第13章

Havingthusprovedthecompositionoflight，Newtontookupanexhaustivediscussionastocolors，whichcannotbeenteredintoatlengthhere。Someofhisremarksonthesubjectofcompoundcolors，however，maybestatedinpart。Newton’sviewsareofparticularinterestinthisconnection，since，aswehavealreadypointedout，thequestionastowhatconstitutedcolorcouldnotbeagreeduponbythephilosophers。Someheldthatcolorwasanintegralpartofthesubstance；othersmaintainedthatitwassimplyareflectionfromthesurface；andnoscientificexplanationhadbeengenerallyaccepted。Newtonconcludeshispaperasfollows:

"Imightaddmoreinstancesofthisnature，butIshallconcludewiththegeneralonethatthecolorsofallnaturalbodieshavenootheroriginthanthis，thattheyarevariouslyqualifiedtoreflectonesortoflightingreaterplentythananother。AndthisIhaveexperimentedinadarkroombyilluminatingthosebodieswithuncompoundedlightofdiverscolors。Forbythatmeansanybodymaybemadetoappearofanycolor。Theyhavetherenoappropriatecolor，buteverappearofthecolorofthelightcastuponthem，butyetwiththisdifference，thattheyaremostbriskandvividinthelightoftheirowndaylightcolor。

Miniumappeareththereofanycolorindifferentlywithwhich’tisillustrated，butyetmostluminousinred；andsoBiseappearethindifferentlyofanycolorwithwhich’tisillustrated，butyetmostluminousinblue。AndthereforeMiniumreflectethraysofanycolor，butmostcopiouslythoseinduedwithred；andconsequently，whenillustratedwithdaylight——thatis，withallsortsofrayspromiscuouslyblended——thosequalifiedwithredshallaboundmostinthereflectedlight，andbytheirprevalencecauseittoappearofthatcolor。Andforthesamereason，Bise，reflectingbluemostcopiously，shallappearbluebytheexcessofthoseraysinitsreflectedlight；andthelikeofotherbodies。Andthatthisistheentireandadequatecauseoftheircolorsismanifest，becausetheyhavenopowertochangeoralterthecolorsofanysortofraysincidentapart，butputonallcolorsindifferentlywithwhichtheyareenlightened。"[2]

Thisepoch-makingpaperarousedastormofopposition。SomeofNewton’sopponentscriticisedhismethods，othersevendoubtedthetruthofhisexperiments。TherewasoneslightmistakeinNewton’sbeliefthatallprismswouldgiveaspectrumofexactlythesamelength，anditwassometimebeforehecorrectedthiserror。Meanwhilehepatientlymetandansweredtheargumentsofhisopponentsuntilhebegantofeelthatpatiencewasnolongeravirtue。Atonetimeheevenwentsofarastodeclarethat，oncehewas"freeofthisbusiness，"hewouldrenouncescientificresearchforever，atleastinapublicway。Fortunatelyfortheworld，however，hedidnotadheretothisdetermination，butwentontoevengreaterdiscoveries——which，itmaybeadded，involvedstillgreatercontroversies。

IncommentingonNewton’sdiscoveryofthecompositionoflight，Voltairesaid:"SirIsaacNewtonhasdemonstratedtotheeye，bythebareassistanceofaprism，thatlightisacompositionofcoloredrays，which，beingunited，formwhitecolor。Asinglerayisbyhimdividedintoseven，whichallfalluponapieceoflinenorasheetofwhitepaper，intheirorderoneabovetheother，andatequaldistances。Thefirstisred，thesecondorange，thethirdyellow，thefourthgreen，thefifthblue，thesixthindigo，theseventhavioletpurple。Eachoftheseraystransmittedafterwardsbyahundredotherprismswillneverchangethecoloritbears；inlikemannerasgold，whencompletelypurgedfromitsdross，willneverchangeafterwardsinthecrucible。"[3]

XII。NEWTONANDTHELAWOFGRAVITATION

Wecomenowtothestoryofwhatisbycommonconsentthegreatestofscientificachievements。Thelawofuniversalgravitationisthemostfar-reachingprincipleasyetdiscovered。

Ithasapplicationequallytotheminutestparticleofmatterandtothemostdistantsunsintheuniverse，yetitisamazinginitsverysimplicity。Asusuallyphrased，thelawisthis:Thateveryparticleofmatterintheuniverseattractseveryotherparticlewithaforcethatvariesdirectlywiththemassoftheparticlesandinverselyasthesquaresoftheirmutualdistance。

Newtondidnotvaultatoncetothefullexpressionofthislaw，thoughhehadformulateditfullybeforehegavetheresultsofhisinvestigationstotheworld。Wehavenowtofollowthestepsbywhichhereachedthisculminatingachievement。

AttheverybeginningwemustunderstandthattheideaofuniversalgravitationwasnotabsolutelyoriginalwithNewton。

AwaybackintheoldGreekdays，aswehaveseen，Anaxagorasconceivedandclearlyexpressedtheideathattheforcewhichholdstheheavenlybodiesintheirorbitsmaybethesamethatoperatesuponsubstancesatthesurfaceoftheearth。WithAnaxagorasthiswasscarcelymorethanaguess。Afterhisdaytheideaseemsnottohavebeenexpressedbyanyoneuntiltheseventeenthcentury’sawakeningofscience。ThentheconsiderationofKepler’sThirdLawofplanetarymotionsuggestedtomanymindsperhapsindependentlytheprobabilitythattheforcehithertomentionedmerelyascentripetal，throughtheoperationofwhichtheplanetsareheldintheirorbitsisaforcevaryinginverselyasthesquareofthedistancefromthesun。ThisideahadcometoRobertHooke，toWren，andperhapstoHalley，aswellastoNewton；butasyetnoonehadconceivedamethodbywhichthevalidityofthesuggestionmightbetested。

ItwasclaimedlateronbyHookethathehaddiscoveredamethoddemonstratingthetruthofthetheoryofinversesquares，andafterthefullannouncementofNewton’sdiscoveryaheatedcontroversywasprecipitatedinwhichHookeputforwardhisclaimswithaccustomedacrimony。Hooke，however，neverproducedhisdemonstration，anditmaywellbedoubtedwhetherhehadfoundamethodwhichdidmorethanvaguelysuggestthelawwhichtheobservationsofKeplerhadpartiallyrevealed。Newton’sgreatmeritlaynotsomuchinconceivingthelawofinversesquaresasinthedemonstrationofthelaw。Hewasledtothisdemonstrationthroughconsideringtheorbitalmotionofthemoon。Accordingtothefamiliarstory，whichhasbecomeoneoftheclassicmythsofscience，Newtonwasledtotakeuptheproblemthroughobservingthefallofanapple。Voltaireisresponsibleforthestory，whichservesaswellasanother；itstruthorfalsityneednotintheleastconcernus。Sufficeitthatthroughponderingonthefamiliarfactofterrestrialgravitation，Newtonwasledtoquestionwhetherthisforcewhichoperatessotangiblyhereattheearth’ssurfacemaynotextenditsinfluenceoutintothedepthsofspace，soastoinclude，forexample，themoon。

Obviouslysomeforcepullsthemoonconstantlytowardstheearth；

otherwisethatbodywouldflyoffatatangentandneverreturn。

Maynotthisso-calledcentripetalforcebeidenticalwithterrestrialgravitation？SuchwasNewton’squery。ProbablymanyanothermansinceAnaxagorashadaskedthesamequestion，butassuredlyNewtonwasthefirstmantofindananswer。

ThethoughtthatsuggesteditselftoNewton’smindwasthis:Ifwemakeadiagramillustratingtheorbitalcourseofthemoonforanygivenperiod，sayoneminute，weshallfindthatthecourseofthemoondepartsfromastraightlineduringthatperiodbyameasurabledistance——that:istosay，themoonhasbeenvirtuallypulledtowardstheearthbyanamountthatisrepresentedbythedifferencebetweenitsactualpositionattheendoftheminuteunderobservationandthepositionitwouldoccupyhaditscoursebeentangential，as，accordingtothefirstlawofmotion，itmusthavebeenhadnotsomeforcedeflectedittowardstheearth。

Measuringthedeflectioninquestion——whichisequivalenttotheso-calledversedsineofthearctraversed——wehaveabasisfordeterminingthestrengthofthedeflectingforce。Newtonconstructedsuchadiagram，and，measuringtheamountofthemoon’sdeparturefromatangentialrectilinearcourseinoneminute，determinedthistobe，byhiscalculation，thirteenfeet。

Obviously，then，theforceactinguponthemoonisonethatwouldcausethatbodytofalltowardstheearthtothedistanceofthirteenfeetinthefirstminuteofitsfall。Wouldsuchbetheforceofgravitationactingatthedistanceofthemoonifthepowerofgravitationvariesinverselyasthesquareofthedistance？ThatwasthetangibleforminwhichtheproblempresenteditselftoNewton。Themathematicalsolutionoftheproblemwassimpleenough。Itisbasedonacomparisonofthemoon’sdistancewiththelengthoftheearth’sradius。Onmakingthiscalculation，Newtonfoundthatthepullofgravitation——ifthatwerereallytheforcethatcontrolsthemoon——givesthatbodyafallofslightlyoverfifteenfeetinthefirstminute，insteadofthirteenfeet。Herewassurelyasuggestiveapproximation，yet，ontheotherband，thediscrepancyseemedtobetoogreattowarranthiminthesuppositionthathehadfoundthetruesolution。Hethereforedismissedthematterfromhismindforthetimebeing，nordidhereturntoitdefinitelyforsomeyears。

{illustrationcaption=DIAGRAMTOILLUSTRATENEWTON’SLAWOF

GRAVITATION(ErepresentstheearthandAthemoon。Weretheearth’spullonthemoontocease，themoon’sinertiawouldcauseittotakethetangentialcourse，AB。Ontheotherhand，werethemoon’smotiontobestoppedforaninstant，themoonwouldfalldirectlytowardstheearth，alongthelineAD。Themoon’sactualorbit，resultingfromthesecomponentforces，isAC。LetAC

representtheactualflightofthemooninoneminute。ThenBC，whichisobviouslyequaltoAD，representsthedistancewhichthemoonvirtuallyfallstowardstheearthinoneminute。Actualcomputation，basedonmeasurementsofthemoon’sorbit，showedthisdistancetobeaboutfifteenfeet。Anothercomputationshowedthatthisisthedistancethatthemoonwouldfalltowardstheearthundertheinfluenceofgravity，onthesuppositionthattheforceofgravitydecreasesinverselywiththesquareofthedistance；thebasisofcomparisonbeingfurnishedbyfallingbodiesatthesurfaceoftheearth。Theoryandobservationsthuscoinciding，Newtonwasjustifiedindeclaringthattheforcethatpullsthemoontowardstheearthandkeepsitinitsorbit，isthefamiliarforceofgravity，andthatthisvariesinverselyasthesquareofthedistance。)}

ItwastoappearinduetimethatNewton’shypothesiswasperfectlyvalidandthathismethodofattempteddemonstrationwasequallyso。Thedifficultywasthattheearth’sproperdimensionswerenotatthattimeknown。Awrongestimateoftheearth’ssizevitiatedalltheothercalculationsinvolved，sincethemeasurementofthemoon’sdistancedependsupontheobservationoftheparallax，whichcannotleadtoacorrectcomputationunlessthelengthoftheearth’sradiusisaccuratelyknown。Newton’sfirstcalculationwasmadeasearlyas1666，anditwasnotuntil1682thathisattentionwascalledtoanewandapparentlyaccuratemeasurementofadegreeoftheearth’smeridianmadebytheFrenchastronomerPicard。Thenewmeasurementmadeadegreeoftheearth’ssurface69。10miles，insteadofsixtymiles。

Learningofthismateriallyalteredcalculationastotheearth’ssize，Newtonwasledtotakeupagainhisproblemofthefallingmoon。Asheproceededwithhiscomputation，itbecamemoreandmorecertainthatthistimetheresultwastoharmonizewiththeobservedfacts。Asthestorygoes，hewassocompletelyoverwhelmedwithemotionthathewasforcedtoaskafriendtocompletethesimplecalculation。Thatstorymaywellbetrue，for，simplethoughthecomputationwas，itsresultwasperhapsthemostwonderfuldemonstrationhithertoachievedintheentirefieldofscience。Nowatlastitwasknownthattheforceofgravitationoperatesatthedistanceofthemoon，andholdsthatbodyinitsellipticalorbit，anditrequiredbutaslighteffortoftheimaginationtoassumethattheforcewhichoperatesthroughsuchareachofspaceextendsitsinfluenceyetmorewidely。ThatsuchisreallythecasewasdemonstratedpresentlythroughcalculationsastothemoonsofJupiterandbysimilarcomputationsregardingtheorbitalmotionsofthevariousplanets。Allresultsharmonizing，Newtonwasjustifiedinreachingtheconclusionthatgravitationisauniversalpropertyofmatter。Itremained，asweshallsee，fornineteenth-centuryscientiststoprovethatthesameforceactuallyoperatesuponthestars，thoughitshouldbeaddedthatthisdemonstrationmerelyfortifiedabeliefthathadalreadyfoundfullacceptance。

HavingthusepitomizedNewton’sdiscovery，wemustnowtakeupthestepsofhisprogresssomewhatindetail，andstatehistheoriesandtheirdemonstrationinhisownwords。PropositionIV。，theorem4，ofhisPrincipiaisasfollows:

"Thatthemoongravitatestowardstheearthandbytheforceofgravityiscontinuallydrawnofffromarectilinearmotionandretainedinitsorbit。

"Themeandistanceofthemoonfromtheearth，inthesyzygiesinsemi-diametersoftheearth，is，accordingtoPtolemyandmostastronomers，59；accordingtoVendelinandHuygens，60；toCopernicus，601/3；toStreet，602/3；andtoTycho，561/2。ButTycho，andallthatfollowhistablesofrefractions，makingtherefractionsofthesunandmoon(altogetheragainstthenatureoflight)toexceedtherefractionsofthefixedstars，andthatbyfourorfiveminutesNEARTHEHORIZON，didtherebyincreasethemoon’sHORIZONTALparallaxbyalikenumberofminutes，thatis，byatwelfthorfifteenthpartofthewholeparallax。Correctthiserrorandthedistancewillbecomeabout601/2

semi-diametersoftheearth，neartowhatothershaveassigned。

Letusassumethemeandistanceof60diametersinthesyzygies；

andsupposeonerevolutionofthemoon，inrespecttothefixedstars，tobecompletedin27d。7h。43’，asastronomershavedetermined；andthecircumferenceoftheearthtoamountto123，249，600Parisfeet，astheFrenchhavefoundbymensuration。

Andnow，ifweimaginethemoon，deprivedofallmotion，tobeletgo，soastodescendtowardstheearthwiththeimpulseofallthatforcebywhich(byCor。Prop。iii。)itisretainedinitsorb，itwillinthespaceofoneminuteoftimedescribeinitsfall151/12Parisfeet。Fortheversedsineofthatarcwhichthemoon，inthespaceofoneminuteoftime，wouldbyitsmeanmotiondescribeatthedistanceofsixtysemi-diametersoftheearth，isnearly151/12Parisfeet，ormoreaccurately15

feet，1inch，1line4/9。Wherefore，sincethatforce，inapproachingtheearth，increasesinthereciprocal-duplicateproportionofthedistance，anduponthataccount，atthesurfaceoftheearth，is60x60timesgreaterthanatthemoon，abodyinourregions，fallingwiththatforce，oughtinthespaceofoneminuteoftimetodescribe60x60x151/12Parisfeet；andinthespaceofonesecondoftime，todescribe151/12ofthosefeet，ormoreaccurately，15feet，1inch，1line4/9。Andwiththisveryforceweactuallyfindthatbodieshereuponearthdoreallydescend；forapendulumoscillatingsecondsinthelatitudeofPariswillbe3Parisfeet，and8lines1/2inlength，asMr。Huygenshasobserved。Andthespacewhichaheavybodydescribesbyfallinginonesecondoftimeistohalfthelengthofthependulumintheduplicateratioofthecircumferenceofacircletoitsdiameter(asMr。Huygenshasalsoshown)，andistherefore15Parisfeet，1inch，1line4/9。

Andthereforetheforcebywhichthemoonisretainedinitsorbitisthatverysameforcewhichwecommonlycallgravity；

for，weregravityanotherforcedifferentfromthat，thenbodiesdescendingtotheearthwiththejointimpulseofbothforceswouldfallwithadoublevelocity，andinthespaceofonesecondoftimewoulddescribe301/6Parisfeet；altogetheragainstexperience。"[1]

Allthisisbeautifullyclear，anditsvalidityhasneverinrecentgenerationsbeencalledinquestion；yetitshouldbeexplainedthattheargumentdoesnotamounttoanactuallyindisputabledemonstration。Itisatleastpossiblethatthecoincidencebetweentheobservedandcomputedmotionofthemoonmaybeamerecoincidenceandnothingmore。Thisprobability，however，issoremotethatNewtonisfullyjustifiedindisregardingit，and，ashasbeensaid，allsubsequentgenerationshaveacceptedthecomputationasdemonstrative。

LetusproducenowNewton’sfurthercomputationsastotheotherplanetarybodies，passingontohisfinalconclusionthatgravityisauniversalforce。

"PROPOSITIONV。，THEOREMV。

"ThatthecircumjovialplanetsgravitatetowardsJupiter；thecircumsaturnaltowardsSaturn；thecircumsolartowardsthesun；

andbytheforcesoftheirgravityaredrawnofffromrectilinearmotions，andretainedincurvilinearorbits。

"FortherevolutionsofthecircumjovialplanetsaboutJupiter，ofthecircumsaturnalaboutSaturn，andofMercuryandVenusandtheothercircumsolarplanetsaboutthesun，areappearancesofthesamesortwiththerevolutionofthemoonabouttheearth；

andtherefore，byRuleii。，mustbeowingtothesamesortofcauses；especiallysinceithasbeendemonstratedthattheforcesuponwhichthoserevolutionsdependtendtothecentresofJupiter，ofSaturn，andofthesun；andthatthoseforces，inrecedingfromJupiter，fromSaturn，andfromthesun，decreaseinthesameproportion，andaccordingtothesamelaw，astheforceofgravitydoesinrecedingfromtheearth。

"COR。1——Thereis，therefore，apowerofgravitytendingtoalltheplanets；fordoubtlessVenus，Mercury，andtherestarebodiesofthesamesortwithJupiterandSaturn。Andsinceallattraction(byLawiii。)ismutual，Jupiterwillthereforegravitatetowardsallhisownsatellites，Saturntowardshis，theearthtowardsthemoon，andthesuntowardsalltheprimaryplanets。

"COR。2——Theforceofgravitywhichtendstoanyoneplanetisreciprocallyasthesquareofthedistanceofplacesfromtheplanet’scentre。

"COR。3——Alltheplanetsdomutuallygravitatetowardsoneanother，byCor。1and2，andhenceitisthatJupiterandSaturn，whenneartheirconjunction，bytheirmutualattractionssensiblydisturbeachother’smotions。Sothesundisturbsthemotionsofthemoon；andbothsunandmoondisturboursea，asweshallhereafterexplain。

"SCHOLIUM

"Theforcewhichretainsthecelestialbodiesintheirorbitshasbeenhithertocalledcentripetalforce；butitbeingnowmadeplainthatitcanbenootherthanagravitatingforce，weshallhereaftercallitgravity。ForthecauseofthecentripetalforcewhichretainsthemooninitsorbitwillextenditselftoalltheplanetsbyRulesi。，ii。，andiii。

"PROPOSITIONVI。，THEOREMVI。

"Thatallbodiesgravitatetowardseveryplanet；andthattheweightsofthebodiestowardsanythesameplanet，atequaldistancesfromthecentreoftheplanet，areproportionaltothequantitiesofmatterwhichtheyseverallycontain。

"Ithasbeennowalongtimeobservedbyothersthatallsortsofheavybodies(allowancebeingmadefortheinabilityofretardationwhichtheysufferfromasmallpowerofresistanceintheair)descendtotheearthFROMEQUALHEIGHTSinequaltimes；

andthatequalityoftimeswemaydistinguishtoagreataccuracybyhelpofpendulums。Itriedthethingingold，silver，lead，glass，sand，commonsalt，wood，water，andwheat。Iprovidedtwowoodenboxes，roundandequal:Ifilledtheonewithwood，andsuspendedanequalweightofgold(asexactlyasIcould)inthecentreofoscillationoftheother。Theboxeshangingbyelevenfeet，madeacoupleofpendulumsexactlyequalinweightandfigure，andequallyreceivingtheresistanceoftheair。And，placingtheonebytheother，Iobservedthemtoplaytogetherforwardandbackward，foralongtime，withequalvibrations。Andthereforethequantityofmatteringoldwastothequantityofmatterinthewoodastheactionofthemotiveforce(orvismotrix)uponallthegoldtotheactionofthesameuponallthewood——thatis，astheweightoftheonetotheweightoftheother:andthelikehappenedintheotherbodies。Bytheseexperiments，inbodiesofthesameweight，Icouldmanifestlyhavediscoveredadifferenceofmatterlessthanthethousandthpartofthewhole，hadanysuchbeen。But，withoutalldoubt，thenatureofgravitytowardstheplanetsisthesameastowardstheearth。For，shouldweimagineourterrestrialbodiesremovedtotheorbofthemoon，andthere，togetherwiththemoon，deprivedofallmotion，tobeletgo，soastofalltogethertowardstheearth，itiscertain，fromwhatwehavedemonstratedbefore，that，inequaltimes，theywoulddescribeequalspaceswiththemoon，andofconsequencearetothemoon，inquantityandmatter，astheirweightstoitsweight。

"Moreover，sincethesatellitesofJupiterperformtheirrevolutionsintimeswhichobservethesesquiplicateproportionoftheirdistancesfromJupiter’scentre，theiraccelerativegravitiestowardsJupiterwillbereciprocallyasthesquareoftheirdistancesfromJupiter’scentre——thatis，equal，atequaldistances。And，therefore，thesesatellites，ifsupposedtofallTOWARDSJUPITERfromequalheights，woulddescribeequalspacesinequaltimes，inlikemannerasheavybodiesdoonourearth。

And，bythesameargument，ifthecircumsolarplanetsweresupposedtobeletfallatequaldistancesfromthesun，theywould，intheirdescenttowardsthesun，describeequalspacesinequaltimes。Butforceswhichequallyaccelerateunequalbodiesmustbeasthosebodies——thatistosay，theweightsoftheplanets(TOWARDSTHESUNmustbeastheirquantitiesofmatter。

Further，thattheweightsofJupiterandhissatellitestowardsthesunareproportionaltotheseveralquantitiesoftheirmatter，appearsfromtheexceedinglyregularmotionsofthesatellites。Forifsomeofthesebodiesweremorestronglyattractedtothesuninproportiontotheirquantityofmatterthanothers，themotionsofthesatelliteswouldbedisturbedbythatinequalityofattraction。Ifatequaldistancesfromthesunanysatellite，inproportiontothequantityofitsmatter，didgravitatetowardsthesunwithaforcegreaterthanJupiterinproportiontohis，accordingtoanygivenproportion，supposedtoe；thenthedistancebetweenthecentresofthesunandofthesatellite’sorbitwouldbealwaysgreaterthanthedistancebetweenthecentresofthesunandofJupiternearlyinthesubduplicateofthatproportion:asbysomecomputationsIhavefound。Andifthesatellitedidgravitatetowardsthesunwithaforce，lesserintheproportionofetod，thedistanceofthecentreofthesatellite’sorbfromthesunwouldbelessthanthedistanceofthecentreofJupiterfromthesuninthesubduplicateofthesameproportion。Therefore，ifatequaldistancesfromthesun，theaccelerativegravityofanysatellitetowardsthesunweregreaterorlessthantheaccelerativegravityofJupitertowardsthesunbyone-one-thousandthpartofthewholegravity，thedistanceofthecentreofthesatellite’sorbitfromthesunwouldbegreaterorlessthanthedistanceofJupiterfromthesunbyoneone-two-thousandthpartofthewholedistance——thatis，byafifthpartofthedistanceoftheutmostsatellitefromthecentreofJupiter；aneccentricityoftheorbitwhichwouldbeverysensible。ButtheorbitsofthesatellitesareconcentrictoJupiter，andthereforetheaccelerativegravitiesofJupiterandofallitssatellitestowardsthesun，atequaldistancesfromthesun，areastheirseveralquantitiesofmatter；andtheweightsofthemoonandoftheearthtowardsthesunareeithernone，oraccuratelyproportionaltothemassesofmatterwhichtheycontain。

"COR。5——Thepowerofgravityisofadifferentnaturefromthepowerofmagnetism；forthemagneticattractionisnotasthematterattracted。Somebodiesareattractedmorebythemagnet；

othersless；mostbodiesnotatall。Thepowerofmagnetisminoneandthesamebodymaybeincreasedanddiminished；andissometimesfarstronger，forthequantityofmatter，thanthepowerofgravity；andinrecedingfromthemagnetdecreasesnotintheduplicate，butalmostinthetriplicateproportionofthedistance，asnearlyasIcouldjudgefromsomerudeobservations。

"PROPOSITIONVII。，THEOREMVII。

"Thatthereisapowerofgravitytendingtoallbodies，proportionaltotheseveralquantitiesofmatterwhichtheycontain。

Thatalltheplanetsmutuallygravitateonetowardsanotherwehaveprovedbefore；aswellasthattheforceofgravitytowardseveryoneofthemconsideredapart，isreciprocallyasthesquareofthedistanceofplacesfromthecentreoftheplanet。Andthenceitfollows，thatthegravitytendingtowardsalltheplanetsisproportionaltothematterwhichtheycontain。

"Moreover，sinceallthepartsofanyplanetAgravitatestowardsanyotherplanetB；andthegravityofeverypartistothegravityofthewholeasthematterofthepartistothematterofthewhole；andtoeveryactioncorrespondsareaction；

thereforetheplanetBwill，ontheotherhand，gravitatetowardsallthepartsofplanetA，anditsgravitytowardsanyonepartwillbetothegravitytowardsthewholeasthematteroftheparttothematterofthewhole。Q。E。D。

"HENCEITWOULDAPPEARTHATtheforceofthewholemustarisefromtheforceofthecomponentparts。"

NewtonclosesthisremarkableBookiii。withthefollowingwords:

"Hithertowehaveexplainedthephenomenaoftheheavensandofourseabythepowerofgravity，buthavenotyetassignedthecauseofthispower。Thisiscertain，thatitmustproceedfromacausethatpenetratestotheverycentreofthesunandplanets，withoutsufferingtheleastdiminutionofitsforce；thatoperatesnotaccordingtothequantityofthesurfacesoftheparticlesuponwhichitacts(asmechanicalcausesusedtodo)，butaccordingtothequantityofsolidmatterwhichtheycontain，andpropagatesitsvirtueonallsidestoimmensedistances，decreasingalwaysintheduplicateproportionsofthedistances。

Gravitationtowardsthesunismadeupoutofthegravitationstowardstheseveralparticlesofwhichthebodyofthesuniscomposed；andinrecedingfromthesundecreasesaccuratelyintheduplicateproportionofthedistancesasfarastheorbofSaturn，asevidentlyappearsfromthequiescenceoftheaphelionsoftheplanets；nay，andeventotheremotestaphelionsofthecomets，ifthoseaphelionsarealsoquiescent。ButhithertoI

havenotbeenabletodiscoverthecauseofthosepropertiesofgravityfromphenomena，andIframenohypothesis；forwhateverisnotdeducedfromthephenomenaistobecalledanhypothesis；

andhypotheses，whethermetaphysicalorphysical，whetherofoccultqualitiesormechanical，havenoplaceinexperimentalphilosophy……Andtousitisenoughthatgravitydoesreallyexist，andactaccordingtothelawswhichwehaveexplained，andabundantlyservestoaccountforallthemotionsofthecelestialbodiesandofoursea。"[2]

