Shellac　prevents　the　moist　atmosphere　from　affecting
  them。
  MATERIAL　FOR　PROPELLERS。Very　light　propellers
  can　also　be　made　of　thin；　annealed　aluminum
  sheets；　and　the　pins　in　that　case　will　serve　as
  guides　to　enable　you　to　get　the　desired　pitch。
  Fiber　board　may　also　be　used；　but　this　is　more
  difficult　to　handle。
  Another　good　material　is　celluloid　sheets；
  which；　when　cut　into　proper　strips；　is　dipped　in
  hot　water；　for　bending　purposes；　and　it　readily
  retains　its　shape　when　cooled。
  RUBBERSuitable　rubber　for　the　strips　are
  readily　obtainable　in　the　market。　Experiment
  will　soon　show　what　size　and　lengths　are　best
  adapted　for　the　particular　type　of　propellers
  which　you　succeed　in　making。
  PROPELLER　SHAPE　AND　SIZE。A　good　proportion
  of　propeller　is　shown　in　Fig。　89。　This　also
  shows　the　form　and　manner　of　connecting　the
  shaft。　The　latter　A　has　a　hook　B　on　one　end　to
  which　the　rubber　may　be　attached；　and　its　other
  end　is　flattened；　as　at　C；　and　secured　to　the　blade
  by　two…pointed　brads　D；　clinched　on　the　other
  side。
  _Fig。　89。　Shape　and　Size。_
  The　collar　E　is　soldered　on　the　shaft；　and　in
  practice　the　shaft　is　placed　through　the　bearing
  hole　at　the　end　of　the　frame　before　the　hook　is
  bent。
  SUPPORTING　SURFACES。The　supporting　surfaces
  may　be　made　perfectly　flat；　although　in　this
  particular　it　would　be　well　to　observe　the　rules
  with　respect　to　the　camber　of　large　machines。
  CHAPTER　XV
  THE　AEROPLANE　IN　THE　GREAT　WAR
  DURING　the　civil　war　the　Federal　forces　used
  captive　balloons　for　the　purpose　of　discovering
  the　positions　of　the　enemy。　They　were　of　great
  service　at　that　time；　although　they　were　stationed
  far　within　the　lines　to　prevent　hostile　guns　from
  reaching　them。
  BALLOON　OBSERVATIONS。Necessarily；　observations
  from　balloons　were　and　are　imperfect。　It
  was　found　to　be　very　unsatisfactory　during　the
  Russian…Japanese　war；　because　the　angle　of　vision
  is　very　low；　and；　furthermore；　at　such　distances　the
  movements；　or　even　the　location　of　troops　is　not
  observable；　except　under　the　most　favorable　conditions。
  Balloon　observation　during　the　progress　of　a
  battle　is　absolutely　useless；　because　the　smoke
  from　the　firing　line　is；　necessarily；　between　the
  balloon　and　the　enemy；　so　that　the　aerial　scout
  has　no　opportunity　to　make　any　observations；　even
  in　detached　portions　of　the　fighting　zone；　which
  are　of　any　value　to　the　commanders。
  CHANGED　CONDITIONS　OF　WARFARE。Since　our
  great　war；　conditions　pertaining　to　guns　have　been
  revolutionized。　Now　the　ranges　are　so　great　that
  captive　balloons　would　have　to　be　located　far　in
  the　rear；　and　at　such　a　great　distance　from　the
  firing　line　that　even　the　best　field　glasses　would
  be　useless。
  The　science　of　war　has　also　evolved　another
  condition。　Soldiers　are　no　longer　exposed　during
  artillery　attacks。　Uniforms　are　made　to　imitate
  natural　objects。　The　khaki　suits　were　designed
  to　imitate　the　yellow　veldts　of　South　Africa；
  the　gray…green　garments　of　the　German
  forces　are　designed　to　simulate　the　green　fields
  of　the　north。
  THE　EFFORT　TO　CONCEAL　COMBATANTS。The
  French　have　discarded　the　historic　red　trousers；
  and　the　elimination　of　lace；　white　gloves；　and
  other　telltale　insignias　of　the　officers；　have　been
  dispensed　with　by　special　orders。
  In　the　great　European　war　armies　have　burrowed
  in　the　earth　along　battle　lines　hundreds　of
  miles　in　length；　made　covered　trenches；　prepared
  artificial　groves　to　conceal　batteries；　and　in　many
  ingenious　ways　endeavored　to　make　the　battlefield
  an　imitation　field　of　nature。
  SMOKELESS　POWDER。While　smokeless　powder
  has　been　utilized　to　still　further　hide　a　fighting
  force；　it　has；　in　a　measure；　uncovered　itself；　as
  the　battlefield　is　not　now；　as　in　olden　times；　overspread
  with　masses　of　rolling　smoke。
  Nevertheless；　over　every　battlefield　there　is　a
  haze　which　can　be　penetrated　only　from　above；
  hence　the　possibilities　of　utilizing　the　aeroplane
  in　war　became　the　most　important　study　with　all
  nations；　as　soon　as　flying　became　an　accomplished
  fact。
  INVENTIONS　TO　ATTACK　AERIAL　CRAFT。Before
  any　nation　had　the　opportunity　to　make　an　actual
  test　on　the　battlefield；　inventors　were　at　work　to
  devise　a　means　whereby　an　aerial　foe　could　be
  met。　In　a　measure　the　aerial　gun　has　been　successful；
  but　months　of　war　has　shown　that　the
  aeroplane　is　one　of　the　strongest　arms　of　the
  service　in　actual　warfare。
  It　was　assumed　prior　to　the　European　war　that
  the　chief　function　of　the　aeroplane　would　be　the
  dropping　of　bombs；that　is　for　service　in　attacking
  a　foe。　Actual　practice　has　not　justified
  this　theory。　In　some　places　the　appearance　of
  the　aeroplane　has　caused　terror；　but　it　has　been
  found　the　great　value　is　its　scouting　advantages。
  FUNCTION　OF　THE　AEROPLANE　IN　WAR。While
  bomb　throwing　may　in　the　future　be　perfected；
  it　is　not　at　all　an　easy　problem　for　an　aviator　to
  do　work　which　is　commensurate　with　the　risk
  involved。　The　range　is　generally　too　great；　the
  necessity　of　swift　movement　in　the　machine　too
  speedy　to　assure　accuracy；　and　to　attack　a　foe　at
  haphazard　points　can　never　be　effectual。　Even
  the　slowly…moving　gas　fields；　like　the　Zeppelin；
  cannot　deliver　bombs　with　any　degree　of　precision
  or　accuracy。
  BOMB…THROWING　TESTS。It　is　interesting；　however；
  to　understand　how　an　aviator　knows　where
  or　when　to　drop　the　bomb　from　a　swiftly…moving
  machine。　Several　things　must　be　taken　into　consideration；
  such　as　the　height　of　the　machine　from
  the　earth；　its　speed；　and　the　parabolic　curve　that
  the　bomb　will　take　on　its　flight　to　the　earth。
  When　an　object　is　released　from　a　moving　machine
  it　will　follow　the　machine　from　which　it　is
  dropped；　gradually　receding　from　it；　as　it　descends；
  so　that　the　machine　is　actually　beyond
  the　place　where　the　bomb　strikes　the　earth；　due
  to　the　retarding　motion　of　the　atmosphere　against
  the　missile。
  The　diagram　Fig。　90　will　aid　the　boy　in　grasping
  the　situation。　A　is　the　airship；　B　the　path
  of　its　flight；　a　the　course　of　the　bomb　after　it
  leaves　the　airship；　and　D　the　earth。　The　question
  is　how　to　determine　the　proper　movement
  when　to　release　the　bomb。
  METHOD　FOR　DETERMINING　MOVEMENT　OF　A
  BOMB。Lieut。　Scott；　U。　S。　A。；　of　the　Coast　Survey
  Artillery；　suggested　a　method　for　determining
  these　questions。　It　was　necessary　to　ascertain；
  first；　the　altitude　and　speed。　While　the　barometer
  is　used　to　determine　altitudes；　it　is
  obvious　that　speed　is　a　matter　much　more　difficult
  to　ascertain；　owing　to　the　wind　movements；
  which　in　all　cases　make　it　difficult　for　a　flier　to
  determine；　even　with　instruments　which　have
  been　devised　for　the　purpose。
  _Fig。　90。　Course　of　a　Bomb。_
  Instead；　therefore；　of　relying　on　the　barometer；
  the　ship　is　equipped　with　a　telescope　which　may
  be　instantly　set　at　an　angle　of　45　degrees；　or　vertically。
  Thus；　Fig　91　shows　a　ship　A；　on　which　is
  mounted　a　telescope　B；　at　an　angle　of　45　degrees。
  The　observer　first　notes　the　object　along　the　line
  of　45　degrees；　and　starts　the　time　of　this　observation
  by　a　stop　watch。
  The　telescope　is　then　turned　so　it　is　vertical；
  as　at　C；　and　the　observer　watches　through　the
  telescope　until　the　machine　passes　directly　over
  the　object；　when　the　watch　is　stopped；　to　indicate
  the　time　between　the　two　observations。
  _Fig。　91。　Determining　Altitude　and　Speed。_
  The　height　of　the　machine　along　the　line　D　is
  thus　equal　to　the　line　E　from　B　to　C；　and　the　time
  of　the　flight　from　B　to　a　being　thus　known；　as
  well　as　the　height　of　the　machine；　the　observer
  consults　specially…prepared　tables　which　show
  just　what　kind　of　a　curve　the　bomb　will　make　at
  that　height　and　speed。
  All　that　is　necessary　now　is　to　set　the　sighter
  of　the　telescope　at　the　angle　given　in　the　tables；
  and　when　the　object　to　be　hit　appears　at　the　sight；
  the　bomb　is　dropped。
  THE　GREAT　EXTENT　OF　MODERN　BATTLE　LINES。
  The　great　war　brought　into　the　field　such　stupendous
  masses　of　men　that　the　battle　lines　have
  extended　over　an　unbroken　front　of　over　200
  miles。
  In　the　battle　of　Waterloo；　about　140；000　men
  were　engaged　on　both　sides；　and　the　battle　front
  was　less　than　six　miles。　There　were；　thus　massed；
  along　the　front；　over　20；000　men　every　mile　of
  the　way；　or　10；000　on　each　side。
  In　the　conflict　between　the　Allies　and　the　Germans
  it　is　estimated　that　there　were　less　than
  7500　along　each　mile。　It　was　predicted　in　the
  earlier　stages　of　the　war　that　it　would　be　an　easy
  matter　for　either　side　to　suddenly　mass　such　an
  overwhelming　force　at　one　point　as　to　enable　the
  attacking　party　to　go　through　the　opposing　force
  like　a　wedge。
  Such　tactics　were　often　employed　by　Napoleon
  and　other　great　masters　of　war；　but　in　every　effort
  where　it　has　been　attempted　in　the　present
  conflict；　it　was　foi