propeller；　gives　a　pull　of　less　than　eight　pounds
  for　every　horse　power　exerted。
  FOOT　POUNDS。The　work　produced　by　a　motor
  is　calculated　in　Foot　Pounds。　If　550　pounds
  should　be　lifted；　or　pulled；　one　foot　in　one　second
  of　time；　it　would　be　equal　to　one　horse　power。
  But　here　we　have　a　case　where　one　horse　power
  pulls　only　eight　pounds；　a　distance　of　one　foot
  within　one　second　of　time；　and　we　have　utilized
  less　than　one　sixty…fifth　of　the　actual　energy　produced。
  SMALL　AMOUNT　OF　POWER　AVAILABLE。This　is
  due　to　two　things：　First；　the　exceeding　lightness
  of　the　air；　and　its　great　elasticity；　and；　second；
  the　difficulty　of　making　a　surface　which；　when　it
  strikes　the　air；　will　get　a　sufficient　grip　to　effect
  a　proper　pull。
  Now　it　must　be　obvious；　that　where　only　such
  a　small　amount　of　energy　can　be　made　available；
  in　a　medium　as　elusive　as　air；　the　least　change；　or
  form；　of　the　propeller；　must　have　an　important
  bearing　in　the　general　results。
  HIGH　PROPELLER　SPEED　IMPORTANT。Furthermore；
  all　things　considered；　high　speed　is　important
  in　the　rotation　of　the　propeller；　up　to　a　certain
  point；　beyond　which　the　pull　decreases　in
  proportion　to　the　speed。　High　speed　makes　a
  vacuum　behind　the　blade　and　thus　decreases　the
  effective　pull　of　the　succeeding　blade。
  WIDTH　AND　PITCH　OF　BLADES。If　the　blade　is
  too　wide　the　speed　of　the　engine　is　cut　down　to　a
  point　where　it　cannot　exert　the　proper　energy；　if
  the　pitch　is　very　small　then　it　must　turn　further　to
  get　the　same　thrust；　so　that　the　relation　of　diameter；
  pitch　and　speed；　are　three　problems　far　from
  being　solved。
  It　may　be　a　question　whether　the　propeller　form；
  as　we　now　know　it；　is　anything　like　the　true　or
  ultimate　shape；　which　will　some　day　be　discovered。
  EFFECT　OF　INCREASING　PROPELLER　PULL。If　the
  present　pull　could　be　doubled　what　a　wonderful
  revolution　would　take　place　in　aerial　navigation；
  and　if　it　were　possible　to　get　only　a　quarter　of
  the　effective　pull　of　an　engine；　the　results　would
  be　so　stupendous　that　the　present　method　of　flying
  would　seem　like　child's　play　in　comparison。
  It　is　in　this　very　matter；the　application　of
  the　power；　that　the　bird；　and　other　flying　creatures
  so　far　excel　what　man　has　done。　Calculations
  made　with　birds　as　samples；　show　that　many
  of　them　are　able　to　fly　with　such　a　small　amount
  of　power　that；　if　the　same　energy　should　be　applied
  to　a　flying　machine；　it　would　scarcely　drive
  it　along　the　ground。
  DISPOSITION　OF　THE　PLANES。The　second　factor
  is　the　disposition　or　arrangement　of　the　planes
  with　relation　to　the　weight。　Let　us　illustrate　this
  with　a　concrete　example：
  We　have　an　aeroplane　with　a　sustaining　surface
  of　300　square　feet　which　weighs　900　pounds；
  or　30　pounds　per　square　foot　of　surface。
  DIFFERENT　SPEEDS　WITH　SAME　POWER。Now；　we
  may　be　able　to　do　two　things　with　an　airship　under
  those　conditions。　It　may　be　propelled　through
  the　air　thirty　miles　an　hour；　or　sixty　miles；　with
  the　expenditure　of　the　same　power。
  An　automobile；　if　propelled　at　sixty；　instead　of
  thirty　miles　an　hour；　would　require　an　additional
  power　in　doing　so；　but　an　airship　acts　differently；
  within　certain　limitations。
  When　it　is　first　set　in　motion　its　effective　pull
  may　not　be　equal　to　four　pounds　for　each　horse
  power；　due　to　the　slow　speed　of　the　propeller；　and
  also　owing　to　the　great　angle　of　incidence　which
  resists　the　forward　movement　of　the　ship。
  INCREASE　OF　SPEED　ADDS　TO　RESISTANCE。Finally；
  as　speed　increases；　the　angle　of　the　planes
  decrease；　resistance　is　less；　and　up　to　a　certain
  point　the　pull　of　the　propeller　increases；　but　beyond
  that　the　vacuum　behind　the　blades　becomes
  so　great　as　to　bring　down　the　pull；　and　there　is
  thus　a　balance；a　sort　of　mutual　governing　motion
  which；　together；　determine　the　ultimate　speed
  of　the　aeroplane。
  HOW　POWER　DECREASES　WITH　SPEED。If　now；
  with　the　same　propeller；　the　speed　should　be
  doubled；　the　ship　would　go　no　faster；　because　the
  bite　of　the　propeller　on　the　air　would　be　ineffective；
  hence　it　will　be　seen　that　it　is　not　the　amount
  of　power　in　itself；　that　determines　the　speed；　but
  the　shape　of　the　propeller；　which　must　be　so　made
  that　it　will　be　most　effective　at　the　speed　required
  for　the　ship。
  While　that　is　true　when　speed　is　the　matter　of
  greatest　importance；　it　is　not　the　case　where　it　is
  desired　to　effect　a　launching。　In　that　case　the
