Frank smiled to himself. He had made a discovery a day or two before that had not escaped his mind. That afternoon he managed to make certain preparations unobserved. And when night came he was ready to hazard his own liberty, and his life, if that should prove to be necessary, in an attempt to rescue Henri. He knew the room in which Henri was confined. It was on the side of the Hotel de Ville that overlooked the river. No sentries were posted there, and it was easy for Frank to get to a spot directly underneath Henri's window. The other bank of the river was well guarded, and that was why no sentries watched the side on which was the town hall. 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. And then Henri slipped down beside him, taking the short drop by hanging from the window with his hands. He seized Frank's hand. "I knew you would try to help me," he whispered. "But I had better go back. We cannot escape. There are sentries on the other bank of the Somme. They would catch us together, and you would be a prisoner, too." "Follow me," said Frank. "Take off your shoes. Drop quietly into the water--make no sound of a splash. Swim after me. I shall show you something you do not expect to see." Frank slipped into the water. Both boys were expert swimmers, and Frank, leading the way, slipped along in the deep shadow, without a sound. Henri swam after him. At last Frank stopped and whispered to Henri. "You see this buttress? Dive just beyond it, and swim under water for ten feet. Put up your hands then, and rise. There will be room." At once he dived and disappeared, and Henri followed. When they came to the surface they were in a dark, damp hole, that smelled of slime and filth. But in a moment Henri felt steps, and then there was a faint light that illuminated a vault full of water. And, to his wonder, he saw a boat, covered, except at one end, with a dark cloth. "In with you!" whispered Frank. "Under the cloth, and lie still!" Frank followed when Henri had obeyed. And then the boat began to move in a direction different from that by which they had entered the vault.