Theverymagnitudeoftheimportanceofthetheoryofuniversalgravitationmadeitsgeneralacceptanceamatterofconsiderabletimeaftertheactualdiscovery。ThisoppositionhadofcoursebeenforeseenbyNewton，and，muchasbedreadedcontroversy，hewaspreparedtofaceitandcombatittothebitterend。Heknewthathistheorywasright；itremainedforhimtoconvincetheworldofitstruth。Heknewthatsomeofhiscontemporaryphilosopherswouldacceptitatonce；otherswouldatfirstdoubt，question，anddispute，butfinallyaccept；whilestillotherswoulddoubtanddisputeuntiltheendoftheirdays。Thishadbeenthehistoryofothergreatdiscoveries；andthiswillprobablybethehistoryofmostgreatdiscoveriesforalltime。

Butinthiscasethediscovererlivedtoseehistheoryacceptedbypracticallyallthegreatmindsofhistime。

DelambreisauthorityforthefollowingestimateofNewtonbyLagrange。"ThecelebratedLagrange，"hesays，"whofrequentlyassertedthatNewtonwasthegreatestgeniusthateverexisted，usedtoadd——’andthemostfortunate，forwecannotfindMORE

THANONCEasystemoftheworldtoestablish。’"Withpardonableexaggerationtheadmiringfollowersofthegreatgeneralizerpronouncedthisepitaph:

"NatureandNature’slawslayhidinnight；

Godsaid`LetNewtonbe！’andallwaslight。"

XIII。INSTRUMENTSOFPRECISIONINTHEAGEOFNEWTON

DuringtheNewtonianepochtherewerenumerousimportantinventionsofscientificinstruments，aswellasmanyimprovementsmadeupontheolderones。Someofthesediscoverieshavebeenreferredtobrieflyinotherplaces，buttheirimportanceinpromotingscientificinvestigationwarrantsafullertreatmentofsomeofthemoresignificant。

Manyoftheerrorsthathadariseninvariousscientificcalculationsbeforetheseventeenthcenturymaybeascribedtothecrudenessandinaccuracyintheconstructionofmostscientificinstruments。Scientistshadnotasyetlearnedthatanapproachtoabsoluteaccuracywasnecessaryineveryinvestigationinthefieldofscience，andthatsuchaccuracymustbeextendedtotheconstructionoftheinstrumentsusedintheseinvestigationsandobservations。Inastronomyitisobviousthatinstrumentsofdelicateexactnessaremostessential；yetTychoBrahe，wholivedinthesixteenthcentury，iscreditedwithbeingthefirstastronomerwhoseinstrumentsshowextremecareinconstruction。

ItseemspracticallysettledthatthefirsttelescopewasinventedinHollandin1608；butthreemen，HansLippershey，JamesMetius，andZachariasJansen，havebeengiventhecreditoftheinventionatdifferenttimes。Itwouldseemfromcertainpapers，nowinthelibraryoftheUniversityofLeyden，andincludedinHuygens’spapers，thatLippersheywasprobablythefirsttoinventatelescopeandtodescribehisinvention。ThestoryistoldthatLippershey，whowasaspectacle-maker，stumbledbyaccidentuponthediscoverythatwhentwolensesareheldatacertaindistanceapart，objectsatadistanceappearnearerandlarger。Havingmadethisdiscovery，befittedtwolenseswithatubesoastomaintainthemattheproperdistance，andthusconstructedthefirsttelescope。

ItwasGalileo，however，asreferredtoinaprecedingchapter，whofirstconstructedatelescopebasedonhisknowledgeofthelawsofrefraction。In1609，havingheardthataninstrumenthadbeeninvented，consistingoftwolensesfixedinatube，wherebyobjectsweremadetoappearlargerandnearer，hesetaboutconstructingsuchaninstrumentthatshouldfollowouttheknowneffectsofrefraction。Hisfirsttelescope，madeoftwolensesfixedinaleadpipe，wassoonfollowedbyothersofimprovedtypes，Galileodevotingmuchtimeandlabortoperfectinglensesandcorrectingerrors。Infact，hisworkindevelopingtheinstrumentwassoimportantthatthetelescopecamegraduallytobeknownasthe"Galileantelescope。"

IntheconstructionofhistelescopeGalileomadeuseofaconvexandaconcavelens；butshortlyafterthisKeplerinventedaninstrumentinwhichboththelensesusedwereconvex。ThistelescopegaveamuchlargerfieldofviewthantheGalileantelescope，butdidnotgiveasclearanimage，andinconsequencedidnotcomeintogeneraluseuntilthemiddleoftheseventeenthcentury。ThefirstpowerfultelescopeofthistypewasmadebyHuygensandhisbrother。Itwasoftwelvefeetfocallength，andenabledHuygenstodiscoveranewsatelliteofSaturn，andtodeterminealsothetrueexplanationofSaturn’sring。

ItwasHuygens，togetherwithMalvasiaandAuzout，whofirstappliedthemicrometertothetelescope，althoughtheinventorofthefirstmicrometerwasWilliamGascoigne，ofYorkshire，about1636。Themicrometerasusedintelescopesenablestheobservertomeasureaccuratelysmallangulardistances。Beforetheinventionofthetelescopesuchmeasurementswerelimitedtotheanglethatcouldbedistinguishedbythenakedeye，andwere，ofcourse，onlyapproximatelyaccurate。Evenverycarefulobservers，suchasTychoBrahe，wereabletoobtainonlyfairlyaccurateresults。ButbyapplyingGascoigne’sinventiontothetelescopealmostabsoluteaccuracybecameatoncepossible。TheprincipleofGascoigne’smicrometerwasthatoftwopointerslyingparallel，andinthispositionpointingtozero。Thesewerearrangedsothattheturningofasinglescrewseparatedorapproximatedthematwill，andtheanglethusformedcouldbedeterminedwithabsoluteaccuracy。

Huygens’smicrometerwasaslipofmetalofvariablebreadthinsertedatthefocusofthetelescope。Byobservingatwhatpointthisexactlycoveredanobjectunderexamination，andknowingthefocallengthofthetelescopeandthewidthofthemetal，hecouldthendeducetheapparentangularbreadthoftheobject。HuygensdiscoveredalsothatanobjectplacedinthecommonfocusofthetwolensesofaKeplertelescopeappearsdistinctandclearlydefined。ThemicrometersofMalvasia，andlaterofAuzoutandPicard，arethedevelopmentofthisdiscovery。Malvasia’smicrometer，whichhedescribedin1662，consistedoffinesilverwiresplacedatright-anglesatthefocusofhistelescope。

Astelescopesincreasedinpower，however，itwasfoundthateventhefinestwire，orsilkfilaments，weremuchtoothickforastronomicalobservations，astheyobliteratedtheimage，andso，finally，thespider-webcameintouseandisstillusedinmicrometersandothersimilarinstruments。Beforethattime，however，thefinecrossedwireshadrevolutionizedastronomicalobservations。"Wemayjudgehowgreatwastheimprovementwhichthesecontrivancesintroducedintotheartofobserving，"saysWhewell，"byfindingthatHeveliusrefusedtoadoptthembecausetheywouldmakealltheoldobservationsofnovalue。Hehadspentalaboriousandactivelifeintheexerciseoftheoldmethods，andcouldnotbeartothinkthatallthetreasureswhichhehadaccumulatedhadlosttheirworthbythediscoveryofanewmineofricherones。"[1]

UntilthetimeofNewton，allthetelescopesinusewereeitheroftheGalileanorKepleriantype，thatis，refractors。Butabouttheyear1670Newtonconstructedhisfirstreflectingtelescope，whichwasgreatlysuperiorto，althoughmuchsmallerthan，thetelescopestheninuse。Hewasledtothisinventionbyhisexperimentswithlightandcolors。In1671hepresentedtotheRoyalSocietyasecondandsomewhatlargertelescope，whichhehadmade；andthistypeofinstrumentwaslittleimproveduponuntiltheintroductionoftheachromatictelescope，inventedbyChesterMoorHallin1733。

Asisgenerallyknown，theelementofaccuratemeasurementsoftimeplaysanimportantpartinthemeasurementsofthemovementsoftheheavenlybodies。Infact，onewasscarcelypossiblewithouttheother，andasithappeneditwasthesameman，Huygens，whoperfectedKepler’stelescopeandinventedthependulumclock。ThegeneralideahadbeensuggestedbyGalileo；

or，betterperhaps，theequaltimeoccupiedbythesuccessiveoscillationsofthependulumhadbeennotedbyhim。Hehadnotbeenable，however，toputthisdiscoverytopracticalaccount。

Butin1656Huygensinventedthenecessarymachineryformaintainingthemotionofthependulumandperfectedseveralaccurateclocks。Theseclockswereofinvaluableassistancetotheastronomers，affordingastheydidameansofkeepingtime"moreaccuratethanthesunitself。"WhenPicardhadcorrectedthevariationcausedbyheatandcoldactinguponthependulumrodbycombiningmetalsofdifferentdegreesofexpansibility，ahighdegreeofaccuracywaspossible。

Butwhilethependulumclockwasanunequalledstationarytime-piece，itwasuselessinsuchunstablesituationsas，forexample，onshipboard。ButhereagainHuygensplayedaprominentpartbyfirstapplyingthecoiledbalance-springforregulatingwatchesandmarineclocks。Theideaofapplyingaspringtothebalance-wheelwasnotoriginalwithHuygens，however，asithadbeenfirstconceivedbyRobertHooke；butHuygens’sapplicationmadepracticalHooke’sidea。InEnglandtheimportanceofsecuringaccuratewatchesormarineclockswassofullyappreciatedthatarewardofL20，000sterlingwasofferedbyParliamentasastimulustotheinventorofsuchatime-piece。

