Final answer:
In 'Black Cat', the police utilize the ransom note, victim's description, and knowledge of crime genre conventions to uncover the criminal, who is inferred to be British based on the distraction of demanding American dollars.
Step-by-step explanation:
In the book 'Black Cat', the police discover the crime through a series of investigative steps. The ransom note demanding 1,000 American dollars for the feline victim, last seen on London's east side, provides an initial lead.
Upon closer inspection of this evidence, it is deduced that the criminal must be a native of Britain, using the mention of American dollars as a distraction. Newspapers from London are sent to aid in discovering insights that may have been overlooked. The investigation also includes aspects typically found within the crime genre, according to LibreTexts, including the need for a crime, a criminal, and a detective.
Clues like the victim's detailed description and the location and time of the disappearance help the police piece together the case. This detail, combined with profiles of potential suspects, such as Mr. Moriarty, helps narrow down the search. Comparisons to the Parisian police's methods suggest a reliance on tried and true investigative techniques.