Harry prepared for escape by tearing up the blankets of his bed and knotting them into ropes.

A portion he wrapped round his shoes, so as to walk noiselessly, and taking advantage of a dark, moonless night, when the fog hung thick upon the low land round Reading, he opened his window, threw out his rope, and slipped down to the ground.
So dark was the fog that it was difficult for him to see two paces in advance, and he soon found that the careful observations which he had taken of the place of the sentries would be altogether useless.

Still, in the darkness and thickness of the night, he thought that the chance of detection was small.

Creeping quietly and noiselessly along, he could hear the constant challenges of the sentries round him.

These, excited by the unusual darkness of the night, were unusually vigilant.