four　cubic　feet。
  SHAPES　OF　THE　PONTOONS。We　are　now　ready
  to　design　the　shapes。　Fig。　75　shows　three　general
  types；　A　being　made　rectangular　in　form；
  with　a　tapering　forward　end；　so　constructed　as　to
  ride　up　on　the　water。
  The　type　B　has　a　rounded　under　body；　the　forward
  end　being　also　skiff…shaped　to　decrease　as
  much　as　possible　the　resistance　of　the　water　impact。
  _Fig。　75。　Hydroplane　Floats。_
  The　third　type　C　is　made　in　the　form　of　a
  closed　boat；　with　both　ends　pointed；　and　the　bottom
  rounded；　or　provided　with　a　keel。　Or；　as　in
  some　cases　the　body　may　be　made　triangular　in
  cross　section　so　that　as　it　is　submerged　its　sustaining
  weight　will　increase　at　a　greater　degree
  as　it　is　pressed　down　than　its　vertical　measurement
  indicates。
  All　this；　however；　is　a　matter　left　to　the　judgment
  of　the　designer；　and　is；　in　a　great　degree；
  dependent　on　the　character　of　the　craft　to　which
  it　is　to　be　applied。
  CHAPTER　XII
  EXPERIMENTAL　WORK　IN　FLYING
  THE　novice　about　to　take　his　first　trial　trip　in
  an　automobile　will　soon　learn　that　the　great　task
  in　his　mind　is　to　properly　start　the　machine。　He
  is　conscious　of　one　thing；　that　it　will　be　an　easy
  matter　to　stop　it　by　cutting　off　the　fuel　supply
  and　applying　the　brakes。
  CERTAIN　CONDITIONS　IN　FLYING。In　an　aeroplane
  conditions　are　reversed。　Shutting　off　the
  fuel　supply　and　applying　the　brakes　only　bring
  on　the　main　difficulty。　He　must　learn　to　stop　the
  machine　after　all　this　is　done；　and　this　is　the
  great　test　of　flying。　It　is　not　the　launching；
  the　ability　to　get　into　the　air；　but　the　landing；　that
  gives　the　pupil　his　first　shock。
  Man　is　so　accustomed　to　the　little　swirls　of　air
  all　about　him；　that　he　does　not　appreciate　what
  they　mean　to　a　machine　which　is　once　free　to
  glide　along　in　the　little　currents　which　are　so　unnoticeable
  to　him　as　a　pedestrian。
  The　contour　of　the　earth；　the　fences；　trees；　little
  elevations　and　other　natural　surroundings；　all
  have　their　effect　on　a　slight　moving　air　current；
  and　these　inequalities　affect　the　air　and　disturb
  it　to　a　still　greater　extent　as　the　wind　increases。
  Even　in　a　still　air；　with　the　sun　shining；　there　are
  air　eddies；　caused　by　the　uneven　heating　of　the
  air　in　space。
  HEAT　IN　AIR。Heat　is　transmitted　through　the
  air　by　what　is　called　convection；　that　is；　the　particles
  of　the　air　transmit　it　from　one　point　to　the
  next。　If　a　room　is　closed　up　tight；　and　a　little
  aperture　provided　so　as　to　let　in　a　streak　of　sunlight；
  it　will　give　some　idea　of　the　unrest　of　the
  atmosphere。　This　may　be　exhibited　by　smoke
  along　the　line　of　the　sun's　rays；　which　indicates
  that　the　particles　of　air　are　constantly　in　motion；
  although　there　may　be　absolutely　nothing　in　the
  room　to　disturb　it。
  MOTION　WHEN　IN　FLIGHT。If　you　can　imagine
  a　small　airship　floating　in　that　space；　you　can
  readily　conceive　that　it　will　be　hurled　hither　and
  thither　by　the　motion　which　is　thus　apparent　to
  the　eye。
  This　motion　is　greatly　accentuated　by　the　surface
  of　the　earth；　independently　of　its　uneven　contour。
  If　a　ball　is　thrown　through　the　air；　its
  dynamic　force　is　measured　by　its　impact。　So
  with　light；　and　heat。　In　the　space　between　the
  planets　it　is　very　cold。　The　sunlight；　or　the　rays
  from　the　sun　are　there；　just　the　same　as　on　the
  earth。
  Unless　the　rays　come　into　contact　with　something；
  they　produce　no　effect。　When　the　beams
  from　the　sun　come　into　contact　with　the　atmosphere
  a　dynamic　force　is　exerted；　just　the　same
  as　when　the　ball　struck　an　object。　When　the　rays
  reach　the　earth；　reflection　takes　place；　and　these
  reflected　beams　act　on　the　air　under　different　conditions。
  CHANGING　ATMOSPHERE。If　the　air　is　full　of
  moisture；　as　it　may　be　at　some　places；　while
  comparatively　dry　at　other　points；　the　reflection
  throughout　the　moist　area　is　much　greater　than　in
  the　dry　places；　hence　evaporation　will　take　place
  and　whenever　a　liquid　vaporizes　it　means　heat。
  On　the　other　hand；　when　the　vapor　is　turning
  to　a　liquid；　condensation　takes　place；　and　that
  means　cooling。　If　the　air　should　be　of　the　same
  degree　of　saturation　throughout；that　is；　have
  the　same　amount　of　moisture　everywhere；　there
  would　be　few　winds。　These　remarks　apply　to
  conditions　which　exist　over　low　altitudes　all　over
  the　earth。
  But　at　high　altitudes　the　conditions　are　entirely
  different。　As　we　ascend　the　air　becomes　rarer。
