisedarknight。’Tishardtoconceivewhatsortofexhalationsshouldrisefromtheearth，eitherbytheactionofthesunorsubterraneanheat，soastosurmounttheextremecoldandrarenessoftheairinthoseupperregions:butthefactisindisputable，andthereforerequiresasolution。\"
Fromthismuchofthepaperitappearsthattherewasageneralbeliefthatthisburningmasswasheatedvaporthrownofffromtheearthinsomemysteriousmanner，yetthisisunsatisfactorytoHalley，foraftercitingvariousothermeteorsthathaveappearedwithinhisknowledge，hegoesontosay:
\"Whatsortofsubstanceitmustbe，thatcouldbesoimpelledandignitedatthesametime；therebeingnoVulcanoorotherSpiraculumofsubterraneousfireinthenortheastpartsoftheworld，thatweeveryetheardof，fromwhenceitmightbeprojected。
\"Ihavemuchconsideredthisappearance，andthinkitoneofthehardestthingstoaccountforthatIhaveyetmetwithinthephenomenaofmeteors，andIaminducedtothinkthatitmustbesomecollectionofmatterformedintheaether，asitwere，bysomefortuitousconcourseofatoms，andthattheearthmetwithitasitpassedalonginitsorb，thenbutnewlyformed，andbeforeithadconceivedanygreatimpetusofdescenttowardsthesun。Forthedirectionofitwasexactlyoppositetothatoftheearth，whichmadeananglewiththemeridianatthattimeofsixty-sevengr。，thatis，itscoursewasfromwestsouthwesttoeastnortheast，whereforethemeteorseemedtomovethecontraryway。Andbesidesfallingintothepoweroftheearth’sgravity，andlosingitsmotionfromtheoppositionofthemedium，itseemsthatitdescendedtowardstheearth，andwasextinguishedintheTyrrheneSea，tothewestsouthwestofLeghorn。Thegreatblowbeinghearduponitsfirstimmersionintothewater，andtherattlinglikethedrivingofacartoverstonesbeingwhatsucceededuponitsquenching；
somethinglikethisisalwayshearduponquenchingaveryhotironinwater。Thesefactsbeingpastdispute，Iwouldbegladtohavetheopinionofthelearnedthereon，andwhatobjectioncanbereasonablymadeagainsttheabovehypothesis，whichIhumblysubmittotheircensure。\"[1]
Thesefewparagraphs，comingastheydofromaleadingeighteenth-centuryastronomer，conveymoreclearlythananycommenttheactualstateofthemeteorologicallearningatthattime。Thatthisballoffire，rushing\"atagreatervelocitythantheswiftestcannon-ball，\"wassimplyamassofheatedrockpassingthroughouratmosphere，didnotoccurtohim，oratleastwasnotcredited。Noristhissurprisingwhenwereflectthatatthattimeuniversalgravitationhadbeenbutrecentlydiscovered；heathadnotasyetbeenrecognizedassimplyaformofmotion；andthunderandlightningwereunexplainedmysteries，nottobeexplainedforanotherthree-quartersofacentury。
InthechapteronmeteorologyweshallseehowthesolutionofthismysterythatpuzzledHalleyandhisassociatesalltheirliveswasfinallyattained。
BRADLEYANDTHEABERRATIONOFLIGHT
HalleywassucceededasastronomerroyalbyamanwhoseusefuladditionstothesciencewerenottoberecognizedorappreciatedfullyuntilbroughttolightbythePrussianastronomerBesselearlyinthenineteenthcentury。ThiswasDr。JamesBradley，anecclesiastic，whoranksasoneofthemosteminentastronomersoftheeighteenthcentury。Hismostremarkablediscoverywastheexplanationofapeculiarmotionofthepole-star，firstobserved，butnotexplained，byPicardacenturybefore。FormanyyearsasatisfactoryexplanationwassoughtunsuccessfullybyBradleyandhisfellow-astronomers，butatlasthewasabletodemonstratethatthestaryDraconis，onwhichhewasmakinghisobservations，described，orappearedtodescribe，asmallellipse。Ifthisobservationwascorrect，itaffordedameansofcomputingtheaberrationofanystaratalltimes。Theexplanationofthephysicalcauseofthisaberration，asBradleythought，andafterwardsdemonstrated，wastheresultofthecombinationofthemotionoflightwiththeannualmotionoftheearth。Bradleyfirstformulatedthistheoryin1728，butitwasnotuntil1748——twentyyearsofcontinuousstruggleandobservationbyhim——thathewaspreparedtocommunicatetheresultsofhiseffortstotheRoyalSociety。ThisremarkablepaperisthoughtbytheFrenchman，Delambre，toentitleitsauthortoaplaceinsciencebesidesuchastronomersasHipparcbusandKepler。
Bradley’sstudiesledhimtodiscoveralsothelibratorymotionoftheearth’saxis。\"AsthisappearanceofgDraconis。indicatedadiminutionoftheinclinationoftheearth’saxistotheplaneoftheecliptic，\"hesays；\"andasseveralastronomershavesupposedTHATinclinationtodiminishregularly；ifthisphenomenondependeduponsuchacause，andamountedto18\"innineyears，theobliquityoftheeclipticwould，atthatrate，alterawholeminuteinthirtyyears；whichismuchfasterthananyobservations，beforemade，wouldallow。Ihadreason，therefore，tothinkthatsomepartofthismotionattheleast，ifnotthewhole，wasowingtothemoon’sactionupontheequatorialpartsoftheearth；which，Iconceived，mightcausealibratorymotionoftheearth’saxis。ButasI
wasunabletojudge，fromonlynineyearsobservations，whethertheaxiswouldentirelyrecoverthesamepositionthatithadintheyear1727，Ifounditnecessarytocontinuemyobservationsthroughawholeperiodofthemoon’snodes；attheendofwhichIhadthesatisfactiontosee，thatthestars，returnedintothesamepositionagain；asiftherehadbeennoalterationatallintheinclinationoftheearth’saxis；whichfullyconvincedmethatIhadguessedrightlyastothecauseofthephenomena。Thiscircumstanceproveslikewise，thatiftherebeagradualdiminutionoftheobliquityoftheecliptic，itdoesnotariseonlyfromanalterationinthepositionoftheearth’saxis，butratherfromsomechangeintheplaneoftheeclipticitself；becausethestars，attheendoftheperiodofthemoon’snodes，appearedinthesameplaces，withrespecttotheequator，astheyoughttohavedone，iftheearth’saxishadretainedthesameinclinationtoaninvariableplane。\"[2]