  propeller　must　be　made　so　that　its　greatest　pull
  will　be　at　a　slow　speed。　This　means　a　wider
  blade；　and　a　greater　pitch；　and　a　comparatively
  greater　pull　at　a　slow　speed。
  No　such　consideration　need　be　given　to　an　automobile。
  The　constant　accretion　of　power　adds
  to　its　speed。　In　flying　machines　the　aviator　must
  always　consider　some　companion　factor　which
  must　be　consulted。
  HOW　TO　CALCULATE　THE　POWER　APPLIED。In　a
  previous　chapter　reference　was　made　to　a　plane
  at　an　angle　of　forty…five　degrees；　to　which　two
  scales　were　attached；　one　to　get　its　horizontal　pull；
  or　drift；　and　the　other　its　vertical　pull；　or　lift。
  PULLING　AGAINST　AN　ANGLE。Let　us　take　the
  same　example　in　our　aeroplane。　Assuming　that
  it　weighs　900　pounds；　and　that　the　angle　of　the
  planes　is　forty…five　degrees。　If　we　suppose　that
  the　air　beneath　the　plane　is　a　solid；　and　frictionless；
  and　a　pair　of　scales　should　draw　it　up　the　incline；
  the　pull　in　doing　so　would　be　one…half　of　its
  weight；　or　450　pounds。
  It　must　be　obvious；　therefore；　that　its　force；　in
  moving　downwardly；　along　the　surface　A；　Fig。　60；
  would　be　450　pounds。
  The　incline　thus　shown　has　thereon　a　weight　B；
  mounted　on　wheels　a；　and　the　forwardly…projecting
  cord　represents　the　power；　or　propeller　pull；
  which　must；　therefore；　exert　a　force　of　450　pounds
  to　keep　it　in　a　stationary　position　against　the　surface
  A。
  In　such　a　case　the　thrust　along　the　diagonal
  line　E　would　be　900　pounds；　being　the　composition
  of　the　two　forces　pulling　along　the　lines　D；　F。
  THE　HORIZONTAL　AND　VERTICAL　PULL。Now　it
  must　be　obvious；　that　if　the　incline　takes　half　of
  the　weight　while　it　is　being　drawn　forwardly；　in
  the　line　of　D；　if　we　had　a　propeller　drawing　along
  that　line；　which　has　a　pull　of　450　pounds；　it　would
  maintain　the　plane　in　flight；　or；　at　any　rate　hold
  it　in　space；　assuming　that　the　air　should　be　moving
  past　the　plane。
  _Fig。　60。　Horizontal　and　Vertical　pull。_
  The　table　of　lift　and　drift　gives　a　fairly　accurate
  method　of　determining　this　factor；　and　we　refer　to
  the　chapter　on　that　subject　which　will　show　the
  manner　of　making　the　calculations。
  THE　POWER　MOUNTING。More　time　and　labor
  has　been　wasted；　in　airship　experiments；　in　poor
  motor　mounting；　than　in　any　other　direction。
  This　is　especially　true　where　two　propellers　are
  used；　or　where　the　construction　is　such　that　the
  propeller　is　mounted　some　distance　from　the　motor。
  SECURING　THE　PROPELLER　TO　THE　SHAFT。But
  even　where　the　propeller　is　mounted　on　the　engine
  shaft；　too　little　care　is　exercised　to　fix　it　securely。
  The　vibratory　character　of　the　mounting
  makes　this　a　matter　of　first　importance。　If　there
  is　a　solid　base　a　poorly　fixed　propeller　will　hold
  much　longer；　but　it　is　the　extreme　vibration　that
  causes　the　propeller　fastening　to　give　way。
  VIBRATIONS。If　experimenters　realized　that　an
  insecure；　shaking；　or　weaving　bed　would　cause　a
  loss　of　from　ten　to　fifteen　per　cent。　in　the　pull　of
  the　propeller；　more　care　and　attention　would　be
  given　to　this　part　of　the　structure。
  WEAKNESSES　IN　MOUNTING。The　general　weaknesses
  to　which　attention　should　be　directed　are；
  first；　the　insecure　attachment　of　the　propeller　to
  the　shaft；　second；　the　liability　of　the　base　to
  weave；　or　permit　of　a　torsional　movement；　third；
  improper　bracing　of　the　base　to　the　main　body　of
  the　aeroplane。
  If　the　power　is　transferred　from　the　cylinder
  to　the　engine　shaft　where　it　could　deliver　its　output
  without　the　use　of　a　propeller；　it　would　not
  be　so　important　to　consider　the　matter　of　vibration；
  but　the　propeller；　if　permitted　to　vibrate；
  or　dance　about；　absorbs　a　vast　amount　of　energy；
  while　at　the　same　time　cutting　down　its　effective
  pull。
  Aside　from　this　it　is　dangerous　to　permit　the
  slightest　displacement　while　the　engine　is　running。
  Any　looseness　is　sure　to　grow　worse；　instead
  of　better；　and　many　accidents　have　been
  registered　by　bolts　which　have　come　loose　from
  excessive　vibration。　It　is　well；　therefore；　to　have
  each　individual　nut　secured；　or　properly　locked；
  which　is　a　matter　easily　done；　and　when　so　secured
  there　is　but　little　trouble　in　going　over　the　machine
  to　notice　just　how　much　more　the　nut　must
  be　taken　up　to　again　make　it　secure。
  THE　GASOLINE　TANK。What　horrid　details　have
  been　told　of　the　pilots　who　have　been　burned　to
  death　with　the　escaping　gasoline　after　an　accident；
  before　help　arrived。　There　is　n