formation　of　the　plane；　have　been　considered　in
  all　their　aspects；　so　that　the　art　in　this　respect　has
  advanced　with　rapid　strides。
  NARROW　PLATES　MOST　EFFECTIVE。It　was
  learned；　in　the　early　stages　of　the　development
  by　practical　experiments；　that　a　narrow　plane；
  fore　and　aft；　produces　a　greater　lift　than　a　wide
  one；　so　that；　assuming　the　plane　has　100　square
  feet　of　sustaining　surface；　it　is　far　better　to　make
  the　shape　five　feet　by　twenty　than　ten　by　ten。
  However；　it　must　be　observed；　that　to　use　the
  narrow　blade　effectively；　it　must　be　projected
  through　the　air　with　the　long　margin　forwardly。
  Its　sustaining　power　per　square　foot　of　surface
  is　much　less　if　forced　through　the　air　lengthwise。
  Experiments　have　shown　why　a　narrow　blade
  has　proportionally　a　greater　lift；　and　this　may
  be　more　clearly　understood　by　examining　the
  illustrations　which　show　the　movement　of　planes
  through　the　air　at　appropriate　angles。
  _Fig。　22。　Stream　lines　along　a　plane。_
  STREAM　LINES　ALONG　A　PLANE。In　Fig。　22；　A
  is　a　flat　plane；　which　we　will　assume　is　10　feet
  from　the　front　to　the　rear　margin。　For　convenience
  seven　stream　lines　of　air　are　shown；
  which　contact　with　this　inclined　surface。　The　first
  line　1；　after　the　contact　at　the　forward　end；　is
  driven　downwardly　along　the　surface；　so　that　it
  forms　what　we　might　term　a　moving　film。
  The　second　air　stream　2；　strikes　the　first　stream；
  followed　successively　by　the　other　streams；　3；　4；
  and　so　on；　each　succeeding　stream　being　compelled
  to　ride　over；　or　along　on　the　preceding　mass　of
  cushioned　air；　the　last　lines；　near　the　lower　end；
  being；　therefore；　at　such　angles；　and　contacting
  with　such　a　rapidly…moving　column；　that　it　produces
  but　little　lift　in　comparison　with　the　1st；
  2d　and　3d　stream　lines。　These　stream　lines　are
  taken　by　imagining　that　the　air　approaches　and
  contacts　with　the　plane　only　along　the　lines　indicated
  in　the　sketch；　although　they　also　in　practice
  are　active　against　every　part　of　the　plane。
  THE　CENTER　OF　PRESSURE。In　such　a　plane　the
  center　of　pressure　is　near　its　upper　end；　probably
  near　the　line　3；　so　that　the　greater　portion　of　the
  lift　is　exerted　by　that　part　of　the　plane　above
  line　3。
  AIR　LINES　ON　THE　UPPER　SIDE　OF　THE　PLANE。
  Now；　another　factor　must　be　considered；　namely；
  the　effect　produced　on　the　upper　side　of　the　plane；
  over　which　a　rarefied　area　is　formed　at　certain
  points；　and；　in　practice；　this　also　produces；　or
  should　be　utilized　to　effect　a　lift。
  RAREFIED　AREA。What　is　called　a　rarefied　area；
  has　reference　to　a　state　or　condition　of　the　atmosphere
  which　has　less　than　the　normal　pressure　or
  quantity　of　air。　Thus；　the　pressure　at　sea　level；
  is　about　14　3/4　per　square　inch
  As　we　ascend　the　pressure　grows　less；　and　the
  air　is　thus　rarer；　or；　there　is　less　of　it。　This　is　a
  condition　which　is　normally　found　in　the　atmosphere。
  Several　things　tend　to　make　a　rarefied
  condition。　One　is　altitude；　to　which　we　have　just
  referred。
  Then　heat　will　expand　air；　making　it　less　dense；
  or　lighter；　so　that　it　will　move　upwardly；　to　be
  replaced　by　a　colder　body　of　air。　In　aeronautics
  neither　of　these　conditions　is　of　any　importance
  in　considering　the　lifting　power　of　aeroplane　surfaces。
  RAREFACTION　PRODUCED　BY　MOTION。The　third
  rarefied　condition　is　produced　by　motion；　and　generally
  the　area　is　very　limited　when　brought　about
  by　this　means。　If；　for　instance；　a　plane　is　held
  horizontally　and　allowed　to　fall　toward　the　earth；
  it　will　be　retarded　by　two　forces；　namely；　compression
  and　rarefaction；　the　former　acting　on　the
  under　side　of　the　plane；　and　the　latter　on　the　upper
  side。
  Of　the　two　rarefaction　is　the　most　effectual；
  and　produces　a　greater　effect　than　compression。
  This　may　be　proven　by　compressing　air　in　a　long
  pipe；　and　noting　the　difference　in　gauge　pressure
  between　the　ends；　and　then　using　a　suction　pump
  on　the　same　pipe。
  When　a　plane　is　forced　through　the　air　at　any
  angle；　a　rarefied　area　is　formed　on　the　side　which
  is　opposite　the　one　having　the　positive　angle　of
  incidence。
  If　the　plane　can　be　so　formed　as　to　make　a　large
  and　effective　area　it　will　add　greatly　to　the　value
  of　the　sustaining　surface。
  Unfortunately；　the　long　fiat　plane　does　not　lend
  any　aid　in　this　particular；　as　the　stream　line　flows
  down　along　the　top；　as　shown　in　Fig。　23；　without
  being　of　any　service。
