Yes，theAirshipIsHere.
Fromallofwhichwemaywellinferthattheflyingmachineinpracticalformhasarrived，andthatitisheretostay.Itisnoexaggerationtosaythatthetimeiscloseathandwhenpeoplewillkeepflyingmachinesjustastheynowkeepautomobiles，andthatpleasurejauntswillbefullyasnumerousandpopular.Withtheimportantitemofpracticabilityfullydemonstrated，"Come，takeatripinmyairship，"willhavemorerealsignificancethannowattachestothevapidwarblingsofthevaudevillevocalist.
Asafurtherevidencethattheairshipisreallyhere，andthatitspresenceisrecognizedinabusinessway，theactionoflifeandaccidentinsurancecompaniesisinteresting.Someofthemarereconstructingtheirpoliciessoastoincludeaspecialwaiverofinsurancebyaviators.Anythingwhichcompelsthesegreatcorporationstomodifytheirpoliciescannotbelookeduponasamerecuriosityortoy.
Itissomeconsolationtoknowthatthemovementinthisdirectionisnotthusfarwidespread.Moreoveritismorethanprobablethatthecompetitionforbusinesswilleventuallyinducethecompaniestoactmoreliberallytowardaviators，especiallyastheartofaviationadvances.
CHAPTERXIX.
LAWOFTHEAIRSHIP.
Successfulaviationhasevokedsomepeculiarthingsinthewayoflegalactionandinterpretationofthelaw.
Itiswellunderstoodthataman’spropertycannotbeusedwithouthisconsent.Thisisanoldestablishedprincipleincommonlawwhichholdsgoodtoday.
Thelimitsofaman’spropertylines，however，havenotbeensowellunderstoodbylaymen.AccordingtoeminentlegalauthoritiessuchasBlackstone，LittletonandCoke，the"fathersofthelaw，"theownerofrealtyalsoholdstitleaboveandbelowthesurface，andthistheoryisgenerallyacceptedwithoutquestionbythecourts.
RightsofPropertyOwners.
Inotherwordstheownerofrealtyalsoownstheskyaboveitwithoutlimitastodistance.Hecandigasdeepintohisland，orgoashighintotheairashedesires，providedhedoesnottrespassuponorinjuresimilarrightsofothers.
Theownerofrealtymayresistbyforce，allothermeanshavingfailed，anytrespassupon，orinvasionofhisproperty.Otherpeople，forinstance，maynotenteruponit，oroverorunderit，withouthisexpresspermissionandconsent.Thereisonlyoneexception，andthisisinthecaseofpublicutilitycorporationssuchasrailwayswhich，underthelawofeminentdomain，maycondemnarightofwayacrossthepropertyofanobstinateownerwhodeclinestoacceptafairpricefortheprivilege.
PrivilegeSharplyConfined.
Thelawofeminentdomainmaybetakenadvantageofonlybycorporationswhichareengagedinservingthepublic.Itisbasedupontheprinciplethattheadvancementandimprovementofacommunityisofmoreimportanceandcarrieswithitmorerightsthantheinterestsoftheindividualowner.Butevenincaseswheretherightofeminentdomainisexercisedtherecanbenoconfiscationoftheindividual’sproperty.
Exercisingtherightofeminentdomainismerelyobtainingbypublicpurchasewhatisheldtobeessentialtothepublicgood，andwhichcannotbesecuredbyprivatepurchase.Wheneminentdomainproceedingsareresortedtothecourtappointsappraiserswhodetermineuponthevalueofthepropertywanted，andthisvalue（inmoney）ispaidtotheowner.
HowItAffectsAviation.
Itshouldbekeptinmindthatthisprivilegeofthe"rightofeminentdomain"isaccordedonlytocorporationswhichareengagedinservingthepublic.Individualscannottakeadvantageofit.Thusfarallaviationhasbeenconductedbyindividuals;therearenoflyingmachineorairshipcorporationsregularlyengagedinthetransportationofpassengers，mailsorfreight.
Thisleadsuptothequestion"Whatwouldhappenifrealtyownersgenerally，orinanyconsiderablenumbers.
shouldprohibitthenavigationoftheairabovetheirholdings？"Itisidletosaysuchapossibilityisridiculous——
itisalreadyanactualityinafewindividualinstances.
OnepropertyownerinNewJersey，ajusticeofthepeace，maintainsalargesignontheroofofhishousewarningaviatorsthattheymustnottrespassuponhisdomain.Thatheisactingwellwithinhisrightsindoingthisisconcededbylegalauthorities.
HardtoCatchOffenders.
But，supposetheallegedtrespassiscommitted，whatisthepropertyownergoingtodoaboutit？Hemustfirstcatchthetrespasserandthiswouldbeaprettyhardjob.Hecertainlycouldnotovertakehim，unlesshekeptaracingaeroplaneforthisspecialpurpose.Itwouldbeequallydifficulttoindentifytheoffenderaftertheoffensehadbeencommitted，evenifhewerelocated，asaeroplanescarrynolicensenumbers.
Allowingthattheoffendershouldbecaughttheonlyrecourseoftherealtyownerisanactionfordamages.
Hemaypreventthecommissionoftheoffensebyforceifnecessary，butafteritiscommittedhecanonlysuefordamages.Andindoingthishewouldhavealotoftrouble.
PointstoBeProven.
Oneofthefirstthingstheplaintiffwouldbecalledupontoprovewouldbetheelevationofthemachine.
Ifitwerereasonablyclosetothegroundtherewould，ofcourse，begraveriskofdamagetofences，shrubbery，andotherproperty，andthecourtwouldbejustifiedinholdingittobeanuisancethatshouldbesuppressed.
If，ontheotherhand;themachinewaswellupintheair，butgoingslowly，orhoveringovertheplaintiff’sproperty，thecourtmightbeinclinedtorulethatitcouldnotpossiblybeanuisance，butrightherethecourtwouldbeinseriousembarrassment.Bydecidingthatitwasnotanuisancehewouldvirtuallyoverridethelawagainstinvasionofaman’spropertywithouthisconsentregardlessofthenatureoftheinvasion.Bythesamedecisionhewouldalsosayineffectthat，ifoneflyingmachinecoulddothisadozenormorewouldhaveequalrighttodothesamething.Whileonemachinehoveringoveracertainpieceofpropertymaybenoactualnuisanceadozenormoreinthesamepositioncouldhardlybeexcused.
DifficulttoFixDamages.
Suchaconditionwouldtendtogreatlyincreasetheriskofaccident，eitherthroughcollision，orbythecarelessnessoftheaviatorsindroppingarticleswhichmightcausedamagestothepeopleorpropertybelow.Insuchacaseitwouldundoubtedlybeanuisance，andinadditiontoafine，theoffenderwouldalsobeliableforthedamages.
Takingitforgrantedthatnoactualdamageisdone，andtheownermerelysuesonaccountoftheinvasionofhisproperty，howistheamountofcompensationtobefixedupon？Theownerhaslostnothing;nopartofhispossessionshasbeentakenaway;nothinghasbeeninjuredordestroyed;everythingisleftinexactlythesameconditionasbeforetheinvasion.Andyet，ifthelawisstrictlyinterpreted，theoffenderisliable.
RightofWayforAirships.
