These men, through intimidation, or money, or the granting of protection for minor derelictions, are able to maintain a staff of "stool pigeons," or crooks, who keep them informed of the doings of other crooks.

It is through this source that most of the professional criminals are apprehended.
"But where does the detective work come in ?" the reader asks.
It is accomplished by the two or three per cent.

of real detectives on the force.
For instance: A burglary has been committed.

Part of the stolen articles have been traced to a pawnshop.

The pawnbroker describes the man who pledged them, but the description might fit any one of a hundred professional crooks.