He led the way from the room, and began to ascend the stair which rose just outside it. Dorothy followed, winding up through the thickness of the wall. And now she could not hear the engine. As she went up, however, certain sounds of it came again, and grew louder till they seemed close to her ears, then gradually died away and once more ceased. But ever, as they ascended, the rushing sound which had seemed connected with it, although so distant, drew nearer and nearer, until, having surmounted three of the five lofty stories of the building, they could scarcely hear each other speak for the roar of water, falling in intermittent jets. At last they came out on the top of the wall, with nothing between them and the moat below but the battlemented parapet, and behold! the mighty tower was roofed with water: a little tarn filled all the space within the surrounding walk. It undulated in the moonlight like a subsiding storm, and beat the encircling banks. For into its depths shot rather than poured a great volume of water from a huge orifice in the wall, and the roar and the rush were tremendous. It was like the birth of a river, bounding at once from its mountain rock, and the sound of its fall indicated the great depth of the water into which it plunged. Solid indeed must be the walls that sustained the outpush of such a weight of water! 'You see now, cousin, what yon fire-souled slave below is labouring at,' said his lordship. 'His task is to fill this cistern, and that he can in a few hours; and yet, such a slave is he, a child who understands his fetters and the joints of his bones can guide him at will.' 'But, my lord,' questioned Dorothy, 'is there not water here to supply the castle for months? And there is the draw-well in the pitched court besides.' 'Enough, I grant you,' he replied, 'for the mere necessities of life. But what would come of its pleasures? Would not the beleaguered ladies miss the bounty of the marble horse? Whence comes the water he gives so freely that he needeth not to drink himself? He would thirst indeed but for my water-commanding fiend below. Or how would the birds fare, were the fountains on the islands dry in the hot summer? And what would the children say if he ceased to spout? And how would my lord's tables fare, with the armed men besetting every gate, the fish-ponds dry, and the fish rotting in the sun? See you, mistress Dorothy? And for the draw-well, know you not wherein lies the good of a tower stronger than all the rest? Is it not built for final retreat, the rest of the castle being at length in the hands of the enemy? Where then is your draw-well?' 'But this tower, large as it is, could not receive those now within the walls of the castle,' said Dorothy. 'They will be fewer ere its shelter is needful.'