Somebodyhassuggestedtheorganizationofflying—
machinecorporationsascommoncarriers，whichwouldgivethemtherightofeminentdomainwithpowertocondemnarightofway.Butwhatwouldtheycondemn？
Thereisnothingtangibleintheair.Railwaysincondemningarightofwayspecifytangibleproperty（realty）withincertainlimits.Howwouldanaviatordesignateanyparticularrightofwaythroughtheairacertainnumberoffeetinwidth，andacertaindistancefromtheground？
Andyet，shouldthehighercourtsholdtotheletterofthelawanddecidethataviatorshavenorighttonavigatetheircraftoverprivateproperty，somethingwillhavetobedonetogetthemoutofthedilemma，asaviationistoofaradvancedtobediscarded.Fortunatelythereislittleprospectofanywidespreadantagonismamongpropertyownerssolongasaviatorsrefrainfrommakingnuisancesofthemselves.
PossibleSolutionOffered.
Onepossiblesolutionisofferedandthatistoconfinethepathofairshipstothepublichighwayssothatnobody’spropertyrightswouldbeinvaded.Inaddition，asamatterofpromotingsafetyforbothoperatorsandthosewhomayhappentobebeneaththeairshipsastheypassoveracourse，adoptionoftheFrenchrulesaresuggested.Theseareasfollows：
Aeroplanes，whenpassing，mustkeeptotheright，andpassatadistanceofatleast150feet.Theyarefreefromthisrulewhenflyingataltitudesofmorethan100
feet.Everymachinewhenflyingatnightorduringfoggyweathermustcarryagreenlightontheright，andaredlightontheleft，andawhiteheadlightonthefront.
Thesearesensiblerules，butmaybeimproveduponbytheadditionofasignalsystemofsomekind，eitherhorn，whistleorbell.
ResponsibilityofAviators.
Mr.JayCarverBossard，inrecentnumbersof_Fly_，bringsoutsomecuriousandinterestinglegalpointsinconnectionwithaviation，amongwhicharethefollowing：
"Privatepartieswhopossessaerialcraft，anddesiretooperatethesameinaerialterritoryotherthantheirown，mustobtainfromlandownersspecialpermissiontodoso，suchpermissiontobegrantedonlybyagreement，foundeduponavalidconsideration.Otherwise，passingoveranother’slandwillineachinstanceamounttoatrespass.
"Leavingthishighlytechnicalsideofthequestion，letusturntoanotherview：thecriminalandtortliabilityofownersandoperatorstoairshippassengers.IfAinvitesBtomakeanascensionwithhiminhismachine，andB，knowingthatAismerelyanenthusiasticamateurandfarfrombeinganexpert，acceptsandisthroughA’sinnocentnegligenceinjured，hehasnogroundsforrecovery.ButifAcontractswithB，totransporthimfromoneplacetoanother，foraconsideration，andBisinjuredbythepoorpilotingofA，AwouldbeliabletoBfordamageswhichwouldresult.
NowinordertosafeguardsuchpeopleasB，curioustothepointofrecklessness，thelawwillhavetorequireallairshipoperatorstohavealicense，andtosecurethislicenseairshippilotswillhavetomeetcertainrequirements.Hereagainisaquestion.Whoisgoingtosaywhetheranapplicantiscompetenttopilotaballoonorairship？
FineforanAeronaut.
"Anaeroplanewhilemaneuveringissuddenlycaughtbyatreacherousgaleandswepttotheground.Acrowdofpeoplehastenovertoseeiftheaeronautisinjured，andindoingsotrampleoverTax—payerSmith’sgarden，muchtothedetrimentofhisgrowingvegetablesandflowers.Whoisliableforthedamages？Queerasitmayseem，acaseverysimilartothiswasdecidedin1823，intheNewYorksupremecourt，anditwasheldthattheaeronautwasliableuponthefollowinggrounds：
’Torenderonemanliableintrespassfortheactsofothers，itmustappeareitherthattheyactedinconcert，orthattheactoftheone，ordinarilyandnaturallyproducedtheactsoftheothers，Ascendinginaballoonisnotanunlawfulact，butitiscertainthattheaeronauthasnocontroloveritsmotionhorizontally，butisatthesportofthewind，andistodescendwhenandhowhecan.Hisreachingtheearthisamatterofhazard.
Ifhisdescentwouldaccordingtothecircumstancesdrawacrowdofpeoplearoundhim，eitheroutofcuriosity，orforthepurposeofrescuinghimfromaperiloussituation，allthisheoughttohaveforeseen，andmustberesponsiblefor.’
AirNotReallyFree.
"Thegeneralbeliefamongpeopleis，thattheairisfree.Notonlyfreetobreatheandenjoy，butfreetotravelin，andthatnoonehasanydefinitejurisdictionover，orinanypartofit.Nowsupposethisweremadealegaldoctrine.Wouldamurderperpetratedabovethecloudshavetogounpunished？Undoubtedly.Forfeloniescommitteduponthehighseasampleprovisionismadefortheirpunishment，butnewprovisionswillhavetobemadeforcrimescommittedintheair.
RelationsofOwnerandEmployee.
"Itisageneralruleoflawthatamasterisboundtoprovidereasonablysafetools，appliancesandmachinesforhisservant.Howthisruleisgoingtobeappliedincasesofaeroplanes，remainstobeseen.Theaeroplaneownerwhohiresaprofessionalaeronaut，thatis，onewhohasqualifiedasanexpert，oweshimverylittlelegaldutytosupplyhimwithaperfectaeroplane.Theexpertissupposedtoknowasmuchregardingthemachineastheowner，ifnotmore，andhisacceptanceofhispositionrelievestheownerfromliability.Whentheownerhiresanamateuraeronauttoruntheaeroplane，andteacheshimhowtomanipulateit，eventhoughtheprescribedmannerofmanipulationwillmakeflightsafe，neverthelessifthemachineisvisiblydefective，orknowntobeso，anyinjurywhichresultstotheaeronauttheownerisliablefor.
AstoAeroplaneContracts.
"Atthepresenttimetherearemanyordersbeingplacedwithaeroplanemanufacturingcompanies.Therearesomeuniquequestionstoberaisedhereunderthelawofcontract.Itisanelementaryprincipleoflawthatnoonecanbecompelledtocompleteacontractwhichinitselfisimpossibletoperform.Forinstance，acontracttorowaboatacrosstheAtlanticintwoweeks，foraconsideration，couldneverbeenforcedbecauseitiswithinjudicialknowledgethatsuchanundertakingisbeyondhumanpower.Again，contractsformedforthedoingofactscontrarytonatureareneverenforcible，andhereiswhereourdifficultycomesin.Isitpossibletobuildamachineorspeciesofcraftwhichwilltransportapersonorgoodsthroughtheair？Thecourtsknowthatballoonsarepractical;thatis，theyknowthatabagfilledwithgashasaliftingpowerandcanmovethroughtheairatanappreciableheight.
Therefore，acontracttotransportapersoninsuchmannerisagoodcontract，andtheconditionsbeingfavorablecouldundoubtedlybeenforced.Butthepassengers’
rightofactionforinjurywouldbeverylimited.
NoRedressforPurchasers.
