Of the origin of the Hound of the Baskervilles there have been many statements, yet as I come in a direct line from Hugo Baskerville, and as I had the story from my father, who also had it from his, I have set it down with all belief that it occurred even as is here set forth. And I would have you believe, my sons, that the same Justice which punishes sin may also most graciously forgive it, and that no ban is so heavy but that by prayer and repentance it may be removed. Learn then from this story not to fear the fruits of the past, but rather to be circumspect in the future, that those foul passions whereby our family has suffered so grievously may not again be loosed to our undoing. (2.16)
Doctor Mortimer takes this old account of the Hound as absolute truth. But the writer wants to use the story to teach a lesson to his sons not to give in to "those foul passions" that have caused the family so much trouble. It's a story with a moral, which makes its objective truth seem a little doubtful. A scare tactic, maybe. This manuscript also gives us our first glimpse of the suspicion and fear that pervades Dartmoor.
"I could not call you in, Mr. Holmes, without disclosing these facts to the world, and I have already given my reasons for not wishing to do so. Besides, besides—"
"Why do you hesitate?"
"There is a realm in which the most acute and most experienced of detectives is helpless."
"You mean that the thing is supernatural?"
"I did not positively say so."
"No, but you evidently think it." (3.52-7)
Out on the moors, it is possible to believe in anything, even demon dogs. Holmes, a city boy, will never admit that there's a "realm in which the most acute and most experienced of detectives is helpless." The two settings of the novel represent completely different world views: one that allows the supernatural and another that emphasizes rationality and science. Anyone who's ever turned on the night light (because of that monster under the bed) can relate to this.
"Put into plain words, the matter is this," said he. "In your opinion there is a diabolical agency which makes Dartmoor an unsafe abode for a Baskerville—that is your opinion?"
"At least I might go the length of saying that there is some evidence that this may be so."
"Exactly. But surely, if your supernatural theory be correct, it could work the young man evil in London as easily as in Devonshire. A devil with merely local powers like a parish vestry would be too inconceivable a thing." (3.75-7)
In this passage, Holmes teases Doctor Mortimer for thinking that, if Sir Henry is really being pursued by the devil, he'll be safer in London than in Devonshire. As Holmes says, "a devil with merely local powers" would be pretty lame. However, once Watson gets out to Baskerville Hall, the notion that Dartmoor and the Grimpen Mire might actually be places where evil thrives seems more possible to him. Maybe that's because Watson is more imaginative and superstitious than Holmes. Dartmoor is starting to seem a lot like Sleepy Hollow.
"Here are two moorland farmhouses, High Tor and Foulmire. Then fourteen miles away the great convict prison of Princetown. Between and around these scattered points extends the desolate, lifeless moor. This, then, is the stage upon which tragedy has been played, and upon which we may help to play it again."
"It must be a wild place."
"Yes, the setting is a worthy one. If the devil did desire to have a hand in the affairs of men—" (3.116-8)
"Foulmire" is a great name for a farmhouse—it's like calling your house Reeking Mud or Gross Bog. Holmes is setting the stage: even though we haven't actually arrived at Dartmoor yet, we know that it's a place with a "desolate, lifeless moor" and a "great convict prison." Not quite the spot for a relaxing vacay. But Holmes' detached tone shows he isn't particularly worried about it.
"On the whole," said Holmes, "I think that your decision is a wise one. I have ample evidence that you are being dogged in London, and amid the millions of this great city it is difficult to discover who these people are or what their object can be. If their intentions are evil they might do you a mischief, and we should be powerless to prevent it. You did not know, Dr. Mortimer, that you were followed this morning from my house?" (5.39)
Here's one benefit of living in the country, according to Holmes: since you know pretty much everyone in the area, you might know the person trying to kill you. In London, "amid the millions of this great city," it can be harder to identify threats. Don't you suburbanites feel better now?