Theimmediateincentiveforthisofferwastheobviousfactthatwithsuchaninstrumentthedeterminationofthelongitudeofplaceswouldbemuchsimplified。Encouragedbytheseoffers，acertaincarpenternamedHarrisonturnedhisattentiontothesubjectofwatch-making，and，aftermanyyearsoflabor，in1758

producedaspringtime-keeperwhich，duringasea-voyageoccupyingonehundredandsixty-onedays，variedonlyoneminuteandfiveseconds。ThisgainedforHarrisonarewardOfL5000

sterlingatonce，andalittlelaterL10，000more，fromParliament。

Whileinventorswerebusywiththeproblemofaccuratechronometers，however，anotherinstrumentfortakinglongitudeatseahadbeeninvented。Thiswasthereflectingquadrant，orsextant，astheimprovedinstrumentisnowcalled，inventedbyJohnHadleyin1731，andindependentlybyThomasGodfrey，apoorglazierofPhiladelphia，in1730。Godfrey’sinvention，whichwasconstructedonthesameprincipleasthatoftheHadleyinstrument，wasnotgenerallyrecognizeduntiltwoyearsafterHadley’sdiscovery，althoughtheinstrumentwasfinishedandactuallyinuseonasea-voyagesomemonthsbeforeHadleyreportedhisinvention。Theprincipleofthesextant，however，seemstohavebeenknowntoNewton，whoconstructedaninstrumentnotveryunlikethatofHadley；butthisinventionwaslostsightofuntilseveralyearsafterthephilosopher’sdeathandsometimeafterHadley’sinvention。

Theintroductionofthesextantgreatlysimplifiedtakingreckoningsatseaaswellasfacilitatingtakingthecorrectlongitudeofdistantplaces。Beforethattimethemarinerwasobligedtodependuponhiscompass，across-staff，oranastrolabe，atableofthesun’sdeclinationandacorrectionforthealtitudeofthepolestar，andveryinadequateandincorrectcharts。SuchweretheinstrumentsusedbyColumbusandVascodaGamaandtheirimmediatesuccessors。

DuringtheNewtonianperiodthemicroscopesgenerallyinusewerethoseconstructedofsimplelenses，foralthoughcompoundmicroscopeswereknown，thedifficultiesofcorrectingaberrationhadnotbeensurmounted，andamuchclearerfieldwasgivenbythesimpleinstrument。Theresultsobtainedbytheuseofsuchinstruments，however，wereverysatisfactoryinmanyways。Byreferringtocertainplatesinthisvolume，whichreproduceillustrationsfromRobertHooke’sworkonthemicroscope，itwillbeseenthatquiteahighdegreeofeffectivenesshadbeenattained。AnditshouldberecalledthatAntonyvonLeeuwenboek，whosedeathtookplaceshortlybeforeNewton’s，haddiscoveredsuchmicro-organismsasbacteria，hadseenthebloodcorpusclesincirculation，andexaminedanddescribedothermicroscopicstructuresofthebody。

XIV。PROGRESSINELECTRICITYFROMGILBERTANDVONGUERICKETO

FRANKLIN

WehaveseenhowGilbert，byhisexperimentswithmagnets，gaveanimpetustothestudyofmagnetismandelectricity。Gilberthimselfdemonstratedsomefactsandadvancedsometheories，butthesystemofgenerallawswastocomelater。Tothisendthediscoveryofelectricalrepulsion，aswellasattraction，byVonGuericke，withhissulphurball，wasastepforward；butsomethinglikeacenturypassedafterGilbert’sbeginningbeforeanythingofmuchimportancewasdoneinthefieldofelectricity。

In1705，however，FrancisHauksbeebeganaseriesofexperimentsthatresultedinsomestartlingdemonstrations。Formanyyearsithadbeenobservedthatapeculiarlightwasseensometimesinthemercurialbarometer，butHauksbeeandtheotherscientificinvestigatorssupposedtheradiancetobeduetothemercuryinavacuum，broughtabout，perhaps，bysomeagitation。Thatthislightmighthaveanyconnectionwithelectricitydidnot，atfirst，occurtoHauksbeeanymorethanithadtohispredecessors。Theproblemthatinterestedhimwaswhetherthevacuuminthetubeofthebarometerwasessentialtothelight；

andinexperimentingtodeterminethis，heinventedhis"mercurialfountain。"Havingexhaustedtheairinareceivercontainingsomemercury，hefoundthatbyallowingairtorushthroughthemercurythemetalbecameajetthrowninalldirectionsagainstthesidesofthevessel，makingagreat，flamingshower，"likeflashesoflightning，"ashesaid。Butitseemedtohimthattherewasadifferencebetweenthislightandtheglownotedinthebarometer。Thiswasabrightlight，whereasthebarometerlightwasonlyaglow。Ponderingoverthis，Hauksbeetriedvariousexperiments，revolvingpiecesofamber，flint，steel，andothersubstancesinhisexhaustedair-pumpreceiver，withnegative，orunsatisfactory，results。Finally，itoccurredtohimtorevolveanexhaustedglasstubeitself。

Mountingsuchaglobeofglassonanaxissothatitcouldberevolvedrapidlybyabeltrunningonalargewheel，hefoundthatbyholdinghisfingersagainstthewhirlingglobeapurplishglowappeared，givingsufficientlightsothatcoarseprintcouldberead，andthewallsofadarkroomsensiblylightenedseveralfeetaway。Asairwasadmittedtotheglobethelightgraduallydiminished，anditseemedtohimthatthisdiminishedglowwasverysimilarinappearancetothepalelightseeninthemercurialbarometer。Coulditbethatitwastheglass，andnotthemercury，thatcausedit？Goingtoabarometerheproceededtorubtheglassabovethecolumnofmercuryoverthevacuum，withoutdisturbingthemercury，when，tohisastonishment，thesamefaintlight，toallappearancesidenticalwiththeglowseeninthewhirlingglobe，wasproduced。

Turningthesedemonstrationsoverinhismind，herecalledthewell-knownfactthatrubbedglassattractedbitsofpaper，leaf-brass，andotherlightsubstances，andthatthisphenomenonwassupposedtobeelectrical。Thisledhimfinallytodeterminethehithertounsuspectedfact，thattheglowinthebarometerwaselectricalaswasalsotheglowseeninhiswhirlingglobe。

Continuinghisinvestigations，hesoondiscoveredthatsolidglassrodswhenrubbedproducedthesameeffectsasthetube。Bymerechance，happeningtoholdarubbedtubetohischeek，hefelttheeffectofelectricityupontheskinlike"anumberoffine，limberhairs，"andthissuggestedtohimthat，sincethemysteriousmanifestationwassoplain，itcouldbemadetoshowitseffectsuponvarioussubstances。Suspendingsomewoollenthreadsoverthewhirlingglasscylinder，hefoundthatassoonashetouchedtheglasswithhishandsthethreads，whichwerewavedaboutbythewindoftherevolution，suddenlystraightenedthemselvesinapeculiarmanner，andstoodinaradicalposition，pointingtotheaxisofthecylinder。

Encouragedbythesesuccesses，hecontinuedhisexperimentswithbreathlessexpectancy，andsoonmadeanotherimportantdiscovery，thatof"induction，"althoughtherealsignificanceofthisdiscoverywasnotappreciatedbyhimor，forthatmatter，byanyoneelseforseveralgenerationsfollowing。Thisdiscoverywasmadebyplacingtworevolvingcylinderswithinaninchofeachother，onewiththeairexhaustedandtheotherunexhausted。

Placinghishandontheunexhaustedtubecausedthelighttoappearnotonlyuponit，butontheothertubeaswell。Alittlelaterhediscoveredthatitisnotnecessarytowhirltheexhaustedtubetoproducethiseffect，butsimplytoplaceitincloseproximitytotheotherwhirlingcylinder。

ThesedemonstrationsofHauksbeeattractedwideattentionandgaveanimpetustoinvestigatorsinthefieldofelectricity；butstillnogreatadvancewasmadeforsomethinglikeaquarterofacentury。PossiblytheenergiesofthescientistswereexhaustedforthemomentinexploringthenewfieldsthrownopentoinvestigationbythecolossalworkofNewton。

THEEXPERIMENTSOFSTEPHENGRAY

In1729StephenGray(diedin1736)，aneccentricandirascibleoldpensioneroftheCharterHouseinLondon，undertooksomeinvestigationsalonglinessimilartothoseofHauksbee。Whileexperimentingwithaglasstubeforproducingelectricity，asHauksbeehaddone，henoticedthatthecorkswithwhichhehadstoppedtheendsofthetubetoexcludethedust，seemedtoattractbitsofpaperandleaf-brassaswellastheglassitself。

Hesurmisedatoncethatthismysteriouselectricity，or"virtue，"asitwascalled，mightbetransmittedthroughothersubstancesasitseemedtobethroughglass。

"Havingbymeanivoryballofaboutoneandthree-tenthsofaninchindiameter，"hewrites，"withaholethroughit，thisI

fixeduponafir-stickaboutfourincheslong，thrustingtheotherendintothecork，anduponrubbingthetubefoundthattheballattractedandrepelledthefeatherwithmorevigorthanthecorkhaddone，repeatingitsattractionsandrepulsionsformanytimestogether。Ithenfixedtheballonlongersticks，firstupononeofeightinches，andafterwardsupononeoftwenty-fourincheslong，andfoundtheeffectthesame。ThenImadeuseofiron，andthenbrasswire，tofixtheballon，insertingtheotherendofthewireinthecork，asbefore，andfoundthattheattractionwasthesameaswhenthefir-sticksweremadeuseof，andthatwhenthefeatherwasheldoveragainstanypartofthewireitwasattractedbyit；butthoughitwasthennearerthetube，yetitsattractionwasnotsostrongasthatoftheball。