"Inthecaseofgivingwarrantiesonaeroplanes，wehaveyettoseejustwhatacourtisgoingtosay.Itiseasyenoughforamanufacturertoguaranteetobuildamachineofcertaindimensionsandaccordingtocertainspecifications，butwhenheinsertsaclauseinthecontracttotheeffectthatthemachinewillraiseitselffromthesurfaceoftheearth，defythelawsofgravity，andsoarintheheavensatthewilloftheaviator，heistosaytheleastcontractingtoperformamiracle.
"Untilaeroplaneshavebeenmadeandacceptedaspractical，nocourtwillforceamanufacturertoturnoutamachineguaranteedtofly.Sopurchaserscanwellrememberthatiftheirmachinesrefusetoflytheyhavenoredressagainstthemaker，forhecanalwayssay，’Theindustryisstillinitsexperimentalstage.’Incontractingforanenginenobuilderwillguaranteethattheparticularenginewillsuccessfullyoperatetheaeroplane.
Infacthecouldneverbeforcedtoliveuptosuchanagreement，shouldheagreetoastipulationofthatsort.Thebestanyenginemakerwillguaranteeistobuildanengineaccordingtospecifications."
CHAPTERXX.
SOARINGFLIGHT.
ByOctaveChanute.
[5]Thereisawonderfulperformancedailyexhibitedinsouthernclimesandoccasionallyseeninnortherlylatitudesinsummer，whichhasneverbeenthoroughlyexplained.Itisthesoaringorsailingflightofcertainvarietiesoflargebirdswhotransportthemselvesonrigid，unflappingwingsinanydesireddirection;whoinwindsof6to20milesperhour，circle，rise，advance，returnandremainaloftforhourswithoutabeatofwing，saveforgettingunderwayorconvenienceinvariousmaneuvers.
Theyappeartoobtainfromthewindaloneallthenecessaryenergy，eventoadvancingdeadagainstthatwind.
Thisfeatissomuchopposedtoourgeneralideasofphysicsthatthosewhohavenotseenitsometimesdenyitsactuality，andthosewhohaveonlyoccasionallywitnesseditsubsequentlydoubttheevidenceoftheirowneyes.Others，whohaveseentheexceptionalperformances，speculateonvariousexplanations，butthemajoritygiveitupasasortof"negativegravity."
[5]Aeronautics.
SoaringPowerofBirds.
Thewriterofthispaperpublishedinthe"AeronauticalAnnual"for1896and1897anarticleuponthesailingflightofbirds，inwhichhegavealistoftheauthorswhohaddescribedsuchflightorhadadvancedtheoriesforitsexplanation，andhepassedtheseinreview.Healsodescribedhisownobservationsandsubmittedsomecomputationstoaccountfortheobservedfacts.Thesecomputationswerecorrectasfarastheywent，buttheywerescanty.Itwas，forinstance，shownconvincinglybyanalysisthatagullweighing2.188pounds，withatotalsupportingsurfaceof2.015squarefeet，amaximumbodycross—sectionof0.126squarefeetandamaximumcross—
sectionofwingedgesof0.098squarefeet，patrollingonrigidwings（soaring）ontheweathersideofasteamerandmaintaininganupwardangleorattitudeof5degreesto7degreesabovethehorizon，inawindblowing12.78
milesanhour，whichwasdeflectedupward10degreesto20degreesbythesideofthesteamer（theseallbeingcarefullyobservedfacts），wasperfectlysustainedatitsown"relativespeed"of17.88milesperhourandextractedfromtheupwardtrendofthewindsufficientenergytoovercomealltheresistances，thisenergyamountingto6.44foot—poundspersecond.
GreatPowerofGulls.
Itwasshownthatthesamebirdinflappingflightincalmair，withanattitudeorincidenceof3degreesto5
degreesabovethehorizonandaspeedof20.4milesanhourwaswellsustainedandexpended5.88foot—poundspersecond，thisbeingattherateof204poundssustainedperhorsepower.Itwasstatedalsothatagullinitsobservedmaneuvers，risingupfromapileheadonunflappingwings，thenplungingforwardagainstthewindandsubsequentlyrisinghigherthanhisstartingpoint，musteithertimehisascentsanddescentsexactlywiththevariationsinwindvelocities，ormustmeetawindbillowrotatingonahorizontalaxisandcometoapoiseonitscrest，thusavailingofanascendingtrend.
Buttheobservationsfailedtodemonstratethatthevariationsofthewindgustsandthemovementsofthebirdwereabsolutelysynchronous，anditwasconjecturedthatthepeculiarshapeofthesoaringwingofcertainbirds，asdifferentiatedfromtheflappingwing，might，whenexperimentedupon，hereafteraccountfortheperformance.
MysterytobeExplained.
Thesecomputations，howeversatisfactorytheywereforthespeedofwindsobserved，failedtoaccountfortheobservedspiralsoaringofbuzzardsinverylightwindsandthewriterwascompelledtoconfess："Now，thisspiralsoaringinsteadybreezesof5to10milesperhourwhichareapparentlyhorizontal，andthroughwhichthebirdmaintainsanaveragespeedofabout20milesanhour，isthemysterytobeexplained.Itisnotaccountedfor，quantitatively，byanyofthetheorieswhichhavebeenadvanced，anditistheoneperformancewhichhasledsomeobserverstoclaimthatitwasdonethrough’aspiration.’i，e.，thatabirdacteduponbyacurrent，actuallydrewforwardintothatcurrentagainstitsexactdirectionofmotion."
BuzzardsSoarinDeadCalm.
Astillgreatermysterywaspropoundedbythefewobserverswhoassertedthattheyhadseenbuzzardssoaringinadeadcalm，maintainingtheirelevationandtheirspeed.AmongtheseobserverswasMr.E.C.Huffaker，atonetimeassistantexperimenterforProfessorLangley.
Thewriterbelievedandsaidthenthathemustinsomewayhavebeenmistaken，yet，tosatisfyhimself，hepaidseveralvisitstoMr.Huffaker，inEasternTennesseeandtookalonghisanemometer.Hesawquiteanumberofbuzzardssailingataheightof75to100feetinbreezesmeasuring5or6milesanhouratthesurfaceoftheground，andoncehesawonebuzzardsoaringapparentlyinadeadcalm.
Thewriterwasfairlybaffled.Thebirdwasnotsimplygliding，utilizinggravityoracquiredmomentum，hewasactuallycirclinghorizontallyindefianceofphysicsandmathematics.Ittooktwoyearsandawholeseriesoffurtherobservationstobringthosetwosciencesintoaccordwiththefacts.
ResultsofCloseObservations.
Curiouslyenoughthekeytotheperformanceofcirclinginalightwindoradeadcalmwasnotfoundthroughtheusualwayofgatheringhumanknowledge，i.e.，throughobservationsandexperiment.ThesehadfailedbecauseIdidnotknowwhattolookfor.Themysterywas，infact，solvedbyaneclecticprocessofconjectureandcomputation，butoncethesecomputationsindicatedwhatobservationsshouldbemade，theresultsgaveatoncethereasonsforthecirclingofthebirds，fortheirthenobservedattitude，andforthenecessityofanindependentinitialsustainingspeedbeforesoaringbegan.
BothMr.HuffakerandmyselfverifiedthedatamanytimesandImadethecomputations.
