When, next day, dusk was deepening into night, a sentry on the wall of the fort abutting on the ravine might possibly have seen, if it had occurred to him to look, a figure moving by almost imperceptible degrees along the face of the cleft opposite. The course followed was along a narrow ledge some feet below the top, leading from the inner end of the cleft to a spot immediately below the stout iron girder whose presence and purpose had given Bob so much food for conjecture. Arriving at the girder, the figure reached up, cautiously passed the looped end of a long rope over it, and, threading the other end through the loop, drew it tight over the girder and pushed it gently with the stripped branch of a sapling towards the centre. Then he retraced his steps along the ledge, carrying the loose end of the rope. The slack, dangling in the cleft, displaced a stone, which fell with a rattle to the bottom. The stealthy form hurried his pace to reduce the length of the slack, fearing for a moment lest the fall of the stone might have been heard in the fort. But there was little risk; stones must frequently drop down the sides of the ravine; and in any case the rope, in the almost total obscurity of the narrow cleft, was invisible from the farther side. At the nearer end of the ledge Bob rejoined Ah-Sam and the chief, who were crouching by a couple of stout saplings that overhung the deep gorge beneath. Making fast to one of the saplings the rope he carried, he sat down and spoke in low tones to the Chinaman, explaining the scheme which he had devised after hours of meditation.