of　Fig。　7；　and　the　drift　is　correspondingly　decreased。
  _Fig。　7。　Equal　Lift　and　Drift　in　Flight。_
  _Fig。　8。　Unequal　Lift　and　Drift。_
  MOVING　PLANES　VS。　WINDS。In　this　way　Boisset；
  Duchemin；　Langley；　and　others；　determined
  the　comparative　drift；　and　those　results　have　been
  largely　relied　upon　by　aviators；　and　assumed　to
  be　correct　when　applied　to　flying　machines。
  That　they　are　not　correct　has　been　proven　by
  the　Wrights　and　others；　the　only　explanation　being
  that　some　errors　had　been　made　in　the　calculations；
  or　that　aviators　were　liable　to　commit　errors
  in　observing　the　true　angle　of　the　planes
  while　in　flight。
  MOMENTUM　NOT　CONSIDERED。The　great　factor
  of　momentum　has　been　entirely　ignored；　and　it　is
  our　desire　to　press　the　important　point　on　those
  who　begin　to　study　the　question　of　flying　machines。
  THE　FLIGHT　OF　BIRDS。Volumes　have　been
  written　concerning　observations　on　the　flight　of
  birds。　The　marvel　has　been　why　do　soaring　birds
  maintain　themselves　in　space　without　flapping
  their　wings。　In　fact；　it　is　a　much　more　remarkable
  thing　to　contemplate　why　birds　which　depend
  on　flapping　wings　can　fly。
  THE　DOWNWARD　BEAT。It　is　argued　that　the
  downward　beat　of　the　wings　is　so　much　more
  rapid　than　the　upward　motion；　that　it　gets　an　action
  on　the　air　so　as　to　force　the　body　upwardly。
  This　is　disposed　of　by　the　wing　motion　of　many
  birds；　notoriously　the　crow；　whose　lazily…flapping
  wings　can　be　readily　followed　by　the　eye；　and　the
  difference　in　movement；　if　any；　is　not　perceptible。
  THE　CONCAVED　WING。It　is　also　urged　that　the
  concave　on　the　under　side　of　the　wing　gives　the
  quality　of　lift。　Certain　kinds　of　beetles；　and　particularly
  the　common　house　fly；　disprove　that　theory；
  as　their　wings　are　perfectly　flat。
  FEATHER　STRUCTURE　CONSIDERED。Then　the
  feather　argument　is　advanced；　which　seeks　to
  show　that　as　each　wing　is　made　up　of　a　plurality
  of　feathers；　overlapping　each　other；　they　form　a
  sort　of　a　valved　surface；　opening　so　as　to　permit
  air　to　pass　through　them　during　the　period　of
  their　upward　movement；　and　closing　up　as　the
  wing　descends。
  It　is　difficult　to　perform　this　experiment　with
  wings；　so　as　to　show　such　an　individual　feather
  movement。　It　is　certain　that　there　is　nothing　in
  the　structure　of　the　wing　bone　and　the　feather
  connection　which　points　to　any　individual　feather
  movement；　and　our　observation　is；　that　each
  feather　is　entirely　too　rigid　to　permit　of　such　an
  opening　up　between　them。
  It　is　obvious　that　the　wing　is　built　up　in　that
  way　for　an　entirely　different　reason。　Soaring
  birds；　which　do　not　depend　on　the　flapping　motion；
  have　the　same　overlapping　feather　formation。
  WEBBED　WINGS。Furthermore；　there　are　numerous
  flying　creatures　which　do　not　have
  feathered　wings；　but　web…like　structures；　or　like　the
  house　fly；　in　one　continuous　and　unbroken
  plane。
  That　birds　which　fly　with　flapping　wings　derive
  their　support　from　the　air；　is　undoubtedly　true；
  and　that　the　lift　produced　is　due；　not　to　the　form；
  or　shape；　or　area　of　the　wing；　is　also　beyond　question。
  The　records　show　that　every　conceivable
  type　of　outlined　structure　is　used　by　nature；　the
  material　and　texture　of　the　wings　themselves　differ
  to　such　a　degree　that　there　is　absolutely　no
  similarity；　some　have　concaved　under　surfaces；
  and　others　have　not；　some　fly　with　rapidly　beating
  wings；　and　others　with　slow　and　measured
  movements；　many　of　them　fly　with　equal　facility
  without　flapping　movements；　and　the　proportions
  of　weight　to　wing　surface　vary　to　such　an　extent
  that　it　is　utterly　impossible　to　use　such　data　as　a
  guide　in　calculating　what　the　proper　surface
  should　be　for　a　correct　flying　machine。
  THE　ANGLE　OF　MOVEMENT。How；　then；　it　may
  be　asked；　do　they　get　their　support？　There　must
  be　something；　in　all　this　variety　and　diversity　of
  form；　of　motion；　and　of　characteristics；　which
  supplies　the　true　answer。　The　answer　lies　in　the
  angle　of　movement　of　every　wing　motion；　which
  is　at　the　control　of　the　bird；　and　if　this　is　examined
  it　will　be　found　that　it　supplies　the　correct
  answer　to　every　type　of　wing　which　nature　has
  made。
  AN　INITIAL　IMPULSE　OR　MOVEMENT　NECESSARY。
  Let　A；　Fig。　9；　represent　the　section　of　a　bird's
  wing。　All　birds；　whether　of　the　soaring　or　the
  flapping　kind；　must　have　an　initial　forward　movement
  in　order　to　attain　flight。　This　impulse　is
