being　in　range　of　a　circle　with　degrees
  indicated　thereon；　and　the　base　attached　to　the
  frame　of　the　machine；　can　always　be　observed；
  and　the　conditions　noted　at　the　time　the　changes
  take　place。
  PENDULUM　STABILIZER。In　many　respects　the
  use　of　a　pendulum　has　advantages　over　the　gyroscope。
  The　latter　requires　power　to　keep　it　in
  motion。　The　pendulum　is　always　in　condition
  for　service。　While　it　may　be　more　difficult　to
  adjust　the　pendulum；　so　that　it　does　not　affect
  the　planes　by　too　rapid　a　swing；　or　an　oscillation
  which　is　beyond　the　true　angle　desired；　still；　these
  are　matters　which；　in　time；　will　make　the　pendulum
  a　strong　factor　in　lateral　stability。
  _Fig。　67。　Simple　Pendulum　Stabilizer。_
  It　is　an　exceedingly　simple　matter　to　attach　the
  lead　wires　from　an　aileron　to　the　pendulum。　In
  Fig。　67　one　plan　is　illustrated。　The　pendulum
  A　swings　from　the　frame　B　of　the　machine；　the
  ailerons　a　being　in　this　case　also　shown　at　right
  angles　to　their　true　positions。
  The　other；　Fig。　68；　assumes　that　the　machine　is
  exactly　horizontal；　and　as　the　pendulum　is　in　a
  vertical　position；　the　forward　edges　of　both　ailerons
  are　elevated；　but　when　the　pendulum　swings
  both　ailerons　will　be　swung　with　their　forward
  margins　up　or　down　in　unison；　and　thus　the　proper
  angles　are　made　to　right　the　machine。
  STEERING　AND　CONTROLLING　WHEEL。For　the
  purpose　of　concentrating　the　control　in　a　single
  wheel；　which　has　not　alone　a　turning　motion；　but
  is　also　mounted　in　such　a　manner　that　it　will　oscillate
  to　and　fro；　is　very　desirable；　and　is　adapted
  for　any　kind　of　machine。
  _Fig。　68。　Pendulum　Stabilizers。_
  Fig。　69　shows　such　a　structure；　in　which　A
  represents　the　frame　of　the　machine；　and　B　a
  segment　for　the　stem　of　the　wheel；　the　segment
  being　made　of　two　parts；　so　as　to　form　a　guideway
  for　the　stem　a　to　travel　between；　and　the　segment
  is　placed　so　that　the　stem　will　travel　in　a
  fore　and　aft　direction。
  The　lower　end　of　the　stem　is　mounted　in　a
  socket；　at　D；　so　that　while　it　may　be　turned；　it
  will　also　permit　this　oscillating　motion。　Near　its
  lower　end　is　a　cross　bar　E　from　which　the　wires
  run　to　the　vertical　control　plane；　and　also　to　the
  ailerons；　if　the　machine　is　equipped　with　them；　or
  to　the　warping　ends　of　the　planes。
  _Fig。　69。　Steering　and　Control　Wheel。_
  Above　the　cross　arms　is　a　loose　collar　F　to
  which　the　fore　and　aft　cords　are　attached　that　go
  to　the　elevators；　or　horizontal　planes。　The　upper
  end　of　the　stem　has　a　wheel　G；　which　may　also　be
  equipped　with　the　throttle　and　spark　levers。
  AUTOMATIC　STABILIZING　WINGS。Unquestionably；
  the　best　stabilizer　is　one　which　will　act　on
  its　own　initiative。　The　difficulty　with　automatic
  devices　is；　that　they　act　too　late；　as　a　general
  thing；　to　be　effective。　The　device　represented　in
  Fig。　70　is　very　simple；　and　in　practice　is　found　to
  be　most　efficient。
  In　this　Fig。　70　A　and　B　represent　the　upper
  and　the　lower　planes；　respectively。　Near　the　end
  vertical　standards　a；　D；　are　narrow　wings　E　E；
  F　F；　hinged　on　a　fore　and　aft　line　close　below
  each　of　the　planes；　the　wings　being　at　such　distances
  from　the　standards　C　D　that　when　they
  swing　outwardly　they　will　touch　the　standards；
  and　when　in　that　position　will　be　at　an　angle　of
  about　35　degrees　from　the　planes　A　B。
  _Fig。　70。　Automatic　Stabilizing　Wings。_
  _Fig。　71。　Action　of　Stabilizing　Wings。_
  Inwardly　they　are　permitted　to　swing　up　and
  lie　parallel　with　the　planes；　as　shown　in　Fig。　71
  where　the　planes　are　at　an　angle。　In　turning；　all
  machines　skid；that　is　they　travel　obliquely
  across　the　field；　and　this　is　also　true　when　the
  ship　is　sailing　at　right　angles　to　the　course　of　the
  wind。
  This　will　be　made　clear　by　reference　to　Fig。
  72；　in　which　the　dart　A　represents　the　direction
  of　the　movement　of　the　aeroplane；　and　B　the
  direction　of　the　wind；　the　vertical　rudder　a　being
  almost　at　right　angles　to　the　course　of　the　wind。
  _Fig。　72。　Into　the　Wind　at　an　Angle。_
  In　turning　a　circle　the　same　thing　takes　place
  as　shown　in　Fig。　73；　with　the　tail　at　a　different
  angle；　so　as　to　give　a　turning　movement　to　the
  plane。　It　will　be　seen　that　in　the　circling　movement
  the　tendency　of　the　aeroplane　is　to　fly　out
  at　a　tangent；　shown　by　the　line　D；　so　that　the
  planes　of　the　machine　are　not　radially…disposed
  with　reference　to　the　center　of　the　circle；　the　line
  E　showing　the　true　radial　line。
