Suddenly, she knew not whence, arose a frightful roaring, