of　going　into　a　〃hole。〃
  RESPONSIBILITY　FOR　ACCIDENTS。These　so…called
  〃holes〃　are　responsible　for　many　accidents。　The
  outstretched　wings；　many　of　them　over　forty　feet
  from　tip　to　tip；　offer　opportunities　for　a　tilt　at　one
  end　or　the　other；　which　has　sent　so　many　machines
  to　destruction。
  The　high　center　of　gravity　in　all　machines　makes
  the　weight　useless　to　counterbalance　the　rising
  end　or　to　hold　up　the　depressed　wing。
  All　aviators　agree　that　these　unequal　areas　of
  density　extend　over　small　spaces；　and　it　is；　therefore；
  obvious　that　a　machine　which　is　of　such　a
  structure　that　it　moves　through　the　air　broadside
  on；　will　be　more　liable　to　meet　these　inequalities
  than　one　which　is　narrow　and　does　not　take　in　such
  a　wide　path。
  Why；　therefore；　persist　in　making　a　form　which；
  by　its　very　nature；　invites　danger？　Because　birds
  fly　that　way！
  THE　TURNING　MOVEMENT。This　structural　arrangement
  accentuates　the　difficulty　when　the　machine
  turns。　The　air　pressure　against　the　wing
  surface　is　dependent　on　the　speed。　The　broad
  outstretched　surfaces　compel　the　wing　at　the　outer
  side　of　the　circle　to　travel　faster　than　the　inner
  one。　As　a　result；　the　outer　end　of　the　aeroplane
  is　elevated。
  CENTRIFUGAL　ACTION。At　the　same　time　the
  running　gear；　and　the　frame　which　carries　it　and
  supports　the　machine　while　at　rest；　being　below
  the　planes；　a　centrifugal　force　is　exerted；　when
  turning　a　circle；　which　tends　to　swing　the　wheels
  and　frame　outwardly；　and　thereby　still　further
  elevating　the　outer　end　of　the　plane。
  THE　WARPING　PLANES。The　only　remedy　to
  meet　this　condition　is　expressed　in　the　mechanism
  which　wraps　or　twists　the　outer　ends　of　the　planes；
  as　constructed　in　the　Wright　machine；　or　the
  ailerons；　or　small　wings　at　the　rear　margins　of　the
  planes；　as　illustrated　by　the　Farman　machine。
  The　object　of　this　arrangement　is　to　decrease　the
  angle　of　incidence　at　the　rising　end；　and　increase
  the　angle　at　the　depressed　end；　and　thus；　by　manually…
  operated　means　keep　the　machine　on　an　even
  keel。
  CHAPTER　IV
  FORE　AND　AFT　CONTROL
  THERE　is　no　phase　of　the　art　of　flying　more　important
  than　the　fore　and　aft　control　of　an　airship。
  Lateral　stability　is　secondary　to　this　feature；　for
  reasons　which　will　appear　as　we　develop　the
  subject。
  THE　BIRD　TYPE　OF　FORE　AND　AFT　CONTROL。
  Every　aeroplane　follows　the　type　set　by　nature
  in　the　particular　that　the　body　is　caused　to　oscillate
  on　a　vertical　fore　and　aft　plane　while　in
  flight。　The　bird　has　one　important　advantage；
  however；　in　structure。　Its　wing　has　a　flexure　at
  the　joint；　so　that　its　body　can　so　oscillate　independently
  of　the　angle　of　the　wings。
  The　aeroplane　has　the　wing　firmly　fixed　to　the
  body；　hence　the　only　way　in　which　it　is　possible
  to　effect　a　change　in　the　angle　of　the　wing　is　by
  changing　the　angle　of　the　body。　To　be　consistent
  the　aeroplane　should　be　so　constructed　that　the
  angle　of　the　supporting　surfaces　should　be　movable；
  and　not　controllable　by　the　body。
  The　bird；　in　initiating　flight　from　a　perch；　darts
  downwardly；　and　changes　the　angle　of　the　body　to
  correspond　with　the　direction　of　the　flying　start。
  When　it　alights　the　body　is　thrown　so　that　its
  breast　banks　against　the　air；　but　in　ordinary　flight
  its　wings　only　are　used　to　change　the　angle　of
  flight。
  ANGLE　AND　DIRECTION　OF　FLIGHT。In　order　to
  become　familiar　with　terms　which　will　be　frequently
  used　throughout　the　book；　care　should　be
  taken　to　distinguish　between　the　terms　angle　and
  direction　of　flight。　The　former　has　reference　to
  the　up　and　down　movement　of　an　aeroplane；
  whereas　the　latter　is　used　to　designate　a　turning
  movement　to　the　right　or　to　the　left。
  WHY　SHOULD　THE　ANGLE　OF　THE　BODY　CHANGE？
  The　first　question　that　presents　itself　is；　why
  should　the　angle　of　the　aeroplane　body　change？
  Why　should　it　be　made　to　dart　up　and　down　and
  produce　a　sinuous　motion？　Why　should　its　nose
  tilt　toward　the　earth；　when　it　is　descending；　and
  raise　the　forward　part　of　the　structure　while　ascending？
  The　ready　answer　on　the　part　of　the　bird…form
  advocate　is；　that　nature　has　so　designed　a　flying
  structure。　The　argument　is　not　consistent；　because
  in　this　respect；　as　in　every　other；　it　is　not
  made　to　conform　to　the　structure　which　they　seek
  to　copy。
  CHANGING　ANGLE　OF　BODY　NOT　SAFE。Furthermore；
  there　is　not　a　single　argument　which　can　be