Theseobservationsdisclosedseveralfacts：
1st.——Thatwindsblowingfivetoseventeenmilesperhourfrequentlyhadrisingtrendsof10degreesto15
degrees，andthatuponoccasionswhenthereseemedtobeabsolutelynowind，therewasoftenneverthelessalocalrisingoftheairestimatedatarateoffourtoeightmilesormoreperhour.Thiswasascertainedbywatchingthistledown，andrisingfogsalongsideoftreesorhillsofknownheight.Everyonewillreadilyrealizethatwhenwalkingattherateoffourtoeightmilesanhourinadeadcalmthe"relativewind"isquiteinappreciabletothesensesandthatsucharisingairwouldnotbenoticed.
2nd.——Thatthebuzzard，sailinginanapparentlydeadhorizontalcalm，progressedatspeedsoffifteentoeighteenmilesperhour，asmeasuredbyhisshadowontheground.Itwasthoughtthattheairwasthenpossiblyrising8.8feetpersecond，orsixmilesperhour.
3rd.——Thatwhensoaringinverylightwindstheangleofincidenceofthebuzzardswasnegativetothehorizon——i.e.，thatwhenseencomingtowardtheeye，theafternoonlightshoneonthebackinsteadofonthebreast，aswouldhavebeenthecasehadtheanglebeeninclinedabovethehorizon.
4th.——Thatthesailingperformanceonlyoccurredafterthebirdhadacquiredaninitialvelocityofatleastfifteenoreighteenmilesperhour，eitherbyindustriousflappingorbydescendingfromaperch.
AnInterestingExperiment.
5th.——Thatthewholeresistanceofastuffedbuzzard，atanegativeangleof3degreesinacurrentofairof15.52milesperhour，was0.27pounds.ThistestwaskindlymadeforthewriterbyProfessorA.F.Zahminthe"windtunnel"oftheCatholicUniversityatWashington，D.C.，who，moreover，statedthattheresistanceofalivebirdmightbeless，asthedriedplumagecouldnotbemadetoliesmooth.
Thisparticularbuzzardweighedinlife4.25pounds，theareaofhiswingsandbodywas4.57squarefeet，themaximumcross—sectionofhisbodywas0.110squarefeet，andthatofhiswingedgeswhenfullyextendedwas0.244squarefeet.
Withthesedata，itbecamesurprisinglyeasytocomputetheperformancewiththecoefficientsofLilienthalforvariousanglesofincidenceandtodemonstratehowthisbuzzardcouldsoarhorizontallyinadeadhorizontalcalm，providedthatitwasnotaverticalcalm，andthattheairwasrisingattherateoffourorsixmilesperhour，thelowestobserved，andquiteinappreciablewithoutactualmeasuring.
SomeDataonBirdPower.
Themostdifficultcaseispurposelyselected.Forifweassumethatthebirdhaspreviouslyacquiredaninitialminimumspeedofseventeenmilesanhour（24.93
feetpersecond，nearlythelowestmeasured），andthattheairwasrisingverticallysixmilesanhour（8.80feetpersecond），thenwehaveasthetrendofthe"relativewind"encountered：
6
——=0.353，orthetangentof19degrees26’.
17
whichbringsthecaseintothecategoryofrisingwindeffects.Butthebirdwasobservedtohaveanegativeangletothehorizonofabout3degrees，asnearascouldbeguessed，sothathisangleofincidencetothe"relativewind"wasreducedto16degrees26’.
Therelativespeedofhissoaringwastherefore：
Velocity=squarerootof（17squared6squared）=18.03milesperhour.
Atthisspeed，usingtheLangleyco—efficientrecentlypracticallyconfirmedbytheaccurateexperimentsofMr.
Eiffel，theairpressurewouldbe：
18.03squaredX0.00327=1.063poundspersquarefoot.
IfweapplyLilienthal’sco—efficientsforanangleof6degrees26’，wehavefortheforceinaction：
Normal：4.57X1.063X0.912=4.42pounds.
Tangential：4.57X1.063X0.074=—0.359pounds，whichlatter，beingnegative，isapropellingforce.
ResultsAstonishScientists.
Thuswehaveabirdweighing4.25poundsnotonlythoroughlysupported，butimpelledforwardbyaforceof0.359pounds，atseventeenmilesperhour，whiletheexperimentsofProfessorA.F.Zahmshowedthattheresistanceat15.52milesperhourwasonly0.27pounds，17squaredor0.27X———————=0.324pounds，atseventeenmilesan15.52squaredhour.
Theseareastonishingresultsfromthedataobtained，andtheyleadtotheinquirywhethertheenergyoftherisingairissufficienttomakeupthelosseswhichoccurbyreasonoftheresistanceandfrictionofthebird’sbodyandwings，which，beingrounded，donotencounterairpressuresinproportiontotheirmaximumcross—section.
Wehavenoaccuratedataupontheco—efficientstoapplyandestimatesmadebymyselfprovedtobemuchsmallerthanthe0.27poundsresistancemeasuredbyProfessorZahm，sothatwewillfigurewiththelatterasmodified.Asthespeedisseventeenmilesperhour，or24.93feetpersecond，wehaveforthework：
Workdone，0.324X24.93=8.07footpoundspersecond.
EndorsedbyProf.Marvin.
Correspondingenergyofrisingairisnotsufficientatfourmilesperhour.Thisamountstobut2.10footpoundspersecond，butifweassumethattheairwasrisingattherateofsevenmilesperhour（10.26feetpersecond），atwhichthepressurewiththeLangleycoefficientwouldbe0.16poundspersquarefoot，wehaveon4.57squarefeetforenergyofrisingair：4.57X0.16X10.26=7.50
footpoundspersecond，whichisseentobestillalittletoosmall，butwellwithinthelimitsoferror，inviewofthehollowshapeofthebird’swings，whichreceivegreaterpressurethantheflatplanesexperimenteduponbyLangley.
ThesecomputationswerechieflymadeinJanuary，1899，andwerecommunicatedtoafewfriends，whofoundnofallacyinthem，butthoughtthatfewaviatorswouldunderstandthemifpublished.TheywerethensubmittedtoProfessorC.F.MarvinoftheWeatherBureau，whoiswellknownasaskillfulphysicistandmathematician.
Hewrotethattheywere，theoretically，entirelysoundandquantitatively，probably，asaccurateasthepresentstateofthemeasurementsofwindpressurespermitted.
Thewriterdetermined，however，towithholdpublicationuntilthefeatofsoaringflighthadbeenperformedbyman，partlybecausehebelievedthat，toensuresafety，itwouldbenecessarythatthemachineshouldbeequippedwithamotorinordertosupplementanydeficiencyinwindforce.
ConditionsUnfavorableforWrights.
Thefeatwouldhavebeenattemptedin1902byWrightbrothersifthelocalcircumstanceshadbeenmorefavorable.
Theywereexperimentingon"KillDevilHill，"
nearKittyHawk，N.C.Thissandhill，about100feethigh，isborderedbyasmoothbeachonthesidewhencecometheseabreezes，buthasmarshygroundattheback.
Wrightbrotherswereapprehensivethatiftheyroseontheascendingcurrentofairatthefrontandbegantocirclelikethebirds，theymightbecarriedbythedescendingcurrentpastthebackofthehillandlandinthemarsh.Theirglidingmachineofferednogreaterheadresistanceinproportionthanthebuzzard，andtheirglidinganglesofdescentarepracticallyasfavorable，butthebirdsperformedhigherupintheairthanthey.
