a　measurement；　or　a　factor　which　may　be　used　to
  measure　force。
  It　will　thus　be　seen　that　it　was　not　the　thrower
  which　gave　the　power；　but　the　article　itself。　A
  feather　ball　thrown　under　the　same　conditions；
  would　produce　a　half　unit　of　work；　and　the　iron
  ball；　therefore；　produced　800　times　more　energy。
  RESISTANCE。Now；　in　the　movement　of　any　body
  through　space；　it　meets　with　an　enemy　at　every
  step；　and　that　is　air　resistance。　This　is　much
  more　effective　against　the　cotton　than　the　iron
  ball：　or；　it　might　be　expressed　in　another　way：
  The　momentum；　or　the　power；　residing　in　the
  metal　ball；　is　so　much　greater　than　that　within　the
  cotton　ball　that　it　travels　farther；　or　strikes　a
  more　effective　blow　on　impact　with　the　wall。
  HOW　RESISTANCE　AFFECTS　THE　SHAPE。It　is　because
  of　this　counterforce；　resistance；　that　shape
  becomes　important　in　a　flying　object。　The　metal
  ball　may　be　flattened　out　into　a　thin　disk；　and　now；
  when　the　same　force　is　applied；　to　project　it　forwardly；
  it　will　go　as　much　farther　as　the　difference
  in　the　air　impact　against　the　two　forms。
  MASS　AND　RESISTANCE。Owing　to　the　fact　that
  resistance　acts　with　such　a　retarding　force　on　an
  object　of　small　mass；　and　it　is　difficult　to　set　up　a
  rapid　motion　in　an　object　of　great　density；　lightness
  in　flying　machine　structures　has　been　considered；
  in　the　past；　the　principal　thing　necessary。
  THE　EARLY　TENDENCY　TO　ELIMINATE　MOMENTUM。
  Builders　of　flying　machines；　for　several
  years；　sought　to　eliminate　the　very　thing
  which　gives　energy　to　a　horizontally…movable
  body；　namely；　momentum。
  Instead　of　momentum；　something　had　to　be
  substituted。　This　was　found　in　so　arranging　the
  machine　that　its　weight；　or　a　portion　of　it；　would
  be　sustained　in　space　by　the　very　element　which
  seeks　to　retard　its　flight；　namely；　the　atmosphere。
  If　there　should　be　no　material　substance；　like
  air；　then　the　only　way　in　which　a　heavier…than…air
  machine　could　ever　fly；　would　be　by　propelling　it
  through　space；　like　the　ball　was　thrown；　or　by
  some　sort　of　impulse　or　reaction　mechanism　on
  the　air…ship　itself。　It　could　get　no　support　from
  the　atmosphere。
  LIGHT　MACHINES　UNSTABLE。Gradually　the
  question　of　weight　is　solving　itself。　Aviators　are
  beginning　to　realize　that　momentum　is　a　wonderful
  property；　and　a　most　important　element　in
  flying。　The　safest　machines　are　those　which　have
  weight。　The　light；　willowy　machines　are　subject
  to　every　caprice　of　the　wind。　They　are　notoriously
  unstable　in　flight；　and　are　dangerous　even
  in　the　hands　of　experts。
  THE　APPLICATION　OF　POWER。The　thing　now　to
  consider　is　not　form；　or　shape；　or　the　distribution
  of　the　supporting　surfaces；　but　HOW　to　apply
  the　power　so　that　it　will　rapidly　transfer　a　machine
  at　rest　to　one　in　motion；　and　thereby　get
  the　proper　support　on　the　atmosphere　to　hold　it
  in　flight。
  THE　SUPPORTING　SURFACES。This　brings　us　to
  the　consideration　of　one　of　the　first　great　problems
  in　flying　machines；　namely；　the　supporting
  surfaces；not　its　form；　shape　or　arrangement；
  （which　will　be　taken　up　in　their　proper　places）；　but
  the　area；　the　dimensions；　and　the　angle　necessary
  for　flight。
  AREA　NOT　THE　ESSENTIAL　THING。The　history
  of　flying　machines；　short　as　it　is；　furnishes　many
  examples　of　one　striking　fact：　That　area　has
  but　little　to　do　with　sustaining　an　aeroplane　when
  once　in　flight。　The　first　Wright　flyer　weighed
  741　pounds；　had　about　400　square　feet　of　plane
  surface；　and　was　maintained　in　the　air　with　a　12
  horse　power　engine。
  True；　that　machine　was　shot　into　the　air　by　a
  catapult。　Motion　having　once　been　imparted　to　it；
  the　only　thing　necessary　for　the　motor　was　to
  maintain　the　speed。
  There　are　many　instances　to　show　that　when
  once　in　flight；　one　horse　power　will　sustain　over
  100　pounds；　and　each　square　foot　of　supporting
  surface　will　maintain　90　pounds　in　flight。
  THE　LAW　OF　GRAVITY。As　the　effort　to　fly
  may　be　considered　in　the　light　of　a　struggle　to
  avoid　the　laws　of　nature　with　respect　to　matter；
  it　may　be　well　to　consider　this　great　force　as　a
  fitting　prelude　to　the　study　of　our　subject。
  Proper　understanding；　and　use　of　terms　is　very
  desirable；　so　that　we　must　not　confuse　them。
  Thus；　weight　and　mass　are　not　the　same。　Weight
  varies　with　the　latitude；　and　it　is　different　at　various
  altitudes；　but　mass　is　always　the　same。
  If　projected　through　space；　a　certain　mass