  _Fig。　23。　Air　lines　on　the　upper　side　of　a　Plane。_
  THE　CONCAVED　PLANE。These　considerations
  led　to　the　adoption　of　the　concaved　plane　formation；
  and　for　purposes　of　comparison　the　diagram；
  Fig。　24；　shows　the　plane　B　of　the　same　length　and
  angle　as　the　straight　planes。
  In　examining　the　successive　stream　lines　it　will
  be　found　that　while　the　1st；　2d　and　3d　lines　have
  a　little　less　angle　of　impact　than　the　corresponding
  lines　in　the　straight　plane；　the　last　lines；　5；　6
  and　7；　have　much　greater　angles；　so　that　only　line
  4　strikes　the　plane　at　the　same　angle。
  Such　a　plane　structure　would；　therefore；　have
  its　center　of　pressure　somewhere　between　the
  lines　3　and　4；　and　the　lift　being　thus；　practically；
  uniform　over　the　surface；　would　be　more　effective。
  THE　CENTER　OF　PRESSURE。This　is　a　term　used
  to　indicate　the　place　on　the　plane　where　the　air
  acts　with　the　greatest　force。　It　has　reference　to
  a　point　between　the　front　and　rear　margins　only
  of　the　plane。
  _Fig。　24。　Air　lines　below　a　concaved　Plane。_
  UTILIZING　THE　RAREFIED　AREA。This　structure；
  however；　has　another　important　advantage；　as　it
  utilizes　the　rarefied　area　which　is　produced；　and
  which　may　be　understood　by　reference　to　Fig。　25。
  The　plane　B；　with　its　upward　curve；　and　at　the
  same　angle　as　the　straight　plane；　has　its　lower
  end　so　curved；　with　relation　to　the　forward　movement；
  that　the　air；　in　rushing　past　the　upper　end；
  cannot　follow　the　curve　rapidly　enough　to　maintain
  the　same　density　along　C；　hence　this　exerts
  an　upward　pull；　due　to　the　rarefied　area；　which
  serves　as　a　lifting　force；　as　well　as　the　compressed
  mass　beneath　the　plane。
  CHANGING　CENTER　OF　PRESSURE。The　center　of
  pressure　is　not　constant。　It　changes　with　the
  angle　of　the　plane；　but　the　range　is　considerably
  less　on　a　concave　surface　than　on　a　flat　plane。
  _Fig。　25。　Air　lines　above　a　convex　Plane。_
  In　a　plane　disposed　at　a　small　angle；　A；　as　in
  Fig。　26；　the　center　of　pressure　is　nearer　the　forward
  end　of　the　plane　than　with　a　greater　positive
  angle　of　incidence；　as　in　Fig。　27；　and　when
  the　plane　is　in　a　normal　flying　angle；　it　is　at　the
  center；　or　at　a　point　midway　between　the　margins。
  PLANE　MONSTROSITIES。Growing　out　of　the　idea
  that　the　wing　in　nature　must　be　faithfully　copied；
  it　is　believed　by　many　that　a　plane　with　a
  pronounced　thickness　at　its　forward　margin　is　one
  of　the　secrets　of　bird　flight。
  Accordingly　certain　inventors　have　designed
  types　of　wings　which　are　shown　in　Figs。　28　and
  29。
  _Fig。　28　Changing　centers　of　Pressures。_
  _Fig　29。　Bird…wing　structures。_
  Both　of　these　types　have　pronounced　bulges；
  designed　to　〃split〃　the　air；　forgetting；　apparently；
  that　in　other　parts　of　the　machine　every　effort　is
  made　to　prevent　head　resistance。
  THE　BIRD　WING　STRUCTURE。The　advocates　of
  such　construction　maintain　that　the　forward　edge
  of　the　plane　must　forcibly　drive　the　air　column
  apart；　because　the　bird　wing　is　so　made；　and　that
  while　it　may　not　appear　exactly　logical；　still　there
  is　something　about　it　which　seems　to　do　the　work；
  and　for　that　reason　it　is　largely　adopted。
  WHY　THE　BIRD'S　WING　HAS　A　PRONOUNCED
  BULGE。Let　us　examine　this　claim。　The　bone
  which　supports　the　entire　wing　surface；　called　the
  （pectoral）；　has　a　heavy　duty　to　perform。　It　is　so
  constructed　that　it　must　withstand　an　extraordinary
  torsional　strain；　being　located　at　the　forward
  portion　of　the　wing　surface。　Torsion　has
  reference　to　a　twisting　motion。
  In　some　cases；　as　in　the　bat；　this　primary　bone
  has　an　attachment　to　the　rear　of　the　main　joint；
  where　the　rear　margin　of　the　wing　is　attached　to
  the　leg　of　the　animal；　thus　giving　it　a　support
  and　the　main　bone　is；　therefore；　relieved　of　this
  torsional　stress。
  THE　BAT'S　WING。An　examination　of　the　bat's
  wing　shows　that　the　pectoral　bone　is　very　small
  and　thin；　thus　proving　that　when　the　entire　wing
  support　is　thrown　upon　the　primary　bone　it　must
  be　large　enough　to　enable　it　to　carry　out　its　functions。
  It　is　certainly　not　so　made　because　it　is　a
  necessary　shape　which　best　adapts　it　for　flying。
  If　such　were　the　case　then　nature　erred　in　the
  case　of　the　bat；　and　it　made　a　mistake　in　the
  housefly's　wing　which　has　no　such　anterior　enlargement
  to　assist　（？）　it　in　flying。
  AN　ABNORMAL　SHAPE。Another　illustration　is
  shown　in　Fig。　30；　which　has　a　deep　concave　directly
  behind　the　forward　margin；　as　at　A；　so
  that　when