"They won't hear us," said Frank confidently. "The walls are of stone. Anyway, the sound of machinery will drown out any noises from outside. It's our only chance to get into the mill." "Lead the way, then." Frank began to ascend the willow tree. It was difficult work, for although the tree was large, it bent and swayed under his weight. It was impossible for both of them to attempt to climb at the same time, and Joe was forced to wait on guard at the bottom, listening as his brother made his way higher and higher among the springy branches. The topmost branches drooped over the roof of the mill, and when at length Frank had reached them he swung himself over until his feet touched the top of the building. For a second or so he was uncertain of his footing but at length he was able to stand steadily on the sloping surface. He released his grasp and the branches swished back. So far he had been able to move with a minimum of noise and he was confident that his ascent to the roof had been unheard. He called softly to Joe, and in a few minutes a rustling among the branches indicated that his brother was also climbing the tree. Frank waited and directed his brother so that Joe was soon swinging out from the branches. He dropped lightly to the roof of the old mill. "There should be some sort of trapdoor here," said Frank quietly. "If there isn't we'll have to lower ourselves over the edge to one of the upper windows. I noticed a small open window around at the front. But there is probably a trapdoor." The mill roof was not on an abrupt slant, so that the boys were able to make their way along among the shingles without a great deal of difficulty. The roof was in a bad state of repair, and once Frank came upon a wide hole, where the shingles had fallen off and where the wood beneath had rotted away. But there was no trapdoor. "We'll tackle that hole in the roof," he decided. The gap was only about a foot square, but when Frank turned his flashlight on it he saw that immediately beneath them was a sort of attic, the topmost room in the mill. Quietly, they began enlarging the hole in the roof. Fortunately, the effect of rain and wind and weather had been such as to render the roof extremely weak. The shingles broke off easily, and bit by bit they made the hole wider until at last it was a large, black gap. They did not throw the débris to the ground, but piled it carefully up on the roof near by. The work of enlarging the hole in the roof had taken them some time, as they worked cautiously and deliberately with a view to a minimum of noise. Finally they agreed that there was sufficient space to admit the passage of a human body, and Frank began to lower himself through the opening.