wheel　at　certain　speeds。
  A　TANGENT。When　an　object　is　thrown　horizontally
  the　line　of　flight　is　tangential　to　the　earth；
  or　at　right　angles　to　the　force　of　gravity。　Such
  a　course　in　a　flying　machine　finds　less　resistance
  than　if　it　should　be　projected　upwardly；　or　directly
  opposite　the　centripetal　pull。
  _Fig　1。　Tangential　Flight_
  TANGENTIAL　MOTION　REPRESENTS　CENTRIFUGAL
  PULL。A　tangential　motion；　or　a　horizontal
  movement；　seeks　to　move　matter　away　from　the
  center　of　the　earth；　and　any　force　which　imparts
  a　horizontal　motion　to　an　object　exerts　a　centrifugal
  pull　for　that　reason。
  In　Fig。　1；　let　A　represent　the　surface　of　the
  earth；　B　the　starting　point　of　the　flight　of　an　object；
  and　C　the　line　of　flight。　That　represents　a
  tangential　line。　For　the　purpose　of　explaining
  the　phenomena　of　tangential　flight；　we　will　assume
  that　the　missile　was　projected　with　a　sufficient
  force　to　reach　the　vertical　point　D；　which
  is　4000　miles　from　the　starting　point　B。
  In　such　a　case　it　would　now　be　over　5500　miles
  from　the　center　of　the　earth；　and　the　centrifugal
  pull　would　be　decreased　to　such　an　extent　that　the
  ball　would　go　on　and　on　until　it　came　within　the
  sphere　of　influence　from　some　other　celestial
  body。
  EQUALIZING　THE　TWO　MOTIONS。But　now　let　us
  assume　that　the　line　of　flight　is　like　that　shown
  at　E；　in　Fig。　2；　where　it　travels　along　parallel
  with　the　surface　of　the　earth。　In　this　case　the
  force　of　the　ball　equals　the　centripetal　pull；or；
  to　put　it　differently；　the　centrifugal　equals　the
  gravitational　pull。
  The　constant　tendency　of　the　ball　to　fly　off　at
  a　tangent；　and　the　equally　powerful　pull　of
  gravity　acting　against　each　other；　produce　a
  motion　which　is　like　that　of　the　earth；　revolving
  around　the　sun　once　every　three　hundred　and
  sixty…five　days。
  It　is　a　curious　thing　that　neither　Langley；　nor
  any　of　the　scientists；　in　treating　of　the　matter　of
  flight；　have　taken　into　consideration　this　quality
  of　momentum；　in　their　calculations　of　the　elements
  of　flight。
  _Fig。　2　Horizontal　Flight_
  All　have　treated　the　subject　as　though　the
  whole　problem　rested　on　the　angle　at　which　the
  planes　were　placed。　At　45　degrees　the　lift　and
  drift　are　assumed　to　be　equal。
  LIFT　AND　DRIFT。The　terms　should　be　explained；
  in　view　of　the　frequent　allusion　which
  will　be　made　to　the　terms　hereinafter。　Lift
  is　the　word　employed　to　indicate　the　amount
  which　a　plane　surface　will　support　while　in　flight。
  Drift　is　the　term　used　to　indicate　the　resistance
  which　is　offered　to　a　plane　moving　forwardly
  against　the　atmosphere。
  _Fig。　3。　Lift　and　Drift_
  In　Fig。　3　the　plane　A　is　assumed　to　be　moving
  forwardly　in　the　direction　of　the　arrow　B。　This
  indicates　the　resistance。　The　vertical　arrow　C
  shows　the　direction　of　lift；　which　is　the　weight
  held　up　by　the　plane。
  NORMAL　PRESSURE。Now　there　is　another　term
  much　used　which　needs　explanation；　and　that　is
  normal　pressure。　A　pressure　of　this　kind
  against　a　plane　is　where　the　wind　strikes　it　at
  right　angles。　This　is　illustrated　in　Fig。　4；　in
  which　the　plane　is　shown　with　the　wind　striking
  it　squarely。
  It　is　obvious　that　the　wind　will　exert　a　greater
  force　against　a　plane　when　at　its　normal。　On　the
  other　hand；　the　least　pressure　against　a　plane　is
  when　it　is　in　a　horizontal　position；　because　then
  the　wind　has　no　force　against　the　surfaces；　and
  the　only　effect　on　the　drift　is　that　which　takes
  place　when　the　wind　strikes　its　forward　edge。
  _Fig。　4。　Normal　Air　Pressure_
  _Fig。　5。　Edge　Resistance_
  HEAD　RESISTANCE。Fig。　5　shows　such　a　plane；
  the　only　resistance　being　the　thickness　of　the
  plane　as　at　A。　This　is　called　head　resistance；
  and　on　this　subject　there　has　been　much　controversy；
  and　many　theories；　which　will　be　considered
  under　the　proper　headings。
  If　a　plane　is　placed　at　an　angle　of　45　degrees
  the　lift　and　the　drift　are　the　same；　assumedly；　because；
  if　we　were　to　measure　the　power　required
  to　drive　it　forwardly；　it　would　be　found　to　equal
  the　weight　necessary　to　lift　it。　That　is；　suppose
  we　should　hold　a　plane　at　that　angle　with　a　heavy
  wind　blowing　against　it；　and　attach　two　pairs　of
  scales　to　the　plane；　both　would　show　the　same
  pull。
  _Fig。　6。　Measuring　Lift　and　Drift_
  MEASURING　LIFT　AND　DRIFT。In　Fig。　6；　A　is　the
  plane；　B　the　horizontal　line　which　attaches　the
  plane　to　a　scale　C；　and　D　the　line　attaching　it　to
