What could this mean?
While he was puzzling himself Margaret's voice came up to him, low but clear.

"Draw up, Gerard, till you see liberty." At the word Gerard drew the whipcord line up, and drew and drew till he came to another knot, and found a cord of some thickness take the place of the whipcord.

He had no sooner begun to draw this up, than he found that he had now a heavy weight to deal with.

Then the truth suddenly flashed on him, and he went to work and pulled and pulled till the perspiration rolled down him: the weight got heavier and heavier, and at last he was well-nigh exhausted: looking down, he saw in the moonlight a sight that revived him: it was as it were a great snake coming up to him out of the deep shadow cast by the tower.

He gave a shout of joy, and a score more wild pulls, and lo! a stout new rope touched his hand: he hauled and hauled, and dragged the end into his prison, and instantly passed it through both handles of the chest in succession, and knotted it firmly; then sat for a moment to recover his breath and collect his courage.