"That's How You Do That"

They walked down the hallway into the darkness, and then they bumped simultaneously against what seemed to be a wall."What the deuce," said Livingston. "Could Pompey have given us a bum steer?""Livingston," said Harry, "I don't mean to cast blame, but you said that Pompey told us to make a left at the end of that other hallway, but when we came to its end, you made a right.""I did?""Yes, I'm pretty sure.""Why didn't you say something?""I don't know.""You don't know.""No, I suppose I'm not very assertive by nature.""Wait a minute," said Livingston.Harry heard a knocking sound from Livingston's direction in the near total darkness."Do you hear that?" said Livingston."Yes," said Harry."Does that sound hollow to you?""Sort of," said Harry, not wanting to commit himself and possibly commit another blunder, either of commission or omission. He heard another knocking sound."Is that you knocking?" asked Harry."Yes, it's me," replied Livingston. "And I think that's a hollow sound.""Could it be…