Langley’sIdeaofAviation.
ProfessorLangleysaidinconcludinghispaperupon"TheInternalWorkoftheWind"：
"Thefinalapplicationoftheseprinciplestotheartofaerodromicsseems，then，tobe，thatwhileitisnotlikelythattheperfectedaerodromewilleverbeabletodispensealtogetherwiththeabilitytorelyatintervalsonsomeinternalsourceofpower，itwillnotbeindispensablethatthisaerodromeofthefutureshall，inordertogoanydistance——eventocircumnavigatetheglobewithoutalighting——needtocarryaweightoffuelwhichwouldenableittoperformthisjourneyunderconditionsanalogoustothoseofasteamship，butthatthefuelandweightneedonlybesuchastoenableittotakecareofitselfinexceptionalmomentsofcalm."
Nowthatdynamicflyingmachineshavebeenevolvedandarebeingbroughtundercontrol，itseemstobeworthwhiletomakethesecomputationsandthesucceedingexplanationsknown，sothatsomeboldmanwillattemptthefeatofsoaringlikeabird.Thetheoryunderlyingtheperformanceinarisingwindisnotnew，ithasbeensuggestedbyPenaudandothers，butithasattractedlittleattentionbecausetheexactdataandthemaneuversrequiredwerenotknownandthefeathadnotyetbeenperformedbyaman.Thepuzzlehasalwaysbeentoaccountfortheobservedactinverylightwinds，anditishopedthatbythepresentselectionofthemostdifficultcasetoexplain——i.e.，thesoaringinadeadhorizontalcalm——somebodywillattempttheexploit.
RequisitesforSoaringFlights.
Thefollowingaredeemedtobetherequisitesandmaneuverstomasterthesecretsofsoaringflight：
1st——Developadynamicflyingmachineweighingaboutonepoundpersquarefootofarea，withstableequilibriumandunderperfectcontrol，capableofglidingbygravityatanglesofoneinten（53/4degrees）instillair.
2nd.——Selectlocationswheresoaringbirdsaboundandoccasionswhererisingtrendsofgentlewindsarefrequentandtobereliedon.
3rd.——Obtainaninitialvelocityofatleast25feetpersecondbeforeattemptingtosoar.
4th.——Solocatethecenterofgravitythattheapparatusshallassumeanegativeangle，foreandaft，ofabout3degrees.
Calculationsshow，however，thatsufficientpropellingforcemaystillexistat0degrees，butdisappearsentirelyat4degrees.
5th.——Circlelikethebird.Simultaneouslywiththesteering，inclinetheapparatustothesidetowardwhichitisdesiredtoturn，sothatthecentrifugalforceshallbebalancedbythecentripetalforce.Theamountoftherequiredinclinationdependsuponthespeedandontheradiusofthecirclesweptover.
6th.——Risespirallylikethebird.Steerwiththehorizontalrudder，soastodescendslightlywhengoingwiththewindandtoascendwhengoingagainstthewind.Thebirdcirclesoveronespotbecausetherisingtrendsofwindaregenerallyconfinedtosmallareasorlocalchimneys，aspointedoutbySirH.Maximandothers.
7th.——Oncealtitudeisgained，progressmaybemadeinanydirectionbyglidingdownwardbygravity.
Thebird’sflyingapparatusandskillareasyetinfinitelysuperiortothoseofman，butthereareindicationsthatwithinafewyearsthelattermayevolvemoreaccuratelyproportionedapparatusandobtainabsolutecontroloverit.
Itishoped，therefore，thatiftherebefoundnoradicalerrorintheabovecomputations，theywillcarrytheconvictionthatsoaringflightisnotinaccessibletoman，asitpromisesgreateconomiesofmotivepowerinfavorablelocalitiesofrisingwinds.
Thewriterwillbegratefultoexpertswhomaypointoutanymistakecommittedindataorcalculations，andwillfurnishadditionalinformationtoanyaviatorwhomaywishtoattemptthefeatofsoaring.
CHAPTERXXI.
FLYINGMACHINESVS.BALLOONS.
Whilewonderfulsuccesshasattendedthedevelopmentofthedirigible（steerable）balloonthemostardentadvocatesofthisformofaerialnavigationadmitthatithasseriousdrawbacks.Someofthesemaybedescribedasfollows：
ExpenseandOtherItems.
GreatInitialExpense.——Themoderndirigibleballooncostsafortune.TheZeppelin，forinstance，costsmorethan$100，000（theseareofficialfigures）.
ExpenseofInflation.——Gasevaporatesrapidly，andaballoonmustbere—inflated，orpartiallyre—inflated，everytimeitisused.TheZeppelinholds460，000cubicfeetofgaswhich，evenat$1perthousand，wouldcost$460.
DifficultyofObtainingGas.——Ifaballoonsuddenlybecomesdeflated，byaccidentoratmosphericconditions，farfromasourceofgassupply，itispracticallyworthless.
Gasmustbepipedtoit，ortheballooncartedtothegashouse——anexpensiveproceedingineitherevent.
LackofSpeedandControl.
LackofSpeed.——Underthemostfavorableconditionsthemaximumspeedofaballoonis30milesanhour.
Itsgreatbulkmakesthehighspeedattainedbyflyingmachinesimpossible.
DifficultyofControl.——Whilethemoderndirigibleballoonisreadilyhandledincalmorlightwinds，itsbulkmakesitdifficulttocontrolinheavywinds.
TheElementofDanger.——Numerousballoonshavebeendestroyedbylightningandsimilarcauses.OneofthelargestoftheZeppelinswasthuslostatStuttgartin1908.
SomeBalloonPerformances.
Itisonlyamatteroffairnesstostatethat，underfavorableconditions，someverycreditablerecordshavebeenmadewithmodernballoons，viz：
November23d，1907，theFrenchdirigiblePatrie，travelled187milesin6hoursand45minutesagainstalightwind.Thiswasalittleover28milesanhour.
TheClement—Bayard，anotherFrenchmachine，soldtotheRussiangovernment，madeatripof125milesatarateof27milesanhour.
ZeppelinNo.3，carryingeightpassengers，andhavingatotalliftingcapacityof5，500poundsofballastinadditiontopassengers，weightofequipment，etc.，wastestedinOctober，1906，andmade67milesin2hoursand17minutes，about30milesanhour.
Thesearethebestballoontripsonrecord，andshowforcefullythelimitationsofspeed，thegreatestbeingnotover30milesanhour.
SpeedofFlyingMachines.
Opposedtotheballoonperformanceswehaveflyingmachinetrips（ofauthenticrecords）asfollows：
Bleriot——monoplane——in1908——52milesanhour.
Delagrange——June22，1908——101/2milesin16minutes，approximately42milesanhour.
Wrights——October，1905——themachinewastheninitsinfancy——24milesin38minutes，approximately44milesanhour.OnDecember31，1908，theWrightsmade77
milesin2hoursand20minutes.
Lambert，apupiloftheWrights，andusingaWrightbiplane，onOctober18，1909，covered29.82milesin49
minutesand39seconds，beingattherateof36milesanhour.Thisflightwasmadeataheightof1，312feet.
Latham——October21，1909——madeashortflight，about11minutes，intheteethofa40milegale，atBlackpool，Eng.HeusedanAntoniettemonoplane，andtheofficialreportsays："Thisexhibitionofnerve，daringandabilityisunparalledinthehistoryofaviation."
