Why did sir henry get scared while at the moor ? (hound of the baskervilles)

Because there  was a demented HOUND that had been tormented by Sir Henry's enemies to try to kill him. And, his enemies had stolen a shoe, Sir H's scent from which had been sniffed by the HOUND Dartmoor, UK, is a pretty desolate place when it's the 1800s and there's a hound hounding you. Even i you are the heir to "The Baskerville Fortune" ... Sir Arthur Conan Doyle, Sherlock Holmes and Dr Watson

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