"A sentry’s post, I suppose," said Bob to himself, "though it is evidently never required. Only a bird could cross the ravine. I wonder what its width is." It was difficult in the growing darkness to make a guess; still, it could hardly be less than eighty feet across. The width varied a little to right and left of his position, but only by a few feet at the most. Bob saw only too well that the fort was indeed, as his informants had said, on this side inaccessible. The question suggested itself: Could it be reached from the bottom of the ravine at some remote point to left or right? At the spot where he lay the descent on both sides was too precipitous to be scaled even by an expert mountaineer; but it was possible that at some distance the cliffs would be lower as well as less steep. In that case he could descend on one side and ascend on the other. But he might need assistance in his climbing. He remembered that Alpine climbers roped themselves together for safety’s sake; it would be a reasonable precaution to return for Ah-Sam and the chief, so that with their joined straps they might help him in case of need. It was now so nearly dark that the risk of being discovered was much lessened. Rising on hands and knees, he prepared to make his way back up the hillside; but he suddenly remembered the cleft he had noticed on this side of the ravine, at right angles to it, and it occurred to him that he ought to take a cursory look at this also. Crawling within the belt of trees, he rose to his feet and walked rapidly as long as it was safe, then wriggled along from bush to bush till he came to the edge of the opening, and looking over found that it was in the shape of a long wedge, like a slice cut out of a large cake. The sides of this also were precipitous, and the bottom was out of sight. He looked along the cleft from the narrow inner end to the wider outer end, and there noticed a long dark object lying transversely across it. He thought at first that it was possibly a sapling which had been uprooted and fallen, but this idea was negatived on his approaching the spot. The object lay, not on top of the cliff on either side, but a foot or two below. Walking, still with caution, close up to the place, he was surprised to find that the two sides of the cleft were connected by a stout bar of iron, the nearer end of which he felt was firmly embedded in the rock, reminding him of similar poles used at home to support unsteady arches or prop up tottering houses. Obviously no such purpose could have been intended for this bar, and he vaguely wondered why and when it had been placed in position. It had clearly been there a long time; it was much rusted, flakes of rust coming off as he touched it. Leaving the cleft, he returned as quickly as possible to the spot where he had left Ah-Sam and the chief. The former rose to meet him. "Well," said Bob, "has all been quiet?" "Yes, massa; no piecee man come this-side, no bobbely, not’ing."