Whenthewireoftwoorthreefeetlongwasused，itsvibrations，causedbytherubbingofthetube，madeitsomewhattroublesometobemanaged。Thisputmetothinkingwhether，iftheballwashungbyapack-threadandsuspendedbyalooponthetube，theelectricitywouldnotbecarrieddownthelinetotheball；I

foundittosucceedaccordingly；foruponsuspendingtheballonthetubebyapack-threadaboutthreefeetlong，whenthetubehadbeenexcitedbyrubbing，theivoryballattractedandrepelledtheleaf-brassoverwhichitwasheldasfreelyasithaddonewhenitwassuspendedonsticksorwire，asdidalsoaballofcork，andanotherofleadthatweighedonepoundandaquarter。"

Graynextattemptedtodeterminewhatotherbodieswouldattractthebitsofpaper，andforthispurposehetriedcoins，piecesofmetal，andevenatea-kettle，"bothemptyandfilledwithhotorcoldwater"；buthefoundthattheattractivepowerappearedtobethesameregardlessofthesubstanceused。

"Inextproceeded，"hecontinues，"totryatwhatgreaterdistancestheelectricvirtuesmightbecarried，andhavingbymeahollowwalking-cane，whichIsupposewaspartofafishing-rod，twofeetsevenincheslong，Icutthegreatendofittofitintotheboreofthetube，intowhichitwentaboutfiveinches；thenwhenthecanewasputintotheendofthetube，andthisexcited，thecanedrewtheleaf-brasstotheheightofmorethantwoinches，asdidalsotheivoryball，whenbyacorkandstickithadbeenfixedtotheendofthecane……WithseveralpiecesofSpanishcaneandfir-sticksIafterwardsmadearod，which，togetherwiththetube，wassomewhatmorethaneighteenfeetlong，whichwasthegreatestlengthIcouldconvenientlyuseinmychamber，andfoundtheattractionverynearly，ifnotaltogether，asstrongaswhentheballwasplacedontheshorterrods。"

Thisexperimentexhaustedthecapacityofhissmallroom，butongoingtothecountryalittlelaterhewasabletocontinuehisexperiments。"Toapoleofeighteenfeettherewastiedalineofthirty-fourfeetinlength，sothatthepoleandlinetogetherwerefifty-twofeet。WiththepoleandtubeIstoodinthebalcony，theassistantbelowinthecourt，whereheheldtheboardwiththeleaf-brassonit。Thenthetubebeingexcited，asusual，theelectricvirtuepassedfromthetubeupthepoleanddownthelinetotheivoryball，whichattractedtheleaf-brass，andastheballpassedoveritinitsvibrationstheleaf-brasswouldfollowittillitwascarriedofftheboard。"

Graynextattemptedtosendtheelectricityoveralinesuspendedhorizontally。Todothishesuspendedthepack-threadbypiecesofstringloopedovernailsdrivenintobeamsforthatpurpose。

Butwhenthussuspendedhefoundthattheivoryballnolongerexcitedtheleaf-brass，andheguessedcorrectlythattheexplanationofthislayinthefactthat"whentheelectricvirtuecametotheloopthatwassuspendedonthebeamitwentupthesametothebeam，"noneofitreachingtheball。Asweshallseefromwhatfollows，however，Grayhadnotasyetdeterminedthatcertainsubstanceswillconductelectricitywhileotherswillnot。Butbyaluckyaccidenthemadethediscoverythatsilk，forexample，wasapoorconductor，andcouldbeturnedtoaccountininsulatingtheconducting-cord。

AcertainMr。Whelerhadbecomemuchinterestedintheoldpensionerandhiswork，and，asaguestattheWhelerhouse，Grayhadbeenrepeatingsomeofhisformerexperimentswiththefishing-rod，line，andivoryball。Hehadfinallyexhaustedtheheightsfromwhichtheseexperimentscouldbemadebyclimbingtotheclock-towerandexcitingbitsofleaf-brassonthegroundbelow。

"Aswehadnogreaterheightshere，"hesays，"Mr。Whelerwasdesiroustotrywhetherwecouldnotcarrytheelectricvirtuehorizontally。IthentoldhimoftheattemptIhadmadewiththatdesign，butwithoutsuccess，tellinghimthemethodandmaterialsmadeuseof，asmentionedabove。Hethenproposedasilklinetosupportthelinebywhichtheelectricvirtuewastopass。Itoldhimitmightdobetteruponaccountofitssmallness；sothattherewouldbelessvirtuecarriedfromthelineofcommunication。

"Thefirstexperimentwasmadeinthemattedgallery，July2，1729，aboutteninthemorning。Aboutfourfeetfromtheendofthegallerytherewasacrosslinethatwasfixedbyitsendstoeachsideofthegallerybytwonails；themiddlepartofthelinewassilk，therestateachendpack-thread；thenthelinetowhichtheivoryballwashungandbywhichtheelectricvirtuewastobeconveyedtoitfromthetube，beingeightyandone-halffeetinlength，waslaidonthecrosssilkline，sothattheballhungaboutninefeetbelowit。Thentheotherendofthelinewasbyaloopsuspendedontheglasscane，andtheleaf-brassheldundertheballonapieceofwhitepaper；when，thetubebeingrubbed，theballattractedtheleaf-brass，andkeptitsuspendedonitforsometime。"

Thisexperimentsucceededsowellthatthestringwaslengtheneduntilitwassometwohundredandninety-threefeetlong；andstilltheattractiveforcecontinued，apparentlyasstrongasever。Onlengtheningthestringstillmore，however，theextraweightprovedtoomuchforthestrengthofthesilksuspending-thread。"Uponthis，"saysGray，"havingbroughtwithmebothbrassandironwire，insteadofthesilkweputupsmallironwire；butthiswastooweaktobeartheweightoftheline。

Wethentookbrasswireofasomewhatlargersizethanthatofiron。Thissupportedourlineofcommunication；butthoughthetubewaswellrubbed，yettherewasnottheleastmotionorattractiongivenbytheball，neitherwiththegreattube，whichwemadeuseofwhenwefoundthesmallsolidcanetobeineffectual；bywhichwewerenowconvincedthatthesuccesswehadbeforedependeduponthelinesthatsupportedthelineofcommunicationbeingsilk，andnotupontheirbeingsmall，asbeforetrialIhadimagineditmightbe；thesameeffecthappeninghereasitdidwhenthelinethatistoconveytheelectricvirtueissupportedbypack-thread。"

SoonafterthisGrayandhishostsuspendedapack-threadsixhundredandsixty-sixfeetlongonpolesacrossafield，thesepolesbeingslightlyinclinedsothatthethreadcouldbesuspendedfromthetopbysmallsilkcords，thussecuringthenecessaryinsulation。Thispack-threadline，suspendeduponpolesalongwhichGraywasabletotransmittheelectricity，isverysuggestiveofthemoderntelegraph，buttheideaofsignallingormakinguseofitforcommunicatinginanywayseemsnottohaveoccurredtoanyoneatthattime。EventhesuccessorsofGraywhoconstructedlinessomethousandsoffeetlongmadenoattempttousethemforanythingbutexperimentalpurposes——simplytotestthedistancesthatthecurrentcouldbesent。Nevertheless，Grayshouldprobablybecreditedwiththediscoveryoftwoofthemostimportantpropertiesofelectricity——thatitcanbeconductedandinsulated，although，aswehaveseen，GilbertandVonGuerickehadaninklingofboththeseproperties。

EXPERIMENTSOFCISTERNAYDUFAY

SofarEnglandhadproducedthetwoforemostworkersinelectricity。ItwasnowFrance’sturntotakeahand，and，throughtheeffortsofCharlesFrancoisdeCisternayDufay，toadvancethescienceofelectricityverymaterially。Dufaywasahighlyeducatedsavant，whohadbeensoldieranddiplomatbetimes，butwhoseversatilityandabilityasascientistisshownbythefactthathewastheonlymanwhohadevercontributedtotheannalsoftheacademyinvestigationsineveryoneofthesixsubjectsadmittedbythatinstitutionasworthyofrecognition。Dufayupheldhisreputationinthisnewfieldofscience，makingmanydiscoveriesandcorrectingmanymistakesofformerobservers。InthisworkalsoheprovedhimselfagreatdiplomatbyremainingontermsofintimatefriendshipwithDr。

Gray——athingthatfewpeoplewereabletodo。

Almosthisfirststepwastooverthrowthebeliefthatcertainbodiesare"electrics"andothers"non-electrics"——thatis，thatsomesubstanceswhenrubbedshowcertainpeculiaritiesinattractingpiecesofpaperandfoilwhichothersdonot。Dufayprovedthatallbodiespossessthisqualityinacertaindegree。

"Ihavefoundthatallbodies(metallic，soft，orfluidonesexcepted)，"hesays，"maybemadeelectricbyfirstheatingthemmoreorlessandthenrubbingthemonanysortofcloth。Sothatallkindsofstones，aswellpreciousascommon，allkindsofwood，and，ingeneral，everythingthatIhavemadetrialof，becameelectricbybeatingandrubbing，exceptsuchbodiesasgrowsoftbybeat，asthegums，whichdissolveinwater，glue，andsuchlikesubstances。’Tisalsotoberemarkedthatthehardeststonesormarblesrequiremorechafingorheatingthanothers，andthatthesameruleobtainswithregardtothewoods；

sothatbox，lignumvitae，andsuchothersmustbechafedalmosttothedegreeofbrowning，whereasfir，lime-tree，andcorkrequirebutamoderateheat。

