At the end of Sir Arthur Conan Doyle’s short story “The Adventure of the Empty House,” Sherlock Holmes reveals to Watson what clues made him certain that Colonel Moran was the one who killed Adair. One main clue is that Adair had played cards with Moran that night. Holmes tells Watson that “Moran undoubtedly played foul,” which means he was cheating at cards. Knowing this about Moran, Holmes assumed Adair found out that Moran was a cheater. If he had exposed Moran, this would have ruined Moran’s reputation, which gave him a motive to kill Adair.

Another important clue is the type of bullet that the murderer used to kill Adair. “The bullets alone are enough to put his head in a noose,” Holmes says of the Colonel. Moran used soft, expanding bullets from the air gun commissioned by Moriarty. Holmes could also tell that Adair was shot through an open window, so the person who did it must have been skilled at shooting. This is an important part of the plot, because Holmes makes sure to be seen by Moran’s sentinel so that Moran will come and try to kill him through the window with the same weapon. When Moran does attempt to do this, Watson, Holmes, and Lestrade are one step ahead of him and are able to catch him with the murder weapon.