reasoning　upon　the　point；　and　that　mode　_must_　lead　us　to　a　definite
  decision。　…　Let　us　examine；　each　by　each；　the　possible　means　of
  egress。　It　is　clear　that　the　assassins　were　in　the　room　where
  Mademoiselle　L'Espanaye　was　found；　or　at　least　in　the　room　adjoining；
  when　the　party　ascended　the　stairs。　It　is　then　only　from　these　two
  apartments　that　we　have　to　seek　issues。　The　police　have　laid　bare　the
  floors；　the　ceilings；　and　the　masonry　of　the　walls；　in　every
  direction。　No　_secret_　issues　could　have　escaped　their　vigilance。
  But；　not　trusting　to　_their_　eyes；　I　examined　with　my　own。　There
  were；　then；　no　secret　issues。　Both　doors　leading　from　the　rooms　into
  the　passage　were　securely　locked；　with　the　keys　inside。　Let　us　turn
  to　the　chimneys。　These；　although　of　ordinary　width　for　some　eight　or
  ten　feet　above　the　hearths；　will　not　admit；　throughout　their　extent；
  the　body　of　a　large　cat。　The　impossibility　of　egress；　by　means
  already　stated；　being　thus　absolute；　we　are　reduced　to　the　windows。
  Through　those　of　the　front　room　no　one　could　have　escaped　without
  notice　from　the　crowd　in　the　street。　The　murderers　_must_　have
  passed；　then；　through　those　of　the　back　room。　Now；　brought　to　this
  conclusion　in　so　unequivocal　a　manner　as　we　are；　it　is　not　our　part；
  as　reasoners；　to　reject　it　on　account　of　apparent　impossibilities。　It
  is　only　left　for　us　to　prove　that　these　apparent　'impossibilities'
  are；　in　reality；　not　such。
  〃There　are　two　windows　in　the　chamber。　One　of　them　is　unobstructed　by
  furniture；　and　is　wholly　visible。　The　lower　portion　of　the　other　is
  hidden　from　view　by　the　head　of　the　unwieldy　bedstead　which　is　thrust
  close　up　against　it。　The　former　was　found　securely　fastened　from
  within。　It　resisted　the　utmost　force　of　those　who　endeavored　to　raise
  it。　A　large　gimlet…hole　had　been　pierced　in　its　frame　to　the　left；
  and　a　very　stout　nail　was　found　fitted　therein；　nearly　to　the　head。
  Upon　examining　the　other　window；　a　similar　nail　was　seen　similarly
  fitted　in　it；　and　a　vigorous　attempt　to　raise　this　sash；　failed　also。
  The　police　were　now　entirely　satisfied　that　egress　had　not　been　in
  these　directions。　And；　_therefore_；　it　was　thought　a　matter　of
  supererogation　to　withdraw　the　nails　and　open　the　windows。
  〃My　own　examination　was　somewhat　more　particular；　and　was　so　for　the
  reason　I　have　just　given　…　because　here　it　was；　I　knew；　that　all
  apparent　impossibilities　_must_　be　proved　to　be　not　such　in　reality。
  〃I　proceeded　to　think　thus　…　_？posteriori_。　The　murderers　did　escape
  from　one　of　these　windows。　This　being　so；　they　could　not　have
  refastened　the　sashes　from　the　inside；　as　they　were　found　fastened；　…
  the　consideration　which　put　a　stop；　through　its　obviousness；　to　the
  scrutiny　of　the　police　in　this　quarter。　Yet　the　sashes　_were_
  fastened。　They　_must_；　then；　have　the　power　of　fastening　themselves。
  There　was　no　escape　from　this　conclusion。　I　stepped　to　the
  unobstructed　casement；　withdrew　the　nail　with　some　difficulty　and
  attempted　to　raise　the　sash。　It　resisted　all　my　efforts；　as　I　had
  anticipated。　A　concealed　spring　must；　I　now　know；　exist；　and　this
  corroboration　of　my　idea　convinced　me　that　my　premises　at　least；　were
  correct；　however　mysterious　still　appeared　the　circumstances
  attending　the　nails。　A　careful　search　soon　brought　to　light　the
  hidden　spring。　I　pressed　it；　and；　satisfied　with　the　discovery；
  forbore　to　upraise　the　sash。
  〃I　now　replaced　the　nail　and　regarded　it　attentively。　A　person
  passing　out　through　this　window　might　have　reclosed　it；　and　the
  spring　would　have　caught　…　but　the　nail　could　not　have　been　replaced。
  The　conclusion　was　plain；　and　again　narrowed　in　the　field　of　my
  investigations。　The　assassins　_must_　have　escaped　through　the　other
  window。　Supposing；　then；　the　springs　upon　each　sash　to　be　the　same；
  as　was　probable；　there　_must_　be　found　a　difference　between　the
  nails；　or　at　least　between　the　modes　of　their　fixture。　Getting　upon
  the　sacking　of　the　bedstead；　I　looked　over　the　head…board　minutely　at
  the　second　casement。　Passing　my　hand　down　behind　the　board；　I　readily
  discovered　and　pressed　the　spring；　which　was；　as　I　had　supposed；
  identical　in　character　with　its　neighbor。　I　now　looked　at　the　nail。
  It　was　as　stout　as　the　other；　and　apparently　fitted　in　the　same
  manner　…　driven　in　nearly　up　to　the　head。
  〃You　will　say　that　I　was　puzzled；　but；　if　you　think　so；　you　must　have
  misunderstood　the　nature　of　the　inductions。　To　use　a　sporting　phrase；
  I　had　not　been　once　'at　fault。'　The　scent　had　never　for　an　instant
  been　lost。　There　was　no　flaw　in　any　link　of　the　chain。　I　had　traced
  the　secret　to　its　ultimate　result；　…　and　that　result　was　_the　nail。_
  It　had；　I　say；　in　every　respect；　the　appearance　of　its　fellow　in　the
  other　window；　but　this　fact　was　an　absolute　nullity　（conclusive　us　it
  might　seem　to　be）　when　compared　with　the　consideration　that　here；　at
  this　point；　terminated　the　clew。　'There　_must_　be　something　wrong；'　I
  said；　'about　the　nail。'　I　touched　it；　and　the　head；　with　about　a
  quarter　of　an　inch　of　the　shank；　came　off　in　my　fingers。　The　rest　of
  the　shank　was　in　the　gimlet…hole　where　it　had　been　broken　off。　The
  fracture　was　an　old　one　（for　its　edges　were　incrusted　with　rust）；　and
  had　apparently　been　accomplished　by　the　blow　of　a　hammer；　which　had
  partially　imbedded；　in　the　top　of　the　bottom　sash；　the　head　portion
  of　the　nail。　I　now　carefully　replaced　this　head　portion　in　the
  indentation　whence　I　had　taken　it；　and　the　resemblance　to　a　perfect
  nail　was　complete　…　the　fissure　was　invisible。　Pressing　the　spring；　I
  gently　raised　the　sash　for　a　few　inches；　the　head　went　up　with　it；
  remaining　firm　in　its　bed。　I　closed　the　window；　and　the　semblance　of
  the　whole　nail　was　again　perfect。
  〃The　riddle；　so　far；　was　now　unriddled。　The　assassin　had　escaped
  through　the　window　which　looked　upon　the　bed。　Dropping　of　its　own
  accord　upon　his　exit　（or　perhaps　purposely　closed）；　it　had　become
  fastened　by　the　spring；　and　it　was　the　retention　of　this　spring　which
  had　been　mistaken　by　the　police　for　that　of　the　nail；　…　farther
  inquiry　being　thus　considered　unnecessary。
  〃The　next　question　is　that　of　the　mode　of　descent。　Upon　this　point　I
  had　been　satisfied　in　my　walk　with　you　around　the　building。　About
  five　feet　and　a　half　from　the　casement　in　question　there　runs　a
  lightning…rod。　From　this　rod　it　would　have　been　impossible　for　any
  one　to　reach　the　window　itself；　to　say　nothing　of　entering　it。　I
  observed；　however；　that　the　shutters　of　the　fourth　story　were　of　the
  peculiar　kind　called　by　Parisian　carpenters　_ferrades_　…　a　kind
  rarely　employed　at　the　present　day；　but　frequently　seen　upon　very　old
  mansions　at　Lyons　and　Bourdeaux。　They　are　in　the　form　of　an　ordinary
  door；　（a　single；　not　a　folding　door）　except　that　the　lower　half　is
  latticed　or　worked　in　open　trellis　…　thus　affording　an　excellent　hold
  for　the　hands。　In　the　present　instance　these　shutters　are　fully　three
  feet　and　a　half　broad。　When　we　saw　them　from　the　rear　of　the　house；
  they　were　both　about　half　open　…　that　is　to　say；　they　stood　off　at
  right　angles　from　the　wall。　It　is　probable　that　the　police；　as　well
  as　myself；　examined　the　back　of　the　tenement；　but；　if　so；　in　looking
  at　these　_ferrades_　in　the　line　of　their　breadth　（as　they　must　have
  done）；　they　did　not　perceive　this　great　breadth　itself；　or；　at　all
  events；　failed　to　take　it　into　due　consideration。　In　fact；　having
  once　satisfied　themselves　that　no　egress　could　have　been　made　in　this
  quarter；　they　would　naturally　bestow　here　a　very　cursory　examination。
  It　was　clear　to　me；　however；　that　the　shutter　belonging　to　the　window
  at　the　head　of　the　bed；　would；　if　swung　fully　back　to　the　wall；　reach
  to　within　two　feet　of　the　lightning…rod。　It　was　also　evident　that；　by
  exertion　of　a　very　unusual　degree　of　activity　and　courage；　an
  entrance　into　the　window；　from　the　rod；　might　have　been　thus
  effected。　…　By　reaching　to　the　distance　of　two　feet　and　a　half　（we
  now　suppose　the　shutter　open　to　its　whole　extent）　a　robber　might　have
  taken　a　firm　grasp　upon　the　trellis…work。　Letting　go；　then；　his　hold
  upon　the　rod；　placing　his　feet　securely　against　the　wall；　and
  springing　boldly　from　it；　he　might　have　swung　the　shutter　so　as　to
  close　it；　and；　if　we　imagine　the　window　open　at　the　time；　might　even
  have　swung　himself　into　the　room。
  〃I　wish　you　to　bear　especially　in　mind　that　I　have　spoken　of　a　_very_
  unusual　degree　of　activity　as　requisite　to　success　in　so　hazardous
  and　so　difficult　a　feat。　It　is　my　design　to　show　you；　first；　that　the
  thing　might　possibly　have　been　accomplished：　…　but；　secondly　and
  _chiefly_；　I　wish　to　impress　upon　your　understanding　the　_very
  extraordinary_　…　the　almost　pr鎡ernatural　character　of　that　agility
  which　could　have　accomplished　it。
  〃You　will　say；　no　doubt；　using　the　language　of　the　law；　that　'to　make
  out　my　case；'　I　should　rather　undervalue；　than　insist　upon　a　full
  estimation　of　the　activity　required　in　this　matter。　This　may　be　the
  practice　in　law；　but　it　is　not　the　usage　of　reason。　My　ultimate
  object　is　only　the　truth。　My　immediate　purpose　is　to　lead　you　to
  plac