In many of his Sherlock Holmes stories it was the practice of the author, Sir Arthur Conan Doyle, to illustrate his hero's powers of deduction early on, and then to illustrate them again at the end, when Holmes would explain his cold, clear reasoning to his friend Dr. Watson. During the middle part of the story it was not always practical to reveal what Holmes was thinking but only to describe what he was doing. In "The Adventure of the Blue Carbuncle," for example, Holmes shows at the beginning how much he can deduce from examining a battered old hat belonging to a man he has never met. In "The Red-Headed League," Holmes surprises both Watson and his client Jabez Wilson by revealing what he has deduced from Wilson's appearance:
“Beyond the obvious facts that he has at some time done manual labour, that he takes snuff, that he is a Freemason, that he has been in China, and that he has done a considerable amount of writing lately, I can deduce nothing else.”
In the first chapter of "The Hound of the Baskervilles," the great detective demonstrates his deductive powers by examining the walking stick left behind by his visitor, Dr. James Mortimer, M.R.C.S. Holmes deduces correctly that Dr. Mortimer left Charing Cross Hospital in London to take up a country practice, and then he tells the astonished Dr. Watson:
“I think that we might venture a little farther than this. Look at it in this light. On what occasion would it be most probable that such a presentation would be made? When would his friends unite to give him a pledge of their good will? Obviously at the moment when Dr. Mortimer withdrew from the service of the hospital in order to start a practice for himself. We know there has been a presentation. We believe there has been a change from a town hospital to a country practice. Is it, then, stretching our inference too far to say that the presentation was on the occasion of the change?”
“It certainly seems probable.”
“Now, you will observe that he could not have been on the staff of the hospital, since only a man well-established in a London practice could hold such a position, and such a one would not drift into the country. What was he, then? If he was in the hospital and yet not on the staff he could only have been a house-surgeon or a house-physician—little more than a senior student. And he left five years ago—the date is on the stick. So your grave, middle-aged family practitioner vanishes into thin air, my dear Watson, and there emerges a young fellow under thirty, amiable, unambitious, absent-minded, and the possessor of a favourite dog, which I should describe roughly as being larger than a terrier and smaller than a mastiff.”
Holmes remains invisible throughout a large part of "The Hound of the Baskervilles," having persuaded Dr. Watson to go to Baskerville Hall to act in his behalf as investigator and protector of Sir Henry Baskerville. Characteristically, Conan Doyle has Holmes wrap up the story at the end. In Chapter XV, titled "A Retrospection," Holmes and Watson are back at their sitting-room on Baker Street and Holmes is explaining everything that he had been thinking during their adventure with the mysterious hound and its lethal master Mr. Stapleton. So Conan Doyle is following his usual practice of demonstrating Sherlock Holmes' amazing powers of deduction at the beginning of his tale and then having Holmes himself explain his reasoning at greater length when he and his friend are comfortably ensconced before a blazing fire after the mystery has been solved and everything has settled down. The best examples of Sherlock Holmes' powers of deduction are usually to be found in the opening parts of the stories. The reader can assume that the great detective is making many equally significant deductions throughout his investigation, as he does in "The Adventure of the Speckled Band," but these are not usually explained to the reader either by Holmes or Watson until the "adventure" part of the story is over.