  the　scale　E。　When　the　wind　is　of　sufficient　force
  to　hold　up　the　plane；　the　scales　will　show　the　same
  pull；　neglecting；　of　course；　the　weight　of　the
  plane　itself。
  PRESSURE　AT　DIFFERENT　ANGLES。What　every
  one　wants　to　know；　and　a　subject　on　which　a
  great　deal　of　experiment　and　time　have　been　expended；
  is　to　determine　what　the　pressures　are　at
  the　different　angles　between　the　horizontal；　and
  laws　have　been　formulated　which　enable　the　pressures
  to　be　calculated。
  DIFFERENCE　BETWEEN　LIFT　AND　DRIFT　IN　MOTION。The
  first　observation　is　directed　to　the　differences
  that　exist　between　the　lift　and　drift；
  when　the　plane　is　placed　at　an　angle　of　less　than
  45　degrees。　A　machine　weighing　1000　pounds
  has　always　the　same　lift。　Its　mass　does　not
  change。　Remember；　now；　we　allude　to　its　mass；
  or　density。
  We　are　not　now　referring　to　weight；　because
  that　must　be　taken　into　consideration；　in　the
  problem。　As　heretofore　stated；　when　an　object
  moves　horizontally；　it　has　less　weight　than　when
  at　rest。　If　it　had　the　same　weight　it　would　not
  move　forwardly；　but　come　to　rest。
  When　in　motion；　therefore；　while　the　lift；　so
  far　as　its　mass　is　concerned；　does　not　change；　the
  drift　does　decrease；　or　the　forward　pull　is　less
  than　when　at　45　degrees；　and　the　decrease　is　less
  and　less　until　the　plane　assumes　a　horizontal　position；
  where　it　is　absolutely　nil；　if　we　do　not　consider
  head　resistance。
  TABLES　OF　LIFT　AND　DRIFT。All　tables　of　Lift
  and　Drift　consider　only　the　air　pressures。　They
  do　not　take　into　account　the　fact　that　momentum
  takes　an　important　part　in　the　translation　of　an
  object；　like　a　flying　machine。
  A　mass　of　material；　weighing　1000　pounds　while
  at　rest；　sets　up　an　enormous　energy　when　moving
  through　the　air　at　fifty；　seventy…five；　or　one　hundred
  miles　an　hour。　At　the　latter　speed　the　movement
  is　about　160　feet　per　second；　a　motion　which
  is　nearly　sufficient　to　maintain　it　in　horizontal
  flight；　independently　of　any　plane　surface。
  Such　being　the　case；　why　take　into　account　only
  the　angle　of　the　plane？　It　is　no　wonder　that
  aviators　have　not　been　able　to　make　the　theoretical
  considerations　and　the　practical　demonstrations
  agree。
  WHY　TABLES　OF　LIFT　AND　DRIFT　ARE　WRONG。
  A　little　reflection　will　show　why　such　tables　are
  wrong。　They　were　prepared　by　using　a　plane
  surface　at　rest；　and　forcing　a　blast　of　air　against
  the　plane　placed　at　different　angles；　and　for　determining
  air　pressures；　this　is；　no　doubt；　correct。
  But　it　does　not　represent　actual　flying　conditions。
  It　does　not　show　the　conditions　existing
  in　an　aeroplane　while　in　flight。
  To　determine　this；　short　of　actual　experiments
  with　a　machine　in　horizontal　translation；　is　impossible；
  unless　it　is　done　by　taking　into　account
  the　factor　due　to　momentum　and　the　element
  attributable　to　the　lift　of　the　plane　itself　due　to　its
  impact　against　the　atmosphere。
  LANGLEY'S　LAW。The　law　enunciated　by
  Langley　is；　that　the　greater　the　speed　the　less　the
  power　required　to　propel　it。　Water　as　a　propelling
  medium　has　over　seven　hundred　times
  more　force　than　air。　A　vessel　having；　for　instance；
  twenty　horse　power；　and　a　speed　of　ten
  miles　per　hour；　would　require　four　times　that
  power　to　drive　it　through　the　water　at　double　the
  speed。　The　power　is　as　the　square　of　the　speed。
  With　air　the　conditions　are　entirely　different。
  The　boat　submergence　in　the　water　is　practically
  the　same；　whether　going　ten　or　twenty　miles　an
  hour。　The　head　resistance　is　the　same；　substantially；
  at　all　times　in　the　case　of　the　boat；　with　the
  flying　machine　the　resistance　of　its　sustaining
  surfaces　decreases。
  Without　going　into　a　too　technical　description
  of　the　reasoning　which　led　to　the　discovery　of　the
  law　of　air　pressures；　let　us　try　and　understand
  it　by　examining　the　diagram；　Fig。　7。
  A　represents　a　plane　at　an　angle　of　45　degrees；
  moving　forwardly　into　the　atmosphere　in　the
  direction　of　the　arrows　B。　The　measurement
  across　the　plane　vertically；　along　the　line　B；
  which　is　called　the　sine　of　the　angle；　represents
  the　surface　impact　of　air　against　the　plane。
  In　Fig。　8　the　plane　is　at　an　angle　of　27　degrees；
  which　makes　the　distance　in　height　across　the　line
  C　just　one…half　the　length　of　the　line　B　of　Fig。　7；
  hence　the　surface　impact　of　the　air　is　one…half　that
  of　Fig。　7；　and　the　drift　is　correspondingly　decreased。
  _Fig。　7。　Equal　Lift　and　Drift　in　Flig