"HavingreadinoneofMr。Gray’slettersthatwatermaybemadeelectricalbyholdingtheexcitedglasstubenearit(adishofwaterbeingfixedtoastandandthatsetonaplateofglass，oronthebrimofadrinking-glass，previouslychafed，orotherwisewarmed)，Ihavefound，upontrial，thatthesamethinghappenedtoallbodieswithoutexception，whethersolidorfluid，andthatforthatpurpose’twassufficienttosetthemonaglassstandslightlywarmed，oronlydried，andthenbybringingthetubenearthemtheyimmediatelybecameelectrical。Imadethisexperimentwithice，withalightedwood-coal，andwitheverythingthatcameintomymind；andIconstantlyremarkedthatsuchbodiesofthemselvesaswereleastelectricalhadthegreatestdegreeofelectricitycommunicatedtothemattheapprovaloftheglasstube。"

Hisnextimportantdiscoverywasthatcolorshadnothingtodowiththeconductionofelectricity。"Mr。Graysays，towardstheendofoneofhisletters，"hewrites，"thatbodiesattractmoreorlessaccordingtotheircolors。Thisledmetomakeseveralverysingularexperiments。Itookninesilkribbonsofequalsize，onewhite，oneblack，andtheothersevenofthesevenprimitivecolors，andhavinghungthemallinorderinthesameline，andthenbringingthetubenearthem，theblackonewasfirstattracted，thewhiteonenext，andothersinordersuccessivelytotheredone，whichwasattractedleast，andthelastofthemall。Iafterwardscutoutninesquarepiecesofgauzeofthesamecolorswiththeribbons，andhavingputthemoneafteranotheronahoopofwood，withleaf-goldunderthem，theleaf-goldwasattractedthroughallthecoloredpiecesofgauze，butnotthroughthewhiteorblack。Thisinclinedmefirsttothinkthatcolorscontributemuchtoelectricity，butthreeexperimentsconvincedmetothecontrary。Thefirst，thatbywarmingthepiecesofgauzeneithertheblacknorwhitepiecesobstructedtheactionoftheelectricaltubemorethanthoseoftheothercolors。Inlikemanner，theribbonsbeingwarmed，theblackandwhitearenotmorestronglyattractedthantherest。

Thesecondis，thegauzesandribbonsbeingwetted，theribbonsareallattractedequally，andallthepiecesofgauzeequallyintercepttheactionofelectricbodies。Thethirdis，thatthecolorsofaprismbeingthrownonawhitegauze，thereappearnodifferencesofattraction。Whenceitproceedsthatthisdifferenceproceeds，notfromthecolor，asacolor，butfromthesubstancesthatareemployedinthedyeing。ForwhenIcoloredribbonsbyrubbingthemwithcharcoal，carmine，andsuchothersubstances，thedifferencesnolongerprovedthesame。"

Inconnectionwithhisexperimentswithhisthreadsuspendedonglasspoles，Dufaynotedthatacertainamountofthecurrentislost，beinggivenofftothesurroundingair。Herecommended，therefore，thatthecordsexperimentedwithbewrappedwithsomenon-conductor——thatitshouldbe"insulated"("isolee")，ashesaid，firstmakinguseofthisterm。

DUFAYDISCOVERSVITREOUSANDRESINOUSELECTRICITY

IthasbeenshowninanearlierchapterhowVonGuerickediscoveredthatlightsubstanceslikefeathers，afterbeingattractedtothesulphur-ballelectric-machine，wererepelledbyituntiltheytouchedsomeobject。VonGuerickenotedthis，butfailedtoexplainitsatisfactorily。Dufay，repeatingVonGuericke’sexperiments，foundthatif，whiletheexcitedtubeorsulphurballisdrivingtherepelledfeatherbeforeit，theballbetouchedorrubbedanew，thefeathercomestoitagain，andisrepelledalternately，as，thehandtouchestheball，oriswithdrawn。Fromthisheconcludedthatelectrifiedbodiesfirstattractbodiesnotelectrified，"charge"themwithelectricity，andthenrepelthem，thebodysochargednotbeingattractedagainuntilithasdischargeditselectricitybytouchingsomething。

"OnmakingtheexperimentrelatedbyOttovonGuericke，"hesays，"whichconsistsinmakingaballofsulphurrenderedelectricaltorepeladownfeather，Iperceivedthatthesameeffectswereproducednotonlybythetube，butbyallelectricbodieswhatsoever，andIdiscoveredthatwhichaccountsforagreatpartoftheirregularitiesand，ifImayusetheterm，ofthecapricesthatseemtoaccompanymostoftheexperimentsonelectricity。

Thisprincipleisthatelectricbodiesattractallthatarenotso，andrepelthemassoonastheyarebecomeelectricbythevicinityorcontactoftheelectricbody。Thusgold-leafisfirstattractedbythetube，andacquiresanelectricitybyapproachingit，andofconsequenceisimmediatelyrepelledbyit。Norisitreattractedwhileitretainsitselectricquality。Butifwhileitisthussustainedintheairitchancetolightonsomeotherbody，itstraightwaylosesitselectricity，andinconsequenceisreattractedbythetube，which，afterhavinggivenitanewelectricity，repelsitasecondtime，whichcontinuesaslongasthetubekeepsitselectricity。Uponapplyingthisprincipletothevariousexperimentsofelectricity，onewillbesurprisedatthenumberofobscureandpuzzlingfactsthatitclearsup。ForMr。Hauksbee’sfamousexperimentoftheglassglobe，inwhichsilkthreadsareput，isanecessaryconsequenceofit。Whenthesethreadsarearrangedintheformofraysbytheelectricityofthesidesoftheglobe，ifthefingerbeputneartheoutsideoftheglobethesilkthreadswithinflyfromit，asiswellknown，whichhappensonlybecausethefingeroranyotherbodyappliedneartheglassglobeistherebyrenderedelectrical，andconsequentlyrepelsthesilkthreadswhichareendowedwiththesamequality。Withalittlereflectionwemayinthesamemanneraccountformostoftheotherphenomena，andwhichseeminexplicablewithoutattendingtothisprinciple。

"Chancehasthrowninmywayanotherprinciple，moreuniversalandremarkablethantheprecedingone，andwhichthrowsanewlightonthesubjectofelectricity。Thisprincipleisthattherearetwodistinctelectricities，verydifferentfromeachother，oneofwhichIcallvitreouselectricityandtheotherresinouselectricity。Thefirstisthatofglass，rock-crystal，preciousstones，hairofanimals，wool，andmanyotherbodies。Thesecondisthatofamber，copal，gumsack，silkthread，paper，andanumberofothersubstances。Thecharacteristicofthesetwoelectricitiesisthatabodyofthevitreouselectricity，forexample，repelsallsuchasareofthesameelectricity，andonthecontraryattractsallthoseoftheresinouselectricity；sothatthetube，madeelectrical，willrepelglass，crystal，hairofanimals，etc。，whenrenderedelectric，andwillattractsilkthread，paper，etc。，thoughrenderedelectricallikewise。Amber，onthecontrary，willattractelectricglassandothersubstancesofthesameclass，andwillrepelgum-sack，copal，silkthread，etc。Twosilkribbonsrenderedelectricalwillrepeleachother；

twowoollenthreadswilldothelike；butawoollenthreadandasilkenthreadwillmutuallyattracteachother。Thisprincipleverynaturallyexplainswhytheendsofthreadsofsilkorwoolrecedefromeachother，intheformofpencilorbroom，whentheyhaveacquiredanelectricquality。Fromthisprincipleonemaywiththesameeasededucetheexplanationofagreatnumberofotherphenomena；anditisprobablethatthistruthwillleadustothefurtherdiscoveryofmanyotherthings。

"Inordertoknowimmediatelytowhichofthetwoclassesofelectricsbelongsanybodywhatsoever，oneneedonlyrenderelectricasilkthread，whichisknowntobeoftheresinuouselectricity，andseewhetherthatbody，renderedelectrical，attractsorrepelsit。Ifitattractsit，itiscertainlyofthekindofelectricitywhichIcallVITREOUS；if，onthecontrary，itrepelsit，itisofthesamekindofelectricitywiththesilk——thatis，oftheRESINOUS。Ihavelikewiseobservedthatcommunicatedelectricityretainsthesameproperties；forifaballofivoryorwoodbesetonaglassstand，andthisballberenderedelectricbythetube，itwillrepelsuchsubstancesasthetuberepels；butifitberenderedelectricbyapplyingacylinderofgum-sacknearit，itwillproducequitecontraryeffects——namely，preciselythesameasgum-sackwouldproduce。Inordertosucceedintheseexperiments，itisrequisitethatthetwobodieswhichareputneareachother，tofindoutthenatureoftheirelectricity，berenderedaselectricalaspossible，forifoneofthemwasnotatallorbutweaklyelectrical，itwouldbeattractedbytheother，thoughitbeofthatsortthatshouldnaturallyberepelledbyit。Buttheexperimentwillalwayssucceedperfectlywellifbothbodiesaresufficientlyelectrical。"[1]

Aswenowknow，Dufaywaswronginsupposingthatthereweretwodifferentkindsofelectricity，vitreousandresinous。Alittlelaterthematterwasexplainedbycallingone"positive"

electricityandtheother"negative，"anditwasbelievedthatcertainsubstancesproducedonlytheonekindpeculiartothatparticularsubstance。Weshallseepresently，however，thatsometwentyyearslateranEnglishscientistdispelledthisillusionbyproducingbothpositive(orvitreous)andnegative(orresinous)electricityonthesametubeofglassatthesametime。

