How did Holmes explain he knew the snake was coming?
At the end of the story, Watson relates that Holmes explained his thinking as they traveled back to London from Stoke Moran the next day. Holmes tells his friend and biographer he had originally suspected that the gypsies were involved with Julia's death and with an apparent attempt on Helen's life. But when Holmes inspected the interiors and exteriors of the bedrooms, he realized that the danger could not have come from a window or door and therefore had to come from inside the building. It was characteristic of Holmes' thinking processes that he eliminated as many possible explanations as he could. He focused on the bell-pull and the bed in Helen's room adjacent to that of her stepfather.
The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track.
So Holmes was expecting a snake to come crawling down the bell-pull as he sat there in the dark with Dr. Watson, both of them maintaining complete silence. The snake must have been on the bed for several hours without their awareness.
Far away we could hear the deep tones of the parish clock, which boomed out every quarter of an hour. How long they seemed, those quarters! Twelve struck, and one and two and three, and still we sat waiting silently for whatever might befall.
Approximately a half-hour after they heard the parish clock strike three, they saw a light shine briefly through the ventilator and then go out quickly.
Someone in the next room had lit a dark lantern.
Then they heard a soft whistle.
The instant that we heard it, Holmes sprang from the bed, struck a match, and lashed furiously with his cane at the bell-pull.
“You see it, Watson?” he yelled. “You see it?”
Once Holmes sees the snake slithering back up the bell-pull in response to the whistle, he is no longer concerned about maintaining silence. Dr. Roylott certainly must be astonished to hear a man's voice yelling next door, but his snake gets to him almost immediately. Holmes and Watson hear a horrible cry, and when they go into the doctor's room they find him dead.
"The Adventure of the Speckled Band," like almost all the Sherlock Holmes stories, combines ratiocination and adventure. Some readers like to follow the great detective's thought processes, while others read the stories for their interesting characters and exciting action. Holmes is a deep thinker, but his cases usually involve him in strange adventures. The Hound of the Baskervilles is another good example. His cases invariably take him away from his rooms at 221B Baker Street. The stories give the reader a panoramic view of England in late Victorian times.