me；　somehow；　that　there　is　no　time　to　be　lost。〃　　　　　〃Delighted；〃　said　Angus；　rising　also；　〃though　he's　safe　enough　for　the　present；　for　I've　set　four　men　to　watch　the　only　hole　to　his　burrow。〃　　　　　They　turned　out　into　the　street；　the　small　priest　trundling　after　them　with　the　docility　of　a　small　dog。　　He　merely　said；　in　a　cheerful　way；　like　one　making　conversation；　〃How　quick　the　snow　gets　thick　on　the　ground。〃　　　　　As　they　threaded　the　steep　side　streets　already　powdered　with　silver；　Angus　finished　his　story；　and　by　the　time　they　reached　the　crescent　with　the　towering　flats；　he　had　leisure　to　turn　his　attention　to　the　four　sentinels。　　The　chestnut　seller；　both　before　and　after　receiving　a　sovereign；　swore　stubbornly　that　he　had　watched　the　door　and　seen　no　visitor　enter。　　The　policeman　was　even　more　emphatic。　　He　said　he　had　had　experience　of　crooks　of　all　kinds；　in　top　hats　and　in　rags；　he　wasn't　so　green　as　to　expect　suspicious　characters　to　look　suspicious；　he　looked　out　for　anybody；　and；　so　help　him；　there　had　been　nobody。　　And　when　all　three　men　gathered　round　the　gilded　commissionaire；　who　still　stood　smiling　astride　of　the　porch；　the　verdict　was　more　final　still。　　　　　〃I've　got　a　right　to　ask　any　man；　duke　or　dustman；　what　he　wants　in　these　flats；〃　said　the　genial　and　gold…laced　giant；　〃and　I'll　swear　there's　been　nobody　to　ask　since　this　gentleman　went　away。〃　　　　　The　unimportant　Father　Brown；　who　stood　back；　looking　modestly　at　the　pavement；　here　ventured　to　say　meekly；　〃Has　nobody　been　up　and　down　stairs；　then；　since　the　snow　began　to　fall？　　It　began　while　we　were　all　round　at　Flambeau's。〃　　　　　〃Nobody's　been　in　here；　sir；　you　can　take　it　from　me；〃　said　the　official；　with　beaming　authority。　　　　　〃Then　I　wonder　what　that　is？〃　said　the　priest；　and　stared　at　the　ground　blankly　like　a　fish。　　　　　The　others　all　looked　down　also；　and　Flambeau　used　a　fierce　exclamation　and　a　French　gesture。　　For　it　was　unquestionably　true　that　down　the　middle　of　the　entrance　guarded　by　the　man　in　gold　lace；　actually　between　the　arrogant；　stretched　legs　of　that　colossus；　ran　a　stringy　pattern　of　grey　footprints　stamped　upon　the　white　snow。　　　　　〃God！〃　cried　Angus　involuntarily；　〃the　Invisible　Man！〃　　　　　Without　another　word　he　turned　and　dashed　up　the　stairs；　with　Flambeau　following；　but　Father　Brown　still　stood　looking　about　him　in　the　snow…clad　street　as　if　he　had　lost　interest　in　his　query。　　　　　Flambeau　was　plainly　in　a　mood　to　break　down　the　door　with　his　big　shoulders；　but　the　Scotchman；　with　more　reason；　if　less　intuition；　fumbled　about　on　the　frame　of　the　door　till　he　found　the　invisible　button；　and　the　door　swung　slowly　open。　　　　　It　showed　substantially　the　same　serried　interior；　the　hall　had　grown　darker；　though　it　was　still　struck　here　and　there　with　the　last　crimson　shafts　of　sunset；　and　one　or　two　of　the　headless　machines　had　been　moved　from　their　places　for　this　or　that　purpose；　and　stood　here　and　there　about　the　twilit　place。　　The　green　and　red　of　their　coats　were　all　darkened　in　the　dusk；　and　their　likeness　to　human　shapes　slightly　increased　by　their　very　shapelessness。　　