Here's a breakdown of Holmes' crucial deduction:
The Problem: Dr. Roylott's stepdaughter, Helen Stoner, believes she is being murdered by a venomous snake. She describes a strange whistling sound that occurs at night, and a fear of the "speckled band" which she believes is the source of danger.
Holmes' Deduction:
* The Whistle: Holmes realizes the whistle isn't a sound made by a snake, but rather by a person using a special device to mimic a snake's hiss. This is done to distract and frighten the victim.
* The Speckled Band: Holmes correctly deduces that the "speckled band" is not a snake species, but a venomous swamp adder, a snake known for its agility and ability to climb.
* The Method: Holmes surmises that Dr. Roylott has trained the adder to climb a rope or cord attached to the ventilation shaft of Helen's room, allowing the snake to reach her bed at night.
The Solution:
* The Rope: Holmes confirms his theory when he discovers a loose brick in the ventilation shaft that allows access to the room. He also finds a small loop of cord, presumably used to lower the snake.
* The Snake: During a staged reenactment of the night's events, Holmes uses his knowledge of the snake's behavior to anticipate its movements and protect Helen from its attack.
Key Elements of Holmes' Deduction:
* Observation: Holmes meticulously observes details in the room, including the ventilation shaft, the loose brick, and the cord.
* Logical Reasoning: He connects these observations to the information provided by Helen, using logic to piece together the puzzle.
* Scientific Knowledge: Holmes utilizes his knowledge of snakes and their behavior to understand how Dr. Roylott might have used the adder as a weapon.
In essence, Holmes' deduction in "The Adventure of the Speckled Band" reveals his remarkable ability to connect seemingly disparate clues and arrive at a shocking but logical conclusion.