  It　has　less　moisture；　because　a　wet　atmosphere；
  being　heavier；　lies　nearer　the　surface　of　the　earth。
  Being　rarer　the　action　of　sunlight　on　the　particles
  is　less　intense。　Reflection　and　refraction　of　the
  rays　acting　on　the　light　atmosphere　do　not　produce
  such　a　powerful　effect　as　on　the　air　near　the
  ground。
  All　these　conditionsthe　contour　of　the　earth；
  the　uneven　character　of　the　moisture　in　the　air；
  the　inequalities　of　the　convection　currents；　and
  the　unstable；　tenuous；　elastic　nature　of　the　atmosphere；
  make　the　trials　of　the　aviator　a　hazardous
  one；　and　it　has　brought　out　numerous　theories
  connected　with　bird　flight。　One　of　these　assumes
  that　the　bird；　by　means　of　its　finely　organized
  sense；　is　able　to　detect　rising　air　currents；　and　it
  selects　them　in　its　flight；　and　by　that　means　is　enabled
  to　continue　in　flight　indefinitely；　by　soaring；
  or　by　flapping　its　wings。
  ASCENDING　CURRENTS。It　has　not　been　explained
  how　it　happens　that　these　particular　〃ascending
  currents〃　always　appear　directly　in　the　line　of
  the　bird　flight；　or　why　it　is　that　when；　for　instance；
  a　flock　of　wild　geese　which　always　fly　through
  space　in　an　A…shaped　formation；　are　able　to　get
  ascending　air　currents　over　the　wide　scope　of　space
  they　cover。
  ASPIRATE　CURRENTS。Some　years　ago；　in　making
  experiments　with　the　outstretched　wings　of
  one　of　the　large　soaring　birds；　a　French　sailor
  was　surprised　to　experience　a　peculiar　pulling　motion；
  when　the　bird's　wings　were　held　at　a　certain
  angle；　so　that　the　air　actually　seemed　to　draw　it
  into　the　teeth　of　the　current。
  It　is　known　that　if　a　ball　is　suspended　by　a
  string；　and　a　jet　of　air　is　directed　against　it；　in
  a　particular　way；　the　ball　will　move　toward　the
  jet；　instead　of　being　driven　away　from　it。　A　well
  known　spraying　device；　called　the　〃ball　nozzle；〃
  is　simply　a　ball　on　the　end　of　a　nozzle；　and　the
  stream　of　water　issuing　is　not　effectual　to　drive
  the　ball　away。
  From　the　bird　incident　alluded　to；　a　new　theory
  was　propounded；　namely；　that　birds　flew　because
  of　the　aspirated　action　of　the　air；　and　the　wings
  and　body　were　so　made　as　to　cause　the　moving　air
  current　to　act　on　it；　and　draw　it　forwardly。
  OUTSTRETCHED　WINGS。This　only　added　to　the
  〃bird　wing〃　theory　a　new　argument　that　all　flying
  things　must　have　outstretched　wings；　in　order
  to　fly；　forgetting　that　the　ball；　which　has　no
  outstretched　wings；　has　also　the　same　〃aspirate〃
  movement　attributed　to　the　wings　of　the　bird。
  The　foregoing　remarks　are　made　in　order　to　impress
  on　the　novice　that　theories　do　not　make
  flying　machines；　and　that　speculations；　or　analogies
  of　what　we　see　all　about　us；　will　not　make　an
  aviator。　A　flying　machine　is　a　question　of
  dynamics；　just　as　surely　as　the　action　of　the　sun　on
  the　air；　and　the　movements　of　the　currents；　and
  the　knowledge　of　applying　those　forces　in　the　flying
  machine　makes　the　aviator。
  THE　STARTING　POINT。Before　the　uninitiated
  should　attempt　to　even　mount　a　machine　he　should
  know　what　it　is　composed　of；　and　how　it　is　made。
  His　investigation　should　take　in　every　part　of　the
  mechanism；　he　should　understand　about　the　plane
  surface；　what　the　stresses　are　upon　its　surface；
  what　is　the　duty　of　each　strut；　or　brace　or　wire
  and　be　able　to　make　the　proper　repairs。
  THE　VITAL　PART　OF　THE　MACHINE。The　motor；
  the　life　of　the　machine　itself；　should　be　like　a
  book　to　him。　It　is　not　required　that　he　should
  know　all　the　theories　which　is　necessary　in　the
  building；　as　to　the　many　features　which　go　to
  make　up　a　scientifically…designed　motor；　but　he
  must　know　how　and　why　it　works。　He　should　understand
  the　cam　action；　whereby　the　valves　are
  lifted　at　the　proper　time；　what　the　effect　of　the
  spark　advance　means；　the　throttling　of　the　engine；
  air　admission　and　supply；　the　regulation
  of　the　carbureter；　its　mechanism　and　construction；
  the　propeller　should　be　studied；　and　its　action
  at　various　speeds。
  STUDYING　THE　ACTION　OF　THE　MACHINE。Then
  comes　the　study　on　the　seat　of　the　machine　itself。
  It　will　be　a　novel　sensation。　Before　him　is　the
  steering　wheel；　if　it　should　be　so　equipped。　Turning
  it　to　the　right；　swings　the　vertical　tail　plane
  so　the　machine　will　turn　to　the　right。　Certainly；
  he　knows　that；　but　how　far　must　he　turn　the
  wheel　to　give　it　a　certain　angle。
  It　is　not　enough　to　know　that　a　lever　or　a　wheel
  when　moved　a　certain　way　will　move　a　plane　a
  definite　dir