Farman——October20，1909——wasintheairfor1hour，32min.，16seconds，travelling47miles，1，184yards，adurationrecordforEngland.
Paulhan——January18，1901——471/2milesattherateof45milesanhour，maintaininganaltitudeoffrom1，000
to2，000feet.
ExpenseofProducingGas.
Gasisindispensableintheoperationofdirigibleballoons，andgasisexpensive.Besidesthisitisnotalwayspossibletoobtainitinsufficientquantitieseveninlargecities，asthesupplyonhandisgenerallyneededforregularcustomers.Suchascanbehadiseitherwaterorcoalgas，neitherofwhichisasefficientinliftingpowerashydrogen.
Hydrogenisthelightestandconsequentlythemostbuoyantofallknowngases.Itissecuredcommerciallybytreatingzincorironwithdilutesulphuricorhydrochloricacid.Theaveragecostmaybesafelyplacedat$10per1，000feetsothat，toinflateaballoonofthesizeoftheZeppelin，holding460，000cubicfeet，wouldcost$4，600.
ProportionsofMaterialsRequired.
Inmakinghydrogengasitiscustomarytoallow20
percentforlossbetweenthegenerationandtheintroductionofthegasintotheballoon.Thus，whiletheformulacallsforiron28timesheavierthantheweightofthehydrogenrequired，andacid49timesheavier，therealquantitiesare20percentgreater.Hydrogenweighsabout0.09ouncetothecubicfoot.Consequentlyifweneedsay450，000cubicfeetofgaswemusthave2，531.25
poundsinweight.Toproducethis，allowingforthe20
percentloss，wemusthave35timesitsweightiniron，orover44tons.Ofaciditwouldtake60timestheweightofthegas，ornearly76tons.
InTimeofEmergency.
Thesefiguresareappalling，andunderordinaryconditionswouldbeprohibitive，buttherearetimeswhentheballoonoperator，unabletoobtainwaterorcoalgas，mustfootthebills.Inmilitarymaneuvers，wherethefieldofoperationisfixed，itispossibletofurnishsuppliesofhydrogengasinportablecylinders，butonlongtripswheresuddenleakageorothercausemakesdescentinanunexpectedspotunavoidable，itbecomesaquestionofmakingyourownhydrogengasordesertingtheballoon.
Andwhenthisoccurstheballoonistisupagainstanotherseriousproposition——canhefindthenecessaryzincoriron？Canhegettheacid？
BalloonsforCommercialUse.
Despiteallthistheballoonhasitsuses.Ifthereistobesuchathingasaerialnavigationinacommercialway——thecarryingoffreightandpassengers——itwillcomethroughtheemploymentofsuchmonsterballoonsasCountZeppelinisbuilding.Buteventhenthecarryingcapacitymustofnecessitybelimited.ThelatestZeppelincreation，amonsterinsize，is450feetlong，and421/2feetindiameter.Thedimensionsaresuchastomakeallotherballoonslooklikepigmies;evenmanyocean—goingsteamersaremuchsmaller，andyetitspassengercapacityisverysmall.Onits36—hourflightinMay，1909，theZeppelin，carriedonlyeightpassengers.
Thespeed，however，wasquiterespectable，850milesbeingcoveredinthe36hours，atrifleover23milesanhour.Thereservebuoyancy，thatisthetotalliftingcapacityasidefromtheweightoftheairshipanditsequipment，isestimatedatthreetons.
CHAPTERXXII.
PROBLEMSOFAERIALFLIGHT.
InalecturebeforetheRoyalSocietyofArts，reportedinEngineering，F.W.Lanchestertookthepositionthatpracticalflightwasnottheabstractquestionwhichsomeapparentlyconsideredittobe，butaprobleminlocomotiveengineering.Theflyingmachinewasalocomotiveappliance，designednotmerelytoliftaweight，buttotransportitelsewhere，afactwhichshouldbesufficientlyobvious.Neverthelessoneoftheleadingscientificmenofthedayadvocatedatypeinwhichthis，themainfunctionoftheflyingmachine，wasoverlooked.
Whenthemachinewasconsideredasamethodoftransport，theverticalscrewtype，orhelicopter，becameatonceridiculous.Ithad，nevertheless，manyadvocateswhohadsomevagueandill—definednotionofsubsequentmotionthroughtheairaftertheweightwasraised.
HelicopterTypeUseless.
Whenefficiencyoftransportwasdemanded，thehelicoptertypewasentirelyoutofcourt.Almostallofitsadvocatesneglectedtheeffectofthemotionofthemachinethroughtheairontheefficiencyoftheverticalscrews.Theyeitherassumedthatthemotionwassoslowasnottomatter，orthatapatchofstillairaccompaniedthemachineinitsflight.Onlyoneformofthistypehadanypossibilityofsuccess.Inthisthereweretwoscrewsrunningoninclinedaxles——oneoneachsideoftheweighttobelifted.Theactionofsuchinclinedscrewwascurious，andinapreviouslecturehehadpointedoutthatitwasalmostexactlythesameasthatofabird’swing.Inhigh—speedracingcraftsuchinclinedscrewswereofnecessityoftenused，butitwasatasacrificeoftheirefficiency.Inanycasetheefficiencyoftheinclined—screwhelicoptercouldnotcomparewiththatofanaeroplane，andthattypemightbedismissedfromconsiderationsosoonasefficiencybecametherulingfactorofthedesign.
MustCompeteWithLocomotive.
Tojustifyitselftheaeroplanemustcompete，insomeregardorother，withotherlocomotiveappliances，performingoneormoreofthepurposesoflocomotionmoreefficientlythanexistingsystems.Itwouldbenouseunlessabletostemaircurrents，sothatitsvelocitymusthegreaterthanthatoftheworstwindsliabletobeencountered.
Toillustratethelimitationsimposedonthemotionofanaeroplanebywindvelocity，Mr.LanchestergavethediagramsshowninFigs.1to4.Thecircleineachcasewas，hesaid，describedwitharadiusequaltothespeedoftheaeroplaneinstillair，fromacenterplaced"down—wind"fromtheaeroplanebyanamountequaltothevelocityofthewind.
Fig.1thereforerepresentedthecaseinwhichtheairwasstill，andinthiscasetheaeroplanerepresentedby_A_hadperfectlibertyofmovementinanydirectionInFig.2thevelocityofthewindwashalfthatoftheaeroplane，andthelattercouldstillnavigateinanydirection，butitsspeedagainstthewindwasonlyone—
thirdofitsspeedwiththewind.
InFig.3thevelocityofthewindwasequaltothatoftheaeroplane，andthenmotionagainstthewindwasimpossible;butitcouldmovetoanypointofthecircle，butnottoanypointlyingtotheleftofthetangent_A__B_.Finally，whenthewindhadagreaterspeedthantheaeroplane，asinFig.4，themachinecouldmoveonlyindirectionslimitedbythetangents_A__C_
and_A__D_.
MatterofFuelConsumption.
