But the work of this man was that of a spy pure and simple.
Another highly specialized class of detectives is that engaged in police and banking work, who by experience (or even origin) have a wide and intimate acquaintance with criminals of various sorts, and by their familiarity with the latters' whereabouts, associates, work, and methods are able to recognize and run down the perpetrators of particular crimes.
Thus, for example, there are men in the detective bureau of New York City who know by name, and perhaps have a speaking acquaintance with, a large number of the pick-pockets and burglars of the East Side.

They know their haunts and their ties of friendship or marriage.

When any particular job is pulled off they have a pretty shrewd idea of who is responsible for it and lay their plans accordingly.

If necessary, they run in the whole gang and put each of them through a course of interrogation, accusation, and browbeating until some one breaks down or makes a slip that involves him in a tangle.

These men are special policemen whose knowledge makes them detectives by courtesy.