  Referring　now　to　Fig。　71；　it　will　be　seen　that
  this　skidding　motion　of　the　machine　swings　the
  wings　E　F　inwardly；　so　that　they　offer　no　resistance
  to　the　oblique　movement；　but　the　wings　E
  E；　at　the　other　end　of　the　planes　are　swung　outwardly；
  to　provide　an　angle；　which　tends　to　raise
  up　the　inner　end　of　the　planes；　and　thereby　seek
  to　keep　the　planes　horizontal。
  _Fig。　73。　Turning　a　Circle。_
  BAROMETERS。These　instruments　are　used　for
  registering　heights。　A　barometer　is　a　device　for
  measuring　the　weight　or　pressure　of　the　air。
  The　air　is　supposed　to　extend　to　a　height　of　40
  miles　from　the　surface　of　the　sea。　A　column　of
  air　one　inch　square；　and　forty　miles　high；　weighs
  the　same　as　a　column　of　mercury　one　inch　square
  and　30　inches　high。
  Such　a　column　of　air；　or　of　mercury；　weighs
  14　3/4　pounds。　If　the　air　column　should　be
  weighed　at　the　top　of　the　mountain；　that　part
  above　would　weigh　less　than　if　measured　at　the
  sea　level；　hence；　as　we　ascend　or　descend　the　pressure
  becomes　less　or　more；　dependent　on　the　altitude。
  Mercury　is　also　used　to　indicate　temperature；
  but　this　is　brought　about　by　the　expansive　quality
  of　the　mercury；　and　not　by　its　weight。
  _Fig。　74。　Aneroid　Barometer。_
  ANEROID　BAROMETER。The　term　Aneroid　barometer
  is　frequently　used　in　connection　with　air…
  ship　experiments。　The　word　aneroid　means　not
  wet；　or　not　a　fluid；　like　mercury；　so　that；　while
  aneroid　barometers　are　being　made　which　do　use
  mercury；　they　are　generally　made　without。
  One　such　form　is　illustrated　in　Fig。　74；　which
  represents　a　cylindrical　shell　A；　which　has　at　each
  end　a　head　of　concentrically　formed　corrugations。
  These　heads　are　securely　fixed　to　the　ends　of　the
  shell　A。　Within；　one　of　the　disk　heads　has　a
  short　stem　C；　which　is　attached　to　the　short　end
  of　a　lever　D；　this　lever　being　pivoted　at　E。　The
  outer　end　of　this　lever　is　hinged　to　the　short　end
  of　another　lever　F；　and　so　by　compounding　the
  levers；　it　will　be　seen　that　a　very　slight　movement
  of　the　head　B　will　cause　a　considerable　movement
  in　the　long　end　of　the　lever　F。
  This　end　of　the　lever　F　connects　with　one　limb
  of　a　bell…crank　lever　G；　and　its　other　limb　has　a
  toothed　rack　connection　with　a　gear　H；　which
  turns　the　shaft　to　which　the　pointer　I　is　attached。
  Air　is　withdrawn　from　the　interior　of　the　shell；
  so　that　any　change　in　the　pressure；　or　weight　of
  the　atmosphere；　is　at　once　felt　by　the　disk　heads；
  and　the　finger　turns　to　indicate　the　amount　of
  pressure。
  HYDROPLANES。Hydro　means　water；　hence　the
  term　hydroplane　has　been　given　to　machines
  which　have　suitable　pontoons　or　boats；　so　they
  may　alight　or　initiate　flight　from　water。
  There　is　no　particular　form　which　has　been
  adopted　to　attach　to　aeroplanes；　the　object　generally
  being　to　so　make　them　that　they　will　sustain
  the　greatest　amount　of　weight　with　the　least
  submergence；　and　also　offer　the　least　resistance
  while　the　motor　is　drawing　the　machine　along　the
  surface　of　the　water；　preparatory　to　launching　it。
  SUSTAINING　WEIGHT　OF　PONTOONS。A　pontoon
  having　within　nothing　but　air；　is　merely　a　measuring
  device　which　determines　the　difference　between
  the　weight　of　water　and　the　amount　placed
  on　the　pontoon。　Water　weighs　62　1/2　pounds　per
  cubic　foot。　Ordinary　wood；　an　average　of　32
  pounds；　and　steel　500　pounds。
  It　is；　therefore；　an　easy　matter　to　determine
  how　much　of　solid　matter　will　be　sustained　by　a
  pontoon　of　a　given　size；　or　what　the　dimensions
  of　a　pontoon　should　be　to　hold　up　an　aeroplane
  which　weighs；　with　the　pilot；　say；　1100　pounds。
  As　we　must　calculate　for　a　sufficient　excess　to
  prevent　the　pontoons　from　being　too　much　immersed；
  and　also　allow　a　sufficient　difference　in
  weight　so　that　they　will　keep　on　the　surface　when
  the　aeroplane　strikes　the　surface　in　alighting；　we
  will　take　the　figure　of　1500　pounds　to　make　the
  calculations　from。
  If　this　figure　is　divided　by　62　1/2　we　shall　find
  the　cubical　contents　of　the　pontoons；　not　considering；
  of　course；　the　weight　of　the　material　of　which
  they　are　composed。　This　calculation　shows　that
  we　must　have　24　cubic　feet　in　the　pontoons。
  As　there　should　be　two　main　pontoons；　and　a
  smaller　one　for　the　rear；　each　of　the　main　ones
  might　have　ten　cubic　feet；　and　the　smaller　one
  four　cubic　feet。
  SHAPES　OF　THE　PONTOONS。We　are　now　ready
  to　design　the　shapes。　Fig。　75　shows　three　general
  types；　A　being