way　between　the　tip　and　the　hub。
  In　such　a　propeller；　if　the　blade　is　the　same
  width　and　pitch　at　the　two　points　named；　the　pull
  at　the　tips　will　be　four　times　greater　than　at　the
  intermediate　point。
  CHAPTER　XIV
  EXPERIMENTAL　GLIDERS　AND　MODEL　AEROPLANES
  AN　amusing　and　very　instructive　pastime　is
  afforded　by　constructing　and　flying　gliding　machines；
  and　operating　model　aeroplanes；　the　latter
  being　equipped　with　their　own　power。
  Abroad　this　work　has　been　very　successful　as
  a　means　of　interesting　boys；　and；　indeed；　men
  who　have　taken　up　the　science　of　aviation　are
  giving　this　sport　serious　thought　and　study。
  When　a　machine　of　small　dimensions　is　made
  the　boy　wonders　why　a　large　machine　does　not
  bear　the　same　relation　in　weight　as　a　small　machine。
  This　is　one　of　the　first　lessons　to　learn。
  THE　RELATION　OF　MODELS　TO　FLYING　MACHINES。
  A　model　aeroplane；　say　two　feet　in　length；　which
  has；　we　will　assume；　50　square　inches　of　supporting
  surface；　seems　to　be　a　very　rigid　structure；
  in　proportion　to　its　weight。　It　may　be　dropped
  from　a　considerable　height　without　injuring　it；
  since　the　weight　is　only　between　two　and　three
  ounces。
  An　aeroplane　twenty　times　the　length　of　this
  model；　however　strongly　it　may　be　made；　if
  dropped　the　same　distance；　would　be　crushed；　and
  probably　broken　into　fragments。
  If　the　large　machine　is　twenty　times　the　dimensions
  of　the　small　one；　it　would　be　forty　feet　in
  length；　and；　proportionally；　would　have　only
  seven　square　feet　of　sustaining　surface。　But　an
  operative　machine　of　that　size；　to　be　at　all　rigid；
  would　require　more　than　twenty　times　the　material
  in　weight　to　be　equal　in　strength。
  It　would　weigh　about　800　pounds；　that　is；　4800
  times　the　weight　of　the　model；　and　instead　of
  having　twenty　times　the　plane　surface　would　require
  one　thousand　times　the　spread。
  It　is　this　peculiarity　between　models　and　the
  actual　flyers　that　for　years　made　the　question　of
  flying　a　problem　which；　on　the　basis　of　pure　calculation
  alone；　seemed　to　offer　a　negative；　and
  many　scientific　men　declared　that　practical　flying
  was　an　impossibility。
  LESSONS　FROM　MODELS。Men；　and　boys；　too；
  can　learn　a　useful　lesson　from　the　model　aeroplanes
  in　other　directions；　however；　and　the　principal
  thing　is　the　one　of　stability。
  When　everything　is　considered　the　form　or
  shape　of　a　flying　model　will　serve　to　make　a　large
  flyer。　The　manner　of　balancing　one　will　be　a
  good　criterion　for　the　other　in　practice；　and
  experimenting　with　these　small　devices　is；　therefore；
  most　instructive。
  The　difference　between　gliders　and　model　aeroplanes
  is；　that　gliders　must　be　made　much　lighter
  because　they　are　designed　to　be　projected　through
  the　air　by　a　kick　of　some　kind。
  FLYING　MODEL　AEROPLANES。Model　aeroplanes
  contain　their　own　power　and　propellers　which；
  while　they　may　run　for　a　few　seconds　only；　serve
  the　purpose　of　indicating　how　the　propeller　will
  act；　and　in　what　respect　the　sustaining　surfaces
  are　efficient　and　properly　arranged。
  It　is　not　our　purpose　to　give　a　treatise　on　this
  subject　but　to　confine　this　chapter　to　an　exposition
  of　a　few　of　the　gliders　and　model　forms　which
  are　found　to　be　most　efficient　for　experimental
  work。
  AN　EFFICIENT　GLIDER。Probably　the　simplest
  and　most　efficient　glider；　and　one　which　can　be
  made　in　a　few　moments；　is　to　make　a　copy　of　the
  deltoid　kite；　previously　referred　to。
  This　is　merely　a　triangularly…shaped　piece　of
  paper；　or　stiff　cardboard　A；　Fig。　84；　creased　in
  the　middle；　along　the　dotted　line　B；　the　side　wings
  C；　C；　being　bent　up　so　as　to　form；　what　are　called
  diedral　angles。　This　may　be　shot　through　the
  air　by　a　flick　of　the　finger；　with　the　pointed　end
  foremost；　when　used　as　a　glider。
  _Fig。　85。　Deltoid　Glider。_
  THE　DELTOID　FORMATION。This　same　form　may
  be　advantageously　used　as　a　model　aeroplane；　but
  in　that　case　the　broad　end　should　be　foremost。
  _Fig。　86。　The　Deltoid　Racer。_
  Fig。　86　shows　the　deltoid　glider；　or　aeroplane；
  with　three　cross　braces；　A；　B；　C；　in　the　two　forward
  braces　of　which　are　journaled　the　propeller
  shaft　D；　so　that　the　propeller　E　is　at　the　broad
  end　of　the　glider。
  A　short　stem　F　through　the　rear　brace　C；　provided
  with　a　crank；　has　its　inner　end　connected
  with　the　rear　end　of　the　shaft　D　by　a　rubber　band
  G；　by　which　the　propeller　is　driven。
  A　tail　may　be　attached　to　the　rear　end；　or　at
