midnight
It was argued , Frank supposed , that anyone escaping must attempt to swim the river and that when they tried to climb the other bank it would be easy to find them . In principle , too , that was a good idea . What it did not take into account was the discovery that Frank had made -- and kept to himself . It was just before midnight when he began a faint tapping at Henri 's window . He used a light bamboo cane , tipped with soft cloth , so that the sound , audible to anyone in the room , would not carry more than a few feet . And he tapped out his signal in the Morse code very slowly , knowing that Henri would hear and understand . In a few moments there was the sound of the window opening very gently .