Phantom- Trap Door Mirror

  “It takes some time to release the counterbalance, when you press on the spring from the inside of the room. It is different when you are behind the wall and can act directly on the counterbalance. Then the mirror turns at once and is moved with incredible rapidity.”

  “What counterbalance?” asked Raoul.

  “Why, the counterbalance that lifts the whole of this wall on to its pivot. You surely don’t expect it to move of itself, by enchantment! If you watch, you will see the mirror first rise an inch or two and then shift an inch or two from left to right. It will then be on a pivot and will swing round.”

  “It’s not turning!” said Raoul impatiently.

  “Oh, wait! You have time enough to be impatient, sir! The mechanism has obviously become rusty, or else the spring isn’t working … Unless it is something else,” added the Persian, anxiously.

  “What?”

  “He may simply have cut the cord of the counterbalance and blocked the whole apparatus.”
...
  “But why do these walls obey him alone? He did not build them!”

  “Yes, sir, that is just what he did!”

-Chapter XIX