It　is　also　doubted　whether　the　monoplane　can
  be　made　as　strong　structurally　as　the　other　form；
  owing　to　the　lack　of　the　truss　formation　which　is
  the　strong　point　with　the　superposed　frame。　A
  truss　is　a　form　of　construction　where　braces　can
  be　used　from　one　member　to　the　next；　so　as　to
  brace　and　stiffen　the　whole。
  THE　BIPLANE。Nature　does　not　furnish　a　type
  of　creature　which　has　superposed　wings。　In　this
  particular　the　inventor　surely　did　not　follow　nature。
  The　reasons　which　led　man　to　employ　this
  type　may　be　summarized　as　follows：
  In　experimenting　with　planes　it　is　found　that
  a　broad　fore　and　aft　surface　will　not　lift　as　much
  as　a　narrow　plane。　This　subject　is　fully　explained
  in　the　chapter　on　The　Lifting　Surfaces　of
  Planes。　In　view　of　that　the　technical　descriptions
  of　the　operation　will　not　be　touched　upon
  at　this　place；　except　so　far　as　it　may　be　necessary
  to　set　forth　the　present　subject。
  This　peculiarity　is　due　to　the　accumulation　of
  a　mass　of　moving　air　at　the　rear　end　of　the　plane；
  which　detracts　from　its　lifting　power。　As　it
  would　be　a　point　of　structural　weakness　to　make
  the　wings　narrow　and　very　long；　Wenham　many
  years　ago　suggested　the　idea　of　placing　one　plane
  above　the　other；　and　later　on　Chanute；　an
  engineer；　used　that　form　almost　exclusively；　in
  experimenting　with　his　gliders。
  It　was　due　to　his　influence　that　the　Wrights
  adopted　that　form　in　their　gliding　experiments；
  and　later　on　constructed　their　successful　flyers
  in　that　manner。　Originally　the　monoplane　was
  the　type　generally　employed　by　experimenters；
  such　as　Lilienthal；　and　others。
  STABILITY　IN　BIPLANES。Biplanes　are　not　naturally
  as　stable　laterally　as　the　monoplane。
  The　reason　is；　that　a　downward　tilt　has　the　benefit
  of　only　a　narrow　surface；　comparable　with　the
  monoplane；　which　has　broadness　of　wing。
  To　illustrate　this；　let　us　assume　that　we　have
  a　biplane　with　planes　five　feet　from　front　to　rear；
  and　thirty…six　feet　in　length。　This　would　give
  two　planes　with　a　sustaining　surface　of　360　square
  feet。　The　monoplane　would；　probably；　divide
  this　area　into　one　plane　eight　and　a　half　feet　from
  front　to　rear；　and　42　feet　in　length。
  In　the　monoplane　each　wing　would　project　out
  about　three　feet　more　on　each　side；　but　it　would
  have　eight　and　a　half　feet　fore　and　aft　spread
  to　the　biplane's　five　feet；　and　thus　act　as　a　greater
  support。
  THE　ORTHOPTER。The　term　orthopter；　or　ornithopter；
  meaning　bird　wing；　is　applied　to　such
  flying　machines　as　depend　on　wing　motion　to　support
  them　in　the　air。
  Unquestionably；　a　support　can　be　obtained　by
  beating　on　the　air　but　to　do　so　it　is　necessary　to
  adopt　the　principle　employed　by　nature　to　secure
  an　upward　propulsion。　As　pointed　out　elsewhere；
  it　cannot　be　the　concaved　type　of　wing；
  or　its　shape；　or　relative　size　to　the　weight　it　must
  carry。
  As　nature　has　furnished　such　a　variety　of　data
  on　these　points；　all　varying　to　such　a　remarkable
  degree；　we　must　look　elsewhere　to　find　the　secret。
  Only　one　other　direction　offers　any　opportunity；
  and　that　is　in　the　individual　wing　movement。
  NATURE'S　TYPE　NOT　UNIFORM。When　this　is
  examined；　the　same　obscurity　surrounds　the　issue。
  Even　the　speeds　vary　to　such　an　extent　that　when
  it　is　tried　to　differentiate　them；　in　comparison
  with　form；　shape；　and　construction；　the　experimenter
  finds　himself　wrapt　in　doubt　and　perplexity。
  But　birds　do　fly；　notwithstanding　this　wonderful
  array　of　contradictory　exhibitions。　Observation
  has　not　enabled　us　to　learn　why　these　things
  are　so。　High　authorities；　and　men　who　are　expert
  aviators；　tell　us　that　the　bird　flies　because
  it　is　able　to　pick　out　ascending　air　currents。
  THEORIES　ABOUT　FLIGHT　OF　BIRDS。Then　we
  are　offered　the　theory　that　the　bird　has　an　instinct
  which　tells　it　just　how　to　balance　in　the
  air　when　its　wings　are　once　set　in　motion。
  Frequently；　what　is　taken　for　instinct；　is　something
  entirely　different。
  It　has　been　assumed；　for　instance；　that　a　cyclist
  making　a　turn　at　a　rapid　speed；　and　a　bird　flying
  around　a　circle　will　throw　the　upper　part　of　the
  body　inwardly　to　counteract　the　centrifugal　force
  which　tends　to　throw　it　outwardly。
  Experiments　with　the　monorail　car；　which　is
  equipped　with　a　gyroscope　to　hold　it　in　a　vertical
  position；　show　that　when　the　car　approaches　a
  curve　the　car　will　lean　inwardly；　exactly　the　same
