The ways of the New York detective are like those of the famous Ah Sing, "childlike and bland," but no less astute and successful. They aim at prevention, and trust to it even more than to the pursuit subsequent to the commission of crime. It is an axiom with them to know their game by heart; they study the thoughts and idiosyncrasies, the plans and proceedings, of the criminal classes so closely that they can predicate what will be done under any particular circumstances, how the thief will act when planning, when executing, and, above all, when covering up his tracks after he has made his _coup_. One method followed with marked success is to keep their spies and assistants in the heart of the enemy's camp. It is well known that criminals have little or no fidelity to each other, that "honour among thieves" is a mendacious adage provided any of them can see substantial profit in betraying his associates. The best officers make a point of keeping in touch with the "crooks," visiting them frequently in their favourite resorts, and hearing all the movements and the news. Matters in progress, the activity or otherwise of well-known practitioners, are thus ascertained, for the high-flyer in crime generally knows what others of his class are about, and is willing to pass it on for a consideration, or to stand well with the police. New York possesses its Black Museum, its treasure-house of criminal relics akin to that which may be viewed at the headquarters [Illustration: 1. PHOTOGRAPHING A CRIMINAL FOR THE "ROGUES' GALLERY" (NEW YORK). 2. CABINET IN WHICH AMERICAN CRIMINALS ARE REGISTERED. 3. TWO LEAVES OF THE "ROGUES' GALLERY."] of our Metropolitan Police at New Scotland Yard. A brief summary of the exhibits in this strange depository is, in its way, an epitome of contemporary crime. Every item, even the most insignificant, tells of some flagitious act. The sledge hammers, drills, jemmies, masks, and powder flasks tell their own story, so do the marvellously ingenious burglar's implements manufactured by high-class mechanical skill, and hired out to executive agents on a percentage of results. Here are the bogus gold bricks of some famous confidence trick, the well-named _vol à l'Americain_, lithographic stones from which thousands and thousands of counterfeit notes have been struck off, the curious devices used for opening combination locks, the rope ladders, lanterns, revolvers that have figured in various notable operations. [Illustration: COMPLETE SET OF AN AMERICAN BURGLAR'S TOOLS.]