  acquired　either　by　running　along　the　ground；　or
  by　a　leap；　or　in　dropping　from　a　perch。　Soaring
  birds　cannot；　by　any　possibility；　begin　flight；
  unless　there　is　such　a　movement　to　change　from　a
  position　of　rest　to　one　of　motion。
  _Fig。　9。　Wing　Movement　in　Flight。_
  In　the　diagram；　therefore；　the　bird；　in　moving
  forwardly；　while　raising　the　wing　upwardly；　depresses
  the　rear　edge　of　the　wing；　as　in　position
  1；　and　when　the　wing　beats　downwardly　the　rear
  margin　is　raised；　in　relation　to　its　front　margin；
  as　shown　in　position　2。
  A　WEDGING　MOTION。Thus　the　bird；　by　a
  wedge…like　motion；　gives　a　forwardly…propelling
  action；　and　as　the　rear　margin　has　more　or　less
  flexure；　its　action　against　the　air　is　less　during　its
  upward　beat；　and　this　also　adds　to　the　upward　lift
  of　the　body　of　the　bird。
  NO　MYSTERY　IN　THE　WAVE　MOTION。There　is
  no　mystery　in　the　effect　of　such　a　wave…like　motion；
  and　it　must　be　obvious　that　the　humming
  bird；　and　like　flyers；　which　poise　at　one　spot；　are
  able　to　do　so　because；　instead　of　moving　forwardly；
  or　changing　the　position　of　its　body　horizontally；
  in　performing　the　undulatory　motion　of
  the　wing；　it　causes　the　body　to　rock；　so　that　at　the
  point　where　the　wing　joins　the　body；　an　elliptical
  motion　is　produced。
  _Fig。　10。　Evolution　of　Humming…Bird's　Wing。_
  HOW　BIRDS　POISE　WITH　FLAPPING　WINGS。This
  is　shown　in　Fig。　10；　in　which　eight　successive　positions
  of　the　wing　are　shown；　and　wherein　four
  of　the　position；　namely；　1；　2；　3；　and　4；　represent
  the　downward　movement；　and　6；　7；　8；　and　9；　the
  upward　beat。
  All　the　wing　angles　are　such　that　whether　the
  suspension　point　of　each　wing　is　moving　downwardly；
  or　upwardly；　a　support　is　found　in　some
  part　of　the　wing。
  NARROW…WINGED　BIRDS。Birds　with　rapid　flapping
  motions　have　comparatively　narrow　wings；
  fore　and　aft。　Those　which　flap　slowly；　and　are
  not　swift　flyers；　have　correspondingly　broader
  wings。　The　broad　wing　is　also　typical　of　the
  soaring　birds。
  But　how　do　the　latter　overcome　gravitation
  without　exercising　some　sort　of　wing　movement？
  INITIAL　MOVEMENT　OF　SOARING　BIRDS。Acute
  observations　show　that　during　the　early　stages
  of　flight；　before　speed　is　acquired；　they　depend
  on　the　undulating　movement　of　the　wings；　and
  some　of　them　acquire　the　initial　motion　by　flapping。
  When　speed　is　finally　attained　it　is　difficult
  for　the　eye　to　note　the　motion　of　the　wings。
  SOARING　BIRDS　MOVE　SWIFTLY。Now；　the　first
  observation　is；　that　soaring　birds　are　swiftly…
  moving　creatures。　As　they　sail　overhead
  majestically　they　seem　to　be　moving　slowly。　But
  distance　is　deceptive。　The　soaring　bird　travels
  at　great　speeds；　and　this　in　itself　should　be　sufficient
  to　enable　us　to　cease　wondering；　when　it　is
  remembered　that　swift　translation　decreases
  weight；　so　that　this　factor　does　not；　under　those
  conditions；　operate　against　flight。
  MUSCULAR　ENERGY　EXERTED　BY　SOARING　BIRDS。
  It　is　not　conceivable　that　the　mere　will　of　the
  bird　would　impel　it　forwardly；　without　it　exerted
  some　muscular　energy　to　keep　up　its　speed。　The
  distance　at　which　the　bird　performs　this　wonderful
  evolution　is　at　such　heights　from　the　observer
  that　the　eye　cannot　detect　a　movement。
  WINGS　NOT　MOTIONLESS。While　the　wings　appear
  to　be　absolutely　motionless；　it　is　more　reasonable
  to　assume　that　a　slight　sinuous　movement；
  or　a　rocking　motion　is　constantly　kept　up；　which
  wedges　forwardly　with　sufficient　speed　to　compel
  momentum　to　maintain　it　in　flight。　To　do　so　requires
  but　a　small　amount　of　energy。　The　head
  resistance　of　the　bird　formation　is　reduced　to　a
  minimum；　and　at　such　high　speeds　the　angle　of
  incidence　of　the　wings　is　very　small；　requiring　but
  little　aid　to　maintain　it　in　horizontal　flight。
  CHAPTER　II
  PRINCIPLES　OF　AEROPLANE　FLIGHT
  FROM　the　foregoing　chapter；　while　it　may　be
  rightly　inferred　that　power　is　the　true　secret　of
  aeroplane　flight；　it　is　desirable　to　point　out　certain
  other　things　which　must　be　considered。
  SPEED　AS　ONE　OF　THE　ELEMENTSEvery　boy；
  probably；　has　at　some　time　or　other　thrown　small
  flat　stones；　called　〃skippers。〃　He　has　noticed
  that　if　they　are　particularly　thin；　and　large　in
  diameter；　that　there　is　a　peculiar　sailing　motion；
  and　that　they　move　through　the　air　in　an　undulating
  or　wave…like　path。
  Two　things　contribute　to　this　motion；　one　is　the
  size　of　the　skipper；　relative　to　its　weight；　and　the
  other　is　its　speed。　I