Takingthecaseinwhichthewindhadaspeedequaltohalfthatoftheaeroplane，Mr.Lanchestersaidthatforagivenjourneyoutandhome，downwindandback，theaeroplanewouldrequire30percentmorefuelthanifthetripweremadeinstillair;whileifthejourneywasmadeatrightanglestothedirectionofthewindthefuelneededwouldbe15percentmorethaninacalm.This30percentextrawasquiteaheavyenoughadditiontothefuel;andtosecureeventhisfigureitwasnecessarythattheaeroplaneshouldhaveaspeedoftwicethatofthemaximumwindinwhichitwasdesiredtooperatethemachine.Again，asstatedinthelastlecture，toinsuretheautomaticstabilityofthemachineitwasnecessarythattheaeroplanespeedshouldbelargelyinexcessofthatofthegustsofwindliabletobeencountered.
EccentricitiesoftheWind.
Therewas，Mr.Lanchestersaid，alooseconnectionbetweentheaveragevelocityofthewindandthemaximumspeedofthegusts.Whentheaveragespeedofthewindwas40milesperhour，thatofthegustsmightbeequalormore.Atonemomenttheremightbeacalmorthedirectionofthewindevenreversed，followed，thenextmoment，byaviolentgust.Aboutthesameminimumspeedwasdesirableforsecurityagainstgustsaswasdemandedbyotherconsiderations.Sixtymilesanhourwastheleastfiguredesirableinanaeroplane，andthisshouldbeexceededasmuchaspossible.Actually，theWrightmachinehadaspeedof38milesperhour，whileFarman’sVoisinmachineflewat45milesperhour.
Bothmachineswereextremelysensitivetohighwinds，andthespeaker，inspiteofnewspaperreportstothecontrary，hadneverseeneitherflowninmorethanagentlebreeze.Thedampingoutoftheoscillationsoftheflightpath，discussedinthelastlecture，increasedwiththefourthpowerofthenaturalvelocityofflight，andrapiddampingformedtheeasiest，andsometimestheonly，defenseagainstdangerousoscillations.A
machinejuststableat35milesperhourwouldhavereasonablyrapiddampingifitsspeedwereincreasedto60milesperhour.
ThinksUseIsLimited.
Itwas，thelecturerproceeded，inconceivablethatanyveryextendeduseshouldbemadeoftheaeroplaneunlessthespeedwasmuchgreaterthanthatofthemotorcar.
Itmightinspecialcasesbeofservice，apartfromthisincreaseofspeed，asintheexplorationofcountriesdestituteofroads，butitwouldhavenogeneralutility.
Withanautomobileaveraging25to35milesperhour，almostanypartofEurope，Russiaexcepted，wasattainableinaday’sjourney.Aflyingmachineofbutequalspeedwouldhavenoadvantages，butifthespeedcouldberaisedto90or100milesperhour，thewholecontinentofEuropewouldbecomeaplayground，everypartbeingwithinadaylightflightofBerlin.Further，somemarinecraftnowhadspeedsof40milesperhour，andefficientlytofollowupandreportmovementsofsuchvesselsanaeroplaneshouldtravelat60milesperhouratleast.Hencefromallpointsofviewappearedtheimperativedesirabilityofveryhighvelocitiesofflight.Thedifficultiesofachievementwere，however，great.
WeightofLightestMotors.
Asshowninthefirstlectureofhiscourse，theresistancetomotionwasnearlyindependentofthevelocity，sothatthetotalworkdoneintransportingagivenweightwasnearlyconstant.Hencethequestionoffueleconomywasnotabartohighvelocitiesofflight，thoughshouldthesebecomeexcessive，thebodyresistancemightconstitutealargeproportionofthetotal.Thehorsepowerrequiredvariedasthevelocity，sothefactorgoverningthemaximumvelocityofflightwasthehorsepowerthatcouldbedevelopedonagivenweight.Atpresenttheweightperhorsepoweroffeather—weightmotorsappearedtorangefrom21/4poundsupto7
poundsperbrakehorsepower，someactualfiguresbeingasfollows：
Antoinette5lbs.
Fiat3lbs.
GnomeUnder3lbs.
Metallurgic8lbs.
Renault7lbs.
Wright6lbs.
Automobileengines，ontheotherhand，commonlyweighed12poundsto13poundsperbrakehorsepower.
Forshortflightsfueleconomywasoflessimportancethanasavingintheweightoftheengine.Forlongflights，however，thecasewasdifferent.Thus，ifthegasoleneconsumptionwas1/2poundperhorsepowerhour，andtheengineweighed3poundsperbrakehorsepower，thefuelneededforasix—hourflightwouldweighasmuchastheengine，butforhalfanhour’sflightitsweightwouldbeunimportant.
BestMeansofPropulsion.
Thebestmethodofpropulsionwasbythescrew，whichactinginairwassubjecttomuchthesameconditionsasobtainedinmarinework.Itsefficiencydependedonitsdiameterandpitchandonitsposition，whetherinfrontoforbehindthebodypropelled.Fromthistheoryofdynamicsupport，Mr.Lanchesterproceeded，theefficiencyofeachelementofascrewpropellercouldberepresentedbycurvessuchasweregiveninhisfirstlecturebeforethesociety，andfromthesecurvestheover—allefficiencyofanyproposedpropellercouldbecomputed，bymereinspection，withafairdegreeofaccuracy.Thesecurvesshowedthatthetipsoflong—bladedpropellerswereinefficient，aswasalsotheportionofthebladeneartheroot.Inactualmarinepracticethebladefrombosstotipwascommonlyofsuchalengththattheover—allefficiencywas95percentofthatofthemostefficientelementofit.
AdvocatesPropellersinRear.
Fromthesecurvesthediameterandappropriatepitchofascrewcouldbecalculated，andthenumberofrevolutionswasthenfixed.Thus，foraspeedof80feetpersecondthepitchmightcomeoutas8feet，inwhichcasetherevolutionswouldbe600perminute，whichmight，however，betoolowforthemotor.Itwasthennecessaryeithertogeardownthepropeller，aswasdoneintheWrightmachine，or，ifitwasdecidedtodriveitdirect，tosacrificesomeoftheefficiencyofthepropeller.
Ananalogouscasearoseintheapplicationofthesteamturbinetothepropulsionofcargoboats，aproblemasyetunsolved.Thepropellershouldalwaysbeaft，sothatitcouldabstractenergyfromthewakecurrent，andalsosothatitswashwasclearofthebodypropelled.
Thebestpossibleefficiencywasabout70percent，anditwassafetorelyupon66percent.
BenefitsofSoaringFlight.
Therewas，Mr.Lanchesterproceeded，somepossibilityoftheaeronautreducingthepowerneededfortransportbyhisadoptingtheprincipleofsoaringflight，asexemplifiedbysomebirds.Therewere，hecontinued，twodifferentmodesofsoaringflight.Intheonethebirdmadeuseoftheupwardcurrentofairoftentobefoundintheneighborhoodofsteepverticalcliffs.Thesecliffsdeflectedtheairupwardlongbeforeitactuallyreachedthecliff，awholeregionbelowbeingthustheseatofanupwardcurrent.Darwinhasnotedthatthecondorwasonlytobefoundintheneighborhoodofsuchcliffs.
Alongthesouthcoastalsothegullsmadefrequentuseoftheupcurrentsduetothenearlyperpendicularchalkcliffsalongtheshore.