  would　move　so　as　to　produce　momentum；　which
  would　be　equal　at　all　places　on　the　earth's　surface；
  or　at　any　altitude。
  Gravity　has　been　called　weight；　and　weight
  gravity。　The　real　difference　is　plain　if　gravity
  is　considered　as　the　attraction　of　mass　for　mass。
  Gravity　is　generally　known　and　considered　as　a
  force　which　seeks　to　draw　things　to　the　earth。
  This　is　too　narrow。
  Gravity　acts　in　all　directions。　Two　balls　suspended
  from　strings　and　hung　in　close　proximity
  to　each　other　will　mutually　attract　each　other。
  If　one　has　double　the　mass　it　will　have　twice　the
  attractive　power。　If　one　is　doubled　and　the　other
  tripled；　the　attraction　would　be　increased　six
  times。　But　if　the　distance　should　be　doubled　the
  attraction　would　be　reduced　to　one…fourth；　and
  if　the　distance　should　be　tripled　then　the　pull
  would　be　only　one…ninth。
  The　foregoing　is　the　substance　of　the　law；
  namely；　that　all　bodies　attract　all　other　bodies
  with　a　force　directly　in　proportion　to　their　mass；
  and　inversely　as　the　square　of　their　distance　from
  one　another。
  To　explain　this　we　cite　the　following　illustration：
  Two　bodies；　each　having　a　mass　of　4
  pounds；　and　one　inch　apart；　are　attracted　toward
  each　other；　so　they　touch。　If　one　has　twice　the
  mass　of　the　other；　the　smaller　will　draw　the　larger
  only　one…quarter　of　an　inch；　and　the　large　one
  will　draw　the　other　three…quarters　of　an　inch；
  thus　confirming　the　law　that　two　bodies　will　attract
  each　other　in　proportion　to　their　mass。
  Suppose；　now；　that　these　balls　are　placed　two
  inches　apart；that　is；　twice　the　distance。　As
  each　is；　we　shall　say；　four　pounds　in　weight；　the
  square　of　each　would　be　16。　This　does　not　mean
  that　there　would　be　sixteen　times　the　attraction；
  but；　as　the　law　says；　inversely　as　the　square　of
  the　distance；　so　that　at　two　inches　there　is　only
  one…sixteenth　the　attraction　as　at　one　inch。
  If　the　cord　of　one　of　the　balls　should　be　cut；　it
  would　fall　to　the　earth；　for　the　reason　that　the
  attractive　force　of　the　great　mass　of　the　earth　is
  so　much　greater　than　the　force　of　attraction　in
  its　companion　ball。
  INDESTRUCTIBILITY　OF　GRAVITATION。Gravity
  cannot　be　produced　or　destroyed。　It　acts　between
  all　parts　of　bodies　equally；　the　force　being
  proportioned　to　their　mass。　It　is　not　affected　by
  any　intervening　substance；　and　is　transmitted
  instantaneously；　whatever　the　distance　may　be。
  While；　therefore；　it　is　impossible　to　divest　matter
  of　this　property；　there　are　two　conditions
  which　neutralize　its　effect。　The　first　of　these　is
  position。　Let　us　take　two　balls；　one　solid　and
  the　other　hollow；　but　of　the　same　mass；　or　density。
  If　the　cavity　of　the　one　is　large　enough　to　receive
  the　other；　it　is　obvious　that　while　gravity　is　still
  present　the　lines　of　attraction　being　equal　at
  all　points；　and　radially；　there　can　be　no　pull　which
  moves　them　together。
  DISTANCE　REDUCES　GRAVITATIONAL　PULL。Or
  the　balls　may　be　such　distance　apart　that　the　attractive
  force　ceases。　At　the　center　of　the　earth
  an　object　would　not　weigh　anything。　A　pound
  of　iron　and　an　ounce　of　wood；　one　sixteen　times
  the　mass　of　the　other；　would　be　the　same；absolutely
  without　weight。
  If　the　object　should　be　far　away　in　space　it
  would　not　be　influenced　by　the　earth's　gravity；
  so　it　will　be　understood　that　position　plays　an
  important　part　in　the　attraction　of　mass　for　mass。
  HOW　MOTION　ANTAGONIZES　GRAVITY。The　second
  way　to　neutralize　gravity；　is　by　motion。　A
  ball　thrown　upwardly；　antagonizes　the　force　of
  gravity　during　the　period　of　its　ascent。　In　like
  manner；　when　an　object　is　projected　horizontally；
  while　its　mass　is　still　the　same；　its　weight　is　less。
  Motion　is　that　which　is　constantly　combating
  the　action　of　gravity。　A　body　moving　in　a　circle
  must　be　acted　upon　by　two　forces；　one　which　tends
  to　draw　it　inwardly；　and　the　other　which　seeks　to
  throw　it　outwardly。
  The　former　is　called　centripetal；　and　the　latter
  centrifugal　motion。　Gravity；　therefore；　represents
  centripetal；　and　motion　centrifugal　force。
  If　the　rotative　speed　of　the　earth　should　be　retarded；
  all　objects　on　the　earth　would　be　increased
  in　weight；　and　if　the　motion　should　be　accelerated
  objects　would　become　lighter；　and　if　sufficient
  speed　should　be　attained　all　matter　would　fly　off
  the　surface；　just　as　dirt　dies　off　the　rim　of　a
  wheel　at　certain　speeds。
  A　TANGENT。When　an　object　is　thrown　horizontally
  the　line　of