Just at that moment Ermengarde almost jumped off the bed, she was so startled by...

Just at that moment Ermengarde almost jumped off the bed, she was so startled by a sound she heard. It was like two distinct knocks on the wall.
"What is that?" she exclaimed.
Sara got up from the floor and answered quite dramatically:
"It is the prisoner in the next cell."
"Becky!" cried Ermengarde, enraptured.
"Yes," said Sara. "Listen; the two knocks meant, 'Prisoner, are you there?'"
She knocked three times on the wall herself, as if in answer.
"That means, 'Yes, I am here, and all is well.'"
Four knocks came from Becky's side of the wall.
"That means," explained Sara, "'Then, fellow-sufferer, we will sleep in peace. Good night.'"
Ermengarde quite beamed with delight.
"Oh, Sara!" she whispered joyfully. "It is like a story!"
"It a story," said Sara. "
's a story. You are a story--I am a story. Miss Minchin is a story."
And she sat down again and talked until Ermengarde forgot that she was a sort of escaped prisoner herself, and had to be reminded by Sara that she could not remain in the Bastille all night, but must steal noiselessly downstairs again and creep back into her deserted bed.