  as　a　bird；　or　a　cyclist；　and　when　a　straight　stretch
  is　reached；　it　will　again　straighten　up。
  INSTINCT。Now；　either　the　car；　so　equipped
  possesses　instinct；　or　there　must　be　a　principle
  in　the　laws　of　nature　which　produces　the　similarity
  of　action。
  In　like　manner　there　must　be　some　principle
  that　is　entirely　independent　of　the　form　of　matter；
  or　its　arrangement；　which　enables　the　bird
  to　perform　its　evolutions。　We　are　led　to　believe
  from　all　the　foregoing　considerations　that　it　is
  the　manner　or　the　form　of　the　motion。
  MODE　OF　MOTION。In　this　respect　it　seems　to
  be　comparable　in　every　respect　to　the　great　and
  universal　law　of　the　motions　in　the　universe。
  Thus；　light；　heat　and　electricity　are　the　same；　the
  manifestations　being　unlike　only　because　they
  have　different　modes　of　motion。
  Everything　in　nature　manifests　itself　by　motion。
  It　is　the　only　way　in　which　nature　acts。
  Every　transformation　from　one　thing　to　another；
  is　by　way　of　a　movement　which　is　characteristic
  in　itself。
  Why；　then；　should　this　great　mystery　of　nature；
  act　unlike　the　other　portions　of　which　it　is
  a　part？
  THE　WING　STRUCTURE。The　wing　structure　of
  every　flying　creature　that　man　has　examined；　has
  one　universal　point　of　similarity；　and　that　is　the
  manner　of　its　connection　with　the　body。　It　is　a
  sort　of　universal　joint；　which　permits　the　wing
  to　swing　up　and　down；　perform　a　gyratory　movement
  while　doing　so；　and　folds　to　the　rear　when
  at　rest。
  Some　have　these　movements　in　a　greater　or
  less　degree；　or　capable　of　a　greater　range；　but
  the　joint　is　the　same；　with　scarcely　an　exception。
  When　the　stroke　of　the　wing　is　downwardly　the
  rear　margin　is　higher　than　the　front　edge；　so
  that　the　downward　beat　not　only　raises　the　body
  upwardly；　but　also　propels　it　forwardly。
  THE　WING　MOVEMENT。The　moment　the　wing
  starts　to　swing　upwardly　the　rear　end　is
  depressed；　and　now；　as　the　bird　is　moving　forwardly；
  the　wing　surface　has　a　positive　angle　of
  incidence；　and　as　the　wing　rises　while　the　forward
  motion　is　taking　place；　there　is　no　resistance
  which　is　effective　enough　to　counteract　the
  momentum　which　has　been　set　up。
  The　great　problem　is　to　put　this　motion　into　a
  mechanical　form。　The　trouble　is　not　ascribable
  to　the　inability　of　the　mechanic　to　describe　this
  movement。　It　is　an　exceedingly　simple　one。
  The　first　difficulty　is　in　the　material　that　must
  be　used。　Lightness　and　strength　for　the　wing
  itself　are　the　first　requirements。　Then　rigidity
  in　the　joint　and　in　the　main　rib　of　the　wing；　are
  the　next　considerations。
  In　these　respects　the　ability　of　man　is　limited。
  The　wing　ligatures　of　flying　creatures　is　exceedingly
  strong；　and　flexible；　the　hollow　bone　formation
  and　the　feathers　are　extremely　light；　compared
  with　their　sustaining　powers。
  THE　HELICOPTER　MOTION。The　helicopter；　or
  helix…wing；　is　a　form　of　flying　machine　which　depends
  on　revolving　screws　to　maintain　it　in　the
  air。　Many　propellers　are　now　made；　six　feet　in
  length；　which　have　a　pull　of　from　400　to　500
  pounds。　If　these　are　placed　on　vertically…disposed
  shafts　they　would　exert　a　like　power　to
  raise　a　machine　from　the　earth。
  Obviously；　it　is　difficult　to　equip　such　a　machine
  with　planes　for　sustaining　it　in　flight；　after　it　is
  once　in　the　air；　and　unless　such　means　are　provided
  the　propellers　themselves　must　be　the
  mechanism　to　propel　it　horizontally。
  This　means　a　change　of　direction　of　the　shafts
  which　support　the　propellers；　and　the　construction
  is　necessarily　more　complicated　than　if　they
  were　held　within　non…changeable　bearings。
  This　principle；　however；　affords　a　safer　means
  of　navigating　than　the　orthopter　type；　because
  the　blades　of　such　an　instrument　can　be　forced
  through　the　air　with　infinitely　greater　speed　than
  beating　wings；　and　it　devolves　on　the　inventor　to
  devise　some　form　of　apparatus　which　will　permit
  the　change　of　pull　from　a　vertical　to　a　horizontal
  direction　while　in　flight。
  CHAPTER　VI
  THE　LIFTING　SURFACES　OF　AEROPLANES
  THIS　subject　includes　the　form；　shape　and　angle
  of　planes；　used　in　flight。　It　is　the　direction　in
  which　most　of　the　energy　has　been　expended　in
  developing　machines；　and　the　true　form　is　still
  involved　in　doubt　and　uncertainty。
  RELATIVE　SPEED　AND　ANGLE。The　relative
  speed　and　angle；　and　the　camber；　or　the　curved
  formation　of　the　plane；　have　been　considered　in
  all　their　aspects；　so　that　the　art　in　thi