  advanced　in　behalf　of　that　method　of　building；
  which　proves　it　to　be　correct。　Contrariwise；　an
  analysis　of　the　flying　movement　will　show　that　it　is
  the　one　feature　which　has　militated　against　safety；
  and　that　machines　will　never　be　safe　so　long　as
  the　angle　of　the　body　must　be　depended　upon　to
  control　the　angle　of　flying。
  _Fig。　11a　Monoplane　in　Flight。_
  In　Fig。　11a　three　positions　of　a　monoplane　are
  shown；　each　in　horizontal　flight。　Let　us　say　that
  the　first　figure　A　is　going　at　40　miles　per　hour；
  the　second；　B；　at　50；　and　the　third；　C；　at　60　miles。
  The　body　in　A　is　nearly　horizontal；　the　angle　of
  the　plane　D　being　such　that；　with　the　tail　E　also
  horizontal；　an　even　flight　is　maintained。
  When　the　speed　increases　to　50　miles　an　hour；
  the　angle　of　incidence　in　the　plane　D　must　be
  decreased；　so　that　the　rear　end　of　the　frame　must
  be　raised；　which　is　done　by　giving　the　tail　an　angle
  of　incidence；　otherwise；　as　the　upper　side　of　the
  tail　should　meet　the　air　it　would　drive　the　rear
  end　of　the　frame　down；　and　thus　defeat　the　attempt
  to　elevate　that　part。
  _Fig。　12。　Angles　of　Flight。_
  As　the　speed　increases　ten　miles　more；　the　tail
  is　swung　down　still　further　and　the　rear　end　of
  the　frame　is　now　actually　above　the　plane　of　flight。
  In　order；　now；　to　change　the　angle　of　flight；　without
  altering　the　speed　of　the　machine；　the　tail　is
  used　to　effect　the　control。
  Examine　the　first　diagram　in　Fig。　12。　This
  shows　the　tail　E　still　further　depressed；　and　the
  air　striking　its　lower　side；　causes　an　upward　movement
  of　the　frame　at　that　end；　which　so　much　decreases
  the　angle　of　incidence　that　the　aeroplane
  darts　downwardly。
  In　order　to　ascend；　the　tail；　as　shown　in　the　second
  diagram；　is　elevated　so　as　to　depress　the　rear
  end；　and　now　the　sustaining　surface　shoots　upwardly。
  Suppose　that　in　either　of　the　positions　1　or　2；
  thus　described；　the　aviator　should　lose　control　of
  the　mechanism；　or　it　should　become　deranged　or
  〃stick；〃　conditions　which　have　existed　in　the　history
  of　the　art；　what　is　there　to　prevent　an　accident？
  In　the　first　case；　if　there　is　room；　the　machine
  will　loop　the　loop；　and　in　the　second　case　the　machine
  will　move　upwardly　until　it　is　vertical；　and
  then；　in　all　probability；　as　its　propelling　power　is
  not　sufficient　to　hold　it　in　that　position；　like　a
  helicopter；　and　having　absolutely　no　wing　supporting
  surface　when　in　that　position；　it　will　dart
  down　tail　foremost。
  A　NON…CHANGING　BODY。We　may　contrast　the
  foregoing　instances　of　flight　with　a　machine　having
  the　sustaining　planes　hinged　to　the　body　in
  such　a　manner　as　to　make　the　disposition　of　its
  angles　synchronous　with　the　tail。　In　other　words；
  see　how　a　machine　acts　that　has　the　angle　of　flight
  controllable　by　both　planes；that　is；　the　sustaining
  planes；　as　well　as　the　tail。
  _Fig。　13。　Planes　on　Non…changing　Body。_
  In　Fig。　13　let　the　body　of　the　aeroplane　be　horizontal；
  and　the　sustaining　planes　B　disposed　at
  the　same　angle；　which　we　will　assume　to　be　15
  degrees；　this　being　the　imaginary　angle　for　illustrative
  purposes；　with　the　power　of　the　machine
  to　drive　it　along　horizontally；　as　shown　in　position
  1。
  In　position　2　the　angles　of　both　planes　are　now
  at　10　degrees；　and　the　speed　60　miles　an　hour；
  which　still　drives　the　machine　forward　horizontally。
  In　position　3　the　angle　is　still　less；　being　now
  only　5　degrees　but　the　speed　is　increased　to　80
  miles　per　hour；　but　in　each　instance　the　body　of
  the　machine　is　horizontal。
  Now　it　is　obvious　that　in　order　to　ascend；　in
  either　case；　the　changing　of　the　planes　to　a　greater
  angle　would　raise　the　machine；　but　at　the　same
  time　keep　the　body　on　an　even　keel。
  _Fig。　14。　Descent　with　Non…changing　Body。_
  DESCENDING　POSITIONS　BY　POWER　CONTROL。In
  Fig。　14　the　planes　are　the　same　angles　in　the　three
  positions　respectively；　as　in　Fig。　13；　but　now　the
  power　has　been　reduced；　and　the　speeds　are　30；
  25；　and　20　miles　per　hour；　in　positions　A；　B　and　C。
  Suppose　that　in　either　position　the　power　should
  cease；　and　the　control　broken；　so　that　it　would　be
  impossible　to　move　the　planes。　When　the　machine
  begins　to　lose　its　momentum　it　will　descend　on　a
  curve　shown；　for　instance；　in　Fig。　15；　where　position
  1　of　Fig。　14　is　taken　as　the　speed　and　angles
  of　the　plane　when　the　power　ceased。
  _Fig。　15。　Utilizing　Momentum。_
  CUTTING　OFF　THE　POWER。This　curve；　A；　may
  reach　that