  the　apex　of　the　planes；　so　it　can　be　set　for　the
  purpose　of　directing　the　angle　of　flight；　but　it　will
  be　found　that　this　form　has　remarkable　stability
  in　flight；　and　will　move　forwardly　in　a　straight
  line；　always　making　a　graceful　downward　movement
  when　the　power　is　exhausted。
  It　seems　to　be　a　form　which　has　equal　stabilizing
  powers　whether　at　slow　or　at　high　speeds；
  thus　differing　essentially　from　many　forms　which
  require　a　certain　speed　in　order　to　get　the　best
  results。
  RACING　MODELS。Here　and　in　England　many
  racing　models　have　been　made；　generally　of　the
  A…shaped　type；　which　will　be　explained　hereinafter。
  Such　models　are　also　strong；　and　able　to
  withstand　the　torsional　strain　required　by　the
  rubber　which　is　used　for　exerting　the　power。
  It　is　unfortunate　that　there　is　not　some　type　of
  cheap　motor　which　is　light；　and　adapted　to　run
  for　several　minutes；　which　would　be　of　great　value
  in　work　of　this　kind；　but　in　the　absence　of　such
  mechanism　rubber　bands　are　found　to　be　most
  serviceable；　giving　better　results　than　springs　or
  bows；　since　the　latter　are　both　too　heavy　to　be
  available；　in　proportion　to　the　amount　of　power
  developed。
  Unlike　the　large　aeroplanes；　the　supporting
  surfaces；　in　the　models；　are　at　the　rear　end　of
  the　frames；　the　pointed　ends　being　in　front。
  _Fig。　87。　A…Shaped　Racing　Glider。_
  Fig。　87　shows　the　general　design　of　the　A…
  shaped　gliding　plane　or　aeroplane。　This　is　composed
  of　main　frame　pieces　A；　A；　running　fore
  and　aft；　joined　at　their　rear　ends　by　a　cross　bar
  B；　the　ends　of　which　project　out　slightly　beyond
  their　juncture　with　the　side　bars　A；　A。　These
  projecting　ends　have　holes　drilled　therein　to　receive
  the　shafts　a；　a；　of　the　propeller　D；　D。
  A　main　plane　E　is　mounted　transversely　across
  this　frame　at　its　rear　end；　while　at　its　forward
  end　is　a　small　plane；　called　the　elevator。　The
  pointed　end　of　the　frame　has　on　each　side　a　turnbuckle
  G；　for　the　purpose　of　winding　up　the　shaft；
  and　thus　twisting　the　propeller；　although　this　is
  usually　dispensed　with；　and　the　propeller　itself
  is　turned　to　give　sufficient　twist　to　the　rubber　for
  this　purpose。
  THE　POWER　FOR　MODEL　AEROPLANES。One　end
  of　the　rubber　is　attached　to　the　hook　of　the　shaft
  C；　and　the　other　end　to　the　hook　or　to　the　turnbuckle
  G；　if　it　should　be　so　equipped。
  The　rubbers　are　twisted　in　opposite　directions；
  to　correspond　with　the　twist　of　the　propeller
  blades；　and　when　the　propellers　are　permitted　to
  turn；　their　grip　on　the　air　will　cause　the　model　to
  shoot　forwardly；　until　the　rubbers　are　untwisted；
  when　the　machine　will　gradually　glide　to　the
  ground。
  MAKING　THE　PROPELLER。These　should　have
  the　pitch　uniform　on　both　ends；　and　a　simple
  little　device　can　be　made　to　hold　the　twisted　blade
  after　it　has　been　steamed　and　bent。　Birch　and
  holly　are　good　woods　for　the　blades。　The　strips
  should　be　made　thin　and　then　boiled；　or；　what　is
  better　still；　should　be　placed　in　a　deep　pan；　and
  held　on　a　grid　above　the　water；　so　they　will　be
  thoroughly　steamed。
  They　are　then　taken　out　and　bent　by　hand；　or
  secured　between　a　form　specially　prepared　for
  the　purpose。　The　device　shown　in　Fig。　88　shows
  a　base　board　which　has　in　the　center　a　pair　of
  parallel　pins　A；　A；　slightly　separated　from　each
  other。
  _Fig。　88。　Making　the　Propeller。_
  At　each　end　of　the　base　board　is　a　pair　of　holes
  C；　D；　drilled　in　at　an　angle；　the　angles　being　the
  pitch　desired　for　the　ends　of　the　propeller。　In
  one　of　these　holes　a　pin　E　is　placed；　so　the　pins
  at　the　opposite　ends　project　in　different　directions；
  and　the　tips　of　the　propeller　are　held
  against　the　ends　of　these　pins；　while　the　middle
  of　the　propeller　is　held　between　the　parallel　pins
  A；　A。
  The　two　holes；　at　the　two　angles　at　the　ends　of
  the　board；　are　for　the　purpose　of　making　right
  and　left　hand　propellers；　as　it　is　desirable　to　use
  two　propellers　with　the　A…shaped　model。　Two
  propellers　with　the　deltoid　model　are　not　so　necessary。
  After　the　twist　is　made　and　the　blade　properly
  secured　in　position　it　should　be　allowed　to　thoroughly
  dry；　and　afterwards；　if　it　is　coated　with
  shellac；　will　not　untwist；　as　it　is　the　changing
  character　of　the　atmosphere　which　usually　causes
  the　twisted　strips　to　change　their　positions。
  Shellac　prevents　the　moist　atmosphere　from　affecting
  them。
  MATERIAL　FOR　PROPELLERS。Very　light　pr