At every turn Baskerville gave an exclamation of delight, looking eagerly about him and asking countless questions. To his eyes all seemed beautiful, but to me a tinge of melancholy lay upon the countryside, which bore so clearly the mark of the waning year. Yellow leaves carpeted the lanes and fluttered down upon us as we passed. The rattle of our wheels died away as we drove through drifts of rotting vegetation—sad gifts, as it seemed to me, for Nature to throw before the carriage of the returning heir of the Baskervilles. (6.28)
Getting Watson out of London and into a new environment gives him a chance for some impressive description as our narrator. He emphasizes that the setting of Dartmoor holds "a tinge of melancholy." There is something vaguely sad about this whole area. Sir Henry doesn't know it yet, but it's not gonna be a beautiful day in the neighborhood.
The young heir glanced round with a gloomy face.
"It's no wonder my uncle felt as if trouble were coming on him in such a place as this," said he.
"It's enough to scare any man. I'll have a row of electric lamps up here inside of six months, and you won't know it again, with a thousand candle-power Swan and Edison right here in front of the hall door." (6.41-3)
So much for Sir Henry's exclamation of delight. As soon as he gets to his new home, he's creeped out. His plan to light up Baskerville Hall lets us know that this whole area is basically stuck in the past. With his New World background and interest in newfangled technologies, Sir Henry represents a reforming spirit, wanting to bring Baskerville Hall into the brand-new 20th century.
Swan and Edison was the local electric company at the time. The general availability of electrical power was so new and exciting that we doubt that parents were yelling at their kids to turn off the lights, for Pete's sake.
The whole steep slope was covered with gray circular rings of stone, a score of them at least.
"What are they? Sheep-pens?"
"No, they are the homes of our worthy ancestors. Prehistoric man lived thickly on the moor, and as no one in particular has lived there since, we find all his little arrangements exactly as he left them..." (7.82-3)
The history of Dartmoor is literally written on the ground. With all these architectural ruins, we can never forget that The Hound of the Baskervilles strongly associates the moors with the past and prehistory of England. We can compare this with the London of the earlier chapters, where everything's shiny and new. Early 20th-century London was a place where new stuff was constantly being built: hotels, theaters, subways, and department stores (source).
To that Providence, my sons, I hereby commend you, and I counsel you by way of caution to forbear from crossing the moor in those dark hours when the powers of evil are exalted. (2.23)
"Well, good-bye," [Holmes] added as the train began to glide down the platform. "Bear in mind, Sir Henry, one of the phrases in that queer old legend which Dr. Mortimer has read to us, and avoid the moor in those hours of darkness when the powers of evil are exalted." (6.18)
"I say, Watson," said the baronet, "what would Holmes say to this? How about that hour of darkness in which the power of evil is exalted?"
As if in answer to his words there rose suddenly out of the vast gloom of the moor that strange cry which I had already heard upon the borders of the great Grimpen Mire. (9.94-5)
These three lines all echo each other: the first quote comes from Hugo Baskerville's manuscript, as he emphasizes that his sons should avoid crossing the moor "in those dark hours when the power of evil are exalted." The second includes a reference by Holmes to the manuscript and "the powers of evil," which he uses to warn Sir Henry. And then the third and final reference to the "powers of evil" that Sir Henry and Watson fear as they cross the moors.
This repetition across the text reminds us that Conan Doyle is a genius, who keeps an eye out for continuity throughout the novel. He keeps the creepiness going.
"My God, can there be some truth in all these stories? Is it possible that I am really in danger from so dark a cause? You don't believe it, do you, Watson?"
"And yet it was one thing to laugh about it in London, and it is another to stand out here in the darkness of the moor and to hear such a cry as that." (9.115-7)
Here's the clearest illustration in the novel on the contrast between London and Dartmoor. Sir Henry admits that the legend of the Hound is way harder to dismiss when standing "out here in the darkness of the moor." Dartmoor's dark, scary, supernatural atmosphere gives Stapleton's plans a better chance of success. It sure helped him kill off Sir Charles.