AfterthedeathofDufayhisworkwascontinuedbyhisfellow-countrymanDr。JosephDesaguliers，whowasthefirstexperimentertoelectrifyrunningwater，andwhowasprobablythefirsttosuggestthatcloudsmightbeelectrifiedbodies。Butabout，thistime——thatis，justbeforethemiddleoftheeighteenthcentury——thefieldofgreatestexperimentalactivitywastransferredtoGermany，althoughbothEnglandandFrancewerestillactive。ThetwoGermanphilosopherswhoaccomplishedmostatthistimewereChristianAugustHansenandGeorgeMatthiasBose，bothprofessorsinLeipsic。Bothseemtohaveconceivedtheidea，simultaneouslyandindependently，ofgeneratingelectricitybyrevolvingglobesrunbybeltandwheelinmuchthesamemannerastheapparatusofHauksbee。

WithsuchmachinesitwaspossibletogenerateamuchgreateramountofelectricitythanDufayhadbeenabletodowiththerubbedtube，andsoequipped，thetwoGermanprofessorswereabletogenerateelectricsparksandjetsoffireinamoststartlingmanner。Boseinparticularhadaloveforthespectacular，whichheturnedtoaccountwithhisnewelectricalmachineuponmanyoccasions。Ononeoftheseoccasionshepreparedanelaboratedinner，towhichalargenumberofdistinguishedguestswereinvited。Beforethearrivalofthecompany，however，Boseinsulatedthegreatbanquet-tableoncakesofpitch，andthenconnecteditwithahugeelectricalmachineconcealedinanotherroom。Allbeingready，andtheguestsintheirplacesabouttobeseated，Bosegaveasecretsignalforstartingthismachine，when，totheastonishmentoftheparty，flamesoffireshotfromflowers，dishes，andviands，givingamoststartlingbutbeautifuldisplay。

Toaddstillfurthertotheastonishmentofhisguests，Bosethenpresentedabeautifulyounglady，towhomeachoftheyoungmenofthepartywasintroduced。Insomemysteriousmannershewasinsulatedandconnectedwiththeconcealedelectricalmachine，sothataseachgallanttouchedherfingertipshereceivedanelectricshockthat"madehimreel。"Notcontentwiththis，thehostinvitedtheyoungmentokissthebeautifulmaid。Butthosewhowereboldenoughtoattemptitreceivedanelectricshockthatnearly"knockedtheirteethout，"astheprofessortellsit。

LUDOLFF’SEXPERIMENTWITHTHEELECTRICSPARK

ButBosewasonlyoneofseveralGermanscientistswhoweremakingelaborateexperiments。WhileBosewasconstructingandexperimentingwithhishugemachine，anotherGerman，ChristianFriedrichLudolff，demonstratedthatelectricsparksareactualfire——afactlongsuspectedbuthithertounproved。Ludolff’sdiscovery，asitchanced，wasmadeinthelecture-hallofthereorganizedAcademyofSciencesatBerlin，beforeanaudienceofscientistsandgreatpersonages，attheopeninglecturein1744。

Inthecourseofthislectureonelectricity，duringwhichsomeofthewell-knownmanifestationsofelectricitywerebeingshown，itoccurredtoLudolfftoattempttoignitesomeinflammablefluidbyprojectinganelectricsparkuponitssurfacewithaglassrod。Thisideawassuggestedtohimwhileperformingthefamiliarexperimentofproducingasparkonthesurfaceofabowlofwaterbytouchingitwithachargedglassrod。Heannouncedtohisaudiencetheexperimenthewasabouttoattempt，andhavingwarmedaspoonfulofsulphuricether，hetoucheditssurfacewiththeglassrod，causingittoburstintoflame。Thisexperimentleftnoroomfordoubtthattheelectricsparkwasactualfire。

AssoonasthisexperimentofLudolff’swasmadeknowntoBose，heimmediatelyclaimedthathehadpreviouslymadesimilardemonstrationsonvariousinflammablesubstances，bothliquidandsolid；anditseemshighlyprobablethathehaddoneso，ashewasconstantlyexperimentingwiththesparks，andmustalmostcertainlyhavesetcertainsubstancesablazebyaccident，ifnotbyintent。Atallevents，hecarriedonaseriesofexperimentsalongthislinetogoodpurpose，finallysucceedinginexplodinggun-powder，andsomakingthefirstforerunneroftheelectricfusesnowsouniversallyusedinblasting，firingcannon，andothersimilarpurposes。ItwasBosealsowho，observingsomeofthepeculiarmanifestationsinelectrifiedtubes，andnoticingtheirresemblanceto"northernlights，"wasoneofthefirst，ifnotthefirst，tosuggestthattheauroraborealisisofelectricorigin。

Thesespectaculardemonstrationshadtheeffectofcallingpublicattentiontothefactthatelectricityisamostwonderfulandmysteriousthing，tosaytheleast，andkeptbothscientistsandlaymenagogwithexpectancy。Bosehimselfwasaflamewithexcitement，andsodeterminedinhiseffortstoproducestillstrongerelectriccurrents，thathesacrificedthetubeofhistwenty-foottelescopefortheconstructionofamammothelectricalmachine。Withthisgreatmachineadischargeofelectricitywasgeneratedpowerfulenoughtowoundtheskinwhenithappenedtostrikeit。

Untilthistimeelectricityhadbeenlittlemorethanaplaythingofthescientists——or，atleast，nopracticalusehadbeenmadeofit。Asitwasapractisingphysician，Gilbert，whofirstlaidthefoundationforexperimentingwiththenewsubstance，soagainitwasamedicalmanwhofirstattemptedtoputittopracticaluse，andthatinthefieldofhisprofession。GottliebKruger，aprofessorofmedicineatHallein1743，suggestedthatelectricitymightbeofuseinsomebranchesofmedicine；andtheyearfollowingChristianGottliebKratzensteinmadeafirstexperimenttodeterminetheeffectsofelectricityuponthebody。

Hefoundthat"theactionoftheheartwasaccelerated，thecirculationincreased，andthatmusclesweremadetocontractbythedischarge":andhebeganatonceadministeringelectricityinthetreatmentofcertaindiseases。Hefoundthatitactedbeneficiallyinrheumaticaffections，andthatitwasparticularlyusefulincertainnervousdiseases，suchaspalsies。

Thiswasoveracenturyago，andto-dayaboutthemostimportantusemadeoftheparticularkindofelectricitywithwhichheexperimented(thestatic，orfrictional)isforthetreatmentofdiseasesaffectingthenervoussystem。

BythemiddleofthecenturyaperfectmaniaformakingelectricalmachineshadspreadoverEurope，andthewhirling，hand-rubbedglobesweregraduallyreplacedbygreatcylindersrubbedbywoollenclothsorpads，andgeneratingan"enormouspowerofelectricity。"Thesecylinderswererunbybeltsandfoot-treadles，andgaveamorepowerful，constant，andsatisfactorycurrentthanknownheretofore。Whilemakingexperimentswithoneofthesemachines，JohannHeinrichsWinklerattemptedtomeasurethespeedatwhichelectricitytravels。Todothisheextendedacordsuspendedonsilkthreads，withtheendattachedtothemachineandtheendwhichwastoattractthebitsofgold-leafnearenoughtogethersothattheoperatorcouldwatchandmeasuretheintervaloftimethatelapsedbetweenthestartingofthecurrentalongthecordanditsattractingthegold-leaf。Thelengthofthecordusedinthisexperimentwasonlyalittleoverahundredfeet，andthiswas，ofcourse，entirelyinadequate，thecurrenttravellingthatspaceapparentlyinstantaneously。

Theimprovedmethodofgeneratingelectricitythathadcomeintogeneralusemadeseveralofthescientistsagainturntheirattentionmoreparticularlytoattemptputtingittosomepracticalaccount。Theywerestimulatedtotheseeffortsbytheconstantreproachesthatwerebeginningtobeheardonallsidesthatelectricitywasmerelya"philosopher’splaything。"OneofthefirsttosucceedininventingsomethingthatapproachedapracticalmechanicalcontrivancewasAndrewGordon，aScotchBenedictinemonk。Heinventedanelectricbellwhichwouldringautomatically，andalittle"motor，"ifitmaybesocalled。Andwhileneitheroftheseinventionswereofanypracticalimportanceinthemselves，theywereattemptsintherightdirection，andwerethefirstancestorsofmodernelectricbellsandmotors，althoughtheprincipleuponwhichtheyworkedwasentirelydifferentfrommodernelectricalmachines。Themotorwassimplyawheelwithseveralprotrudingmetalpointsarounditsrim。Thesepointswerearrangedtoreceiveanelectricaldischargefromafrictionalmachine，thedischargecausingthewheeltorotate。Therewasverylittleforcegiventothisrotation，however，notenough，infact，tomakeitpossibletomorethanbarelyturnthewheelitself。Twomoregreatdiscoveries，galvanismandelectro-magneticinduction，werenecessarybeforethepracticalmotorbecamepossible。

ThesoberGordonhadatasteforthespectacularalmostequaltothatofBose。Itwashewhoignitedabowlofalcoholbyturningastreamofelectrifiedwateruponit，thuspresentingtheseemingparadoxoffireproducedbyastreamofwater。Gordonalsodemonstratedthepoweroftheelectricaldischargebykillingsmallbirdsandanimalsatadistanceoftwohundredells，theelectricitybeingconveyedthatdistancethroughsmallwires。