But　in　the　middle　of　them　all；　exactly　where　the　paper　with　the　red　ink　had　lain；　there　lay　something　that　looked　like　red　ink　spilt　out　of　its　bottle。　　But　it　was　not　red　ink。　　　　　With　a　French　combination　of　reason　and　violence　Flambeau　simply　said　〃Murder！〃　and；　plunging　into　the　flat；　had　explored；　every　corner　and　cupboard　of　it　in　five　minutes。　　But　if　he　expected　to　find　a　corpse　he　found　none。　　Isidore　Smythe　was　not　in　the　place；　either　dead　or　alive。　　After　the　most　tearing　search　the　two　men　met　each　other　in　the　outer　hall；　with　streaming　faces　and　staring　eyes。　　〃My　friend；〃　said　Flambeau；　talking　French　in　his　excitement；　〃not　only　is　your　murderer　invisible；　but　he　makes　invisible　also　the　murdered　man。〃　　　　　Angus　looked　round　at　the　dim　room　full　of　dummies；　and　in　some　Celtic　corner　of　his　Scotch　soul　a　shudder　started。　　One　of　the　life…size　dolls　stood　immediately　overshadowing　the　blood　stain；　summoned；　perhaps；　by　the　slain　man　an　instant　before　he　fell。　　One　of　the　high…shouldered　hooks　that　served　the　thing　for　arms；　was　a　little　lifted；　and　Angus　had　suddenly　the　horrid　fancy　that　poor　Smythe's　own　iron　child　had　struck　him　down。　　Matter　had　rebelled；　and　these　machines　had　killed　their　master。　　But　even　so；　what　had　they　done　with　him？　　　　　〃Eaten　him？〃　said　the　nightmare　at　his　ear；　and　he　sickened　for　an　instant　at　the　idea　of　rent；　human　remains　absorbed　and　crushed　into　all　that　acephalous　clockwork。　　　　　He　recovered　his　mental　health　by　an　emphatic　effort；　and　said　to　Flambeau；　〃Well；　there　it　is。　　The　poor　fellow　has　evaporated　like　a　cloud　and　left　a　red　streak　on　the　floor。　　The　tale　does　not　belong　to　this　world。〃　　　　　〃There　is　only　one　thing　to　be　done；〃　said　Flambeau；　〃whether　it　belongs　to　this　world　or　the　other。　　I　must　go　down　and　talk　to　my　friend。〃　　　　　They　descended；　passing　the　man　with　the　pail；　who　again　asseverated　that　he　had　let　no　intruder　pass；　down　to　the　commissionaire　and　the　hovering　chestnut　man；　who　rigidly　reasserted　their　own　watchfulness。　　But　when　Angus　looked　round　for　his　fourth　confirmation　he　could　not　see　it；　and　called　out　with　some　nervousness；　〃Where　is　the　policeman？〃　　　　　〃I　beg　your　pardon；〃　said　Father　Brown；　〃that　is　my　fault。　　I　just　sent　him　down　the　road　to　investigate　somethingthat　I　just　thought　worth　investigating。〃　　　　　〃Well；　we　want　him　back　pretty　soon；〃　said　Angus　abruptly；　〃for　the　wretched　man　upstairs　has　not　only　been　murdered；　but　wiped　out。〃　　　　　〃How？〃　asked　the　priest。　　　　　〃Father；〃　said　Flambeau；　after　a　pause；　〃upon　my　soul　I　believe　it　is　more　in　your　department　than　mine。　　No　friend　or　foe　has　entered　the　house；　but　Smythe　is　gone；　as　if　stolen　by　the　fairies。　If　that　is　not　supernatural；　I〃　　　　　As　he　spoke　they　were　all　checked　by　an　unusual　sight；　the　big　blue　policeman　came　round　the　corner　of　the　crescent；　running。　　He　came　straight　up　to　Brown。　　　　　〃You're　right；　sir；〃　he　panted；　〃they've　just　found　poor　Mr。　Smythe's　body　in　the　canal　down　below。〃　　　　　Angus　put　his　hand　wildly　to　his　head。　　〃Did　he　run　down　and　drown　himself？〃　he　asked。　　　　　