Inthetropicsupcurrentswerealsocausedbytemperaturedifferences.Cumulusclouds，moreover，werenearlyalwaystheterminationsofsuchupcurrentsofheatedair，which，oncoolingbyexpansionintheupperregions，depositedtheirmoistureasfog.Thesecloudsmight，perhaps，proveusefulinthefutureinshowingtheaeronautwhereupcurrentsweretohefound.An—
othermodeofsoaringflightwasthatadoptedbythealbatross，whichtookadvantageofthefactthattheairmovedinpulsations，intowhichthebirdfitteditself，beingthusabletoextractenergyfromthewind.
Whetheritwouldbepossiblefortheaeronauttoemployasimilarmethodmustbelefttothefuturetodecide.
MainDifficultiesinAviation.
Inpracticalflightdifficultiesaroseinstartingandinalighting.Therewasalowerlimittothespeedatwhichthemachinewasstable，anditwasinadvisabletoleavethegroundtillthislimitwasattained.Similarly，inalightingitwasinexpedienttoreducethespeedbelowthelimitofstability.Thisfactconstitutedadifficultyintheadoptionofhighspeeds，sincethelengthofrunneededincreasedinproportiontothesquareofthevelocity.Thisdrawbackcould，however，besurmountedbyformingstartingandalightinggroundsofamplesize.
Hethoughtitquitelikelyinthefuturethatsuchgroundswouldbeconsideredasessentialtotheflyingmachineasaseaportwastoanocean—goingsteamerorasaroadwastotheautomobile.
RequisitesofFlyingMachine.
Flyingmachineswerecommonlydividedintomonoplanesandbiplanes，accordingastheyhadoneortwosupportingsurfaces.Thedistinctionwasnot，however，fundamental.Togettherequisitestrengthsomeformofgirderframeworkwasnecessary，anditwasamerequestionofconveniencewhetherthesupportingsurfacewasarrangedalongboththetopandthebottomofthisgirder，oralongthebottomonly.Theframeworkadopteduniversallywasofwoodbracedbytiesofpianofortewire，anarrangementgivingthestiffnessdesiredwiththeleastpossibleweight.Somekindofchassiswasalsonecessary.
CHAPTERXXIII.
AMATEURSMAYUSEWRIGHTPATENTS.
OwingtothefactthattheWrightbrothershaveenjoinedanumberofprofessionalaviatorsfromusingtheirsystemofcontrol，amateurshavebeenslowtoadoptit.Theyrecognizeitsmerits，andwouldliketousethesystem，buthavebeenapprehensivethatitmightinvolvetheminlitigation.Thereisnodangerofthis，aswillbeseenbythefollowingstatementmadebytheWrights：
WhatWrightBrothersSay.
"Anyamateur，anyprofessionalwhoisnotexhibitingformoney，isatlibertytouseourpatenteddevices.
Weshallbegladtohavethemdoso，andtherewillbenointerferenceonourpart，bylegalaction，orotherwise.
Theonlymenweproceedagainstarethosewho，withoutourpermission，withoutevenaskingourconsent，coollyappropriatetheresultsofourlaborsandusethemforthepurposeofmakingmoney.Curtiss，Delagrange，Voisin，andalltherestofthemwhohaveusedourdeviceshavedonesoinmoney—makingexhibitions.Solongasthereisanymoneytobemadebytheuseoftheproductsofourbrains，weproposetohaveitourselves.
Itistheonlywayinwhichwecangetanyreturnfortheyearsofpatientworkwehavegiventotheproblemofaviation.Ontheotherhand，anymanwhowantstousethesedevicesforthepurposeofpleasure，ortheadvancementofscience，iswelcometodoso，withoutmoneyandwithoutprice.Thisisfairenough，isitnot？"
BasisoftheWrightPatents.
Inaflyingmachineanormallyflataeroplanehavinglateralmarginalportionscapableofmovementtodifferentpositionsaboveorbelowthenormalplaneofthebodyoftheaeroplane，suchmovementbeingaboutanaxistransversetothelineofflight，wherebysaidlateralmarginalportionsmaybemovedtodifferentanglesrelativelytothenormalplaneofthebodyoftheaeroplane，soastopresenttotheatmospheredifferentanglesofincidence，andmeansforsomovingsaidlateralmarginalportions，substantiallyasdescribed.
Applicationofverticalstrutsneartheendshavingflexiblejoints.
Meansforsimultaneouslyimpartingsuchmovementtosaidlateralportionstodifferentanglesrelativelytoeachother.
Referstothemovementofthelateralportionsonthesamesidetothesameangle.
Meansforsimultaneouslymovingverticalruddersoastopresenttothewindthatsidethereofnearestthesideoftheaeroplanehavingthesmallestangleofincidence.
Lateralstabilityisobtainedbywarpingtheendwingsbymovingtheleverattherighthandoftheoperator，connectionbeingmadebywiresfromthelevertothewingtips.Theruddermayalsobecurvedorwarpedinsimilarmannerbyleveraction.
WrightsObtainanInjunction.
InJanuary，1910，JudgeHazel，oftheUnitedStatesCircuitCourt，grantedapreliminaryinjunctionrestrainingtheHerring—CurtissCo.，andGlennH.Curtiss，frommanufacturing，selling，orusingforexhibitionpurposesthemachineknownastheCurtissaeroplane.TheinjunctionwasobtainedonthegroundthattheCurtissmachineisaninfringementupontheWrightpatentsinthematterofwingwarpingandruddercontrol.
Itisnotthepurposeoftheauthorstodiscussthesubjectproorcon.Suchdiscussionwouldhavenoproperplaceinavolumeofthiskind.ItisenoughtosaythatCurtissstoutlyinsiststhathismachineisnotaninfringementoftheWrightpatents，althoughJudgeHazelevidentlythinksdifferently.
WhattheJudgeSaid.
Ingrantingthepreliminaryinjunctionthejudgesaid：
"Defendantsclaimgenerallythatthedifferenceinconstructionoftheirapparatuscausestheequilibriumorlateralbalancetobemaintainedanditsaerialmovementsecureduponanentirelydifferentprinciplefromthatofcomplainant;thedefendants’aeroplanesarecurved，firmlyattachedtothestanchionsandhenceareincapableoftwistingorturninginanydirection;thatthesupplementaryplanesorso—calledruddersaresecuredtotheforwardstanchionattheextremelateralendsoftheplanesandareadjustedmidwaybetweentheupperandlowerplaneswiththemarginsextendingbeyondtheedges;thatinmovingthesupplementaryplanesequalanduniformanglesofincidencearepresentedasdistinguishedfromfluctuatinganglesofincidence.Suchclaimedfunctionaleffects，however，arestronglycontradictedbytheexpertwitnessforcomplainant.
SimilartoPlanofWrights.
"UponthiscontentionitissufficienttosaythattheaffidavitsforthecomplainantsoclearlydefinetheprincipleofoperationoftheflyingmachinesinquestionthatIamreasonablysatisfiedthatthereisavariablenessoftheangleofincidenceinthemachineofdefendantswhichisproducedwhenasupplementaryplaneononesideistiltedorraisedandtheotherstimultaneouslytiltedorlowered.Iamalsosatisfiedthattherearrudderisturnedbytheoperatortothesidehavingtheleastangleofincidenceandthatsuchturningisdoneatthetimethesupplementaryplanesareraisedordepressedtopreventtiltingorupsettingthemachine.