Chapter I The Science of Deduction

tiesnecessaryfortheidealdetective.Hehasthepowerofobservationandthatofdeduction.Heisonlywantinginknowledgeandthatmaycomeintime.HeisnowtranslatingmysmallworksintoFrench.” “Yourworks?” “Oh,didn’tyouknow?”hecried,laughing.“Yes,Ihavebeenguiltyofseveralmonographs.Theyareallupontechnicalsubjects.Here,forexample,isone‘UpontheDistinctionbetweentheAshesoftheVariousTobaccoes.’InitIenumerateahundredandfortyformsofcigar-,cigarette-,andpipe-tobacco,withcolouredplatesillustratingthedifferenceintheash.Itisapointwhichiscontinuallyturningupincriminaltrials,andwhichissometimesofsupremeimportanceasaclue.Ifyoucansaydefinitely,forexample,thatsomemurderhasbeendonebyamanwhowassmokinganIndianlunkah,itobviouslynarrowsyourfieldofsearch.TothetrainedeyethereisasmuchdifferencebetweentheblackashofaTrichinopolyandthewhitefluffofbird’s-eyeasthereisbetweenacabbageandapotato.” “Youhaveanextraordinarygeniusforminuti?,”Iremarked. “Iappreciatetheirimportance.Hereismymonographuponthetracingoffootsteps,withsomeremarksupontheusesofplasterofParisasapreserverofimpresses.Here,too,isacuriouslittleworkupontheinfluenceofatradeupontheformofthehand,withlithotypesofthehandsofslaters,sailors,corkcutters,compositors,weavers,anddiamond-polishers.Thatisamatterofgreatpracticalinteresttothescientificdetective,—especiallyincasesofunclaimedbodies,orindiscoveringtheantecedentsofcriminals.ButIwearyyouwithmyhobby.” “Notatall,”Ianswered,earnestly.“Itisofthegreatestinteresttome,especiallysinceIhavehadtheopportunityofobservingyourpracticalapplicationofit.Butyouspokejustnowofobservationanddeduction.Surelytheonetosomeextentimpliestheother.” “Why,hardly,”heanswered,leaningbackluxuriouslyinhisarm-chair,andsendingupthickbluewreathsfromhispipe.“Forexample,observationshowsmethatyouhavebeentotheWigmoreStreetPost-Officethismorning,butdeductionletsmeknowthatwhenthereyoudispatchedatelegram.” “Right!”saidI.“Rightonbothpoints!ButIconfessthatIdon’tseehowyouarrivedatit.Itwasasuddenimpulseuponmypart,andIhavementionedittonoone.” “Itissimplicityitself,”heremarked,chucklingatmysurprise,—“soabsurdlysimplethatanexplanationissuperfluousandyetitmayservetodefinethelimitsofobservationandofdeduction.Observationtellsmethatyouhavealittlereddishmouldadheringtoyourinstep.JustoppositetheWigmoreStreetOfficetheyhavetakenupthepavementandthrownupsomeearthwhichliesinsuchawaythatitisdifficulttoavoidtreadinginitinentering.Theearthisofthispeculiarreddishtintwhichisfound,asfarasIknow,nowhereelseintheneighbourhood.Somuchisobservation.Therestisdeduction.” “How,then,didyoudeducethetelegram?” “Why,ofcourseIknewthatyouhadnotwrittenaletter,sinceIsatoppositetoyouallmorning.Iseealsoinyouropendesktherethatyouhaveasheetofstampsandathickbundleofpost-cards.Whatcouldyougointothepost-offic