〃He　never　came　down；　I'll　swear；〃　said　the　constable；　〃and　he　wasn't　drowned　either；　for　he　died　of　a　great　stab　over　the　heart。〃　　　　　〃And　yet　you　saw　no　one　enter？〃　said　Flambeau　in　a　grave　voice。　　　　　〃Let　us　walk　down　the　road　a　little；〃　said　the　priest。　　　　　As　they　reached　the　other　end　of　the　crescent　he　observed　abruptly；　〃Stupid　of　me！　　I　forgot　to　ask　the　policeman　something。　I　wonder　if　they　found　a　light　brown　sack。〃　　　　　〃Why　a　light　brown　sack？〃　asked　Angus；　astonished。　　　　　〃Because　if　it　was　any　other　coloured　sack；　the　case　must　begin　over　again；〃　said　Father　Brown；　〃but　if　it　was　a　light　brown　sack；　why；　the　case　is　finished。〃　　　　　〃I　am　pleased　to　hear　it；〃　said　Angus　with　hearty　irony。　　〃It　hasn't　begun；　so　far　as　I　am　concerned。〃　　　　　〃You　must　tell　us　all　about　it；〃　said　Flambeau　with　a　strange　heavy　simplicity；　like　a　child。　　　　　Unconsciously　they　were　walking　with　quickening　steps　down　the　long　sweep　of　road　on　the　other　side　of　the　high　crescent；　Father　Brown　leading　briskly；　though　in　silence。　　At　last　he　said　with　an　almost　touching　vagueness；　〃Well；　I'm　afraid　you'll　think　it　so　prosy。　　We　always　begin　at　the　abstract　end　of　things；　and　you　can't　begin　this　story　anywhere　else。　　　　　〃Have　you　ever　noticed　thisthat　people　never　answer　what　you　say？　　They　answer　what　you　meanor　what　they　think　you　mean。　　Suppose　one　lady　says　to　another　in　a　country　house；　‘Is　anybody　staying　with　you？'　the　lady　doesn't　answer　‘Yes；　the　butler；　the　three　footmen；　the　parlourmaid；　and　so　on；'　though　the　parlourmaid　may　be　in　the　room；　or　the　butler　behind　her　chair。　She　says　‘There　is　nobody　staying　with　us；'　meaning　nobody　of　the　sort　you　mean。　　But　suppose　a　doctor　inquiring　into　an　epidemic　asks；　‘Who　is　staying　in　the　house？'　then　the　lady　will　remember　the　butler；　the　parlourmaid；　and　the　rest。　　All　language　is　used　like　that；　you　never　get　a　question　answered　literally；　even　when　you　get　it　answered　truly。　　When　those　four　quite　honest　men　said　that　no　man　had　gone　into　the　Mansions；　they　did　not　really　mean　that　no　man　had　gone　into　them。　　They　meant　no　man　whom　they　could　suspect　of　being　your　man。　　A　man　did　go　into　the　house；　and　did　come　out　of　it；　but　they　never　noticed　him。〃　　　　　〃An　invisible　man？〃　inquired　Angus；　raising　his　red　eyebrows。　〃A　mentally　invisible　man；〃　said　Father　Brown。　　　　　A　minute　or　two　after　he　resumed　in　the　same　unassuming　voice；　like　a　man　thinking　his　way。　　〃Of　course　you　can't　think　of　such　a　man；　until　you　do　think　of　him。　　That's　where　his　cleverness　comes　in。　　But　I　came　to　think　of　him　through　two　or　three　little　things　in　the　tale　Mr。　Angus　told　us。　　First；　there　was　the　fact　that　this　Welkin　went　for　long　walks。　　And　then　there　was　the　vast　lot　of　stamp　paper　on　the　window。　　And　then；　most　of　all；　there　were　the　two　things　the　young　lady　saidthings　that　couldn't　be　true。　Don't　get　annoyed；〃　he　added　hastily；　noting　a　sudden　movement　of　the　Scotchman's　head；　〃she　thought　they　were　true。　　A　person　can't　be　quite　alone　in　a　street　a　second　before　she　receives　a　letter。　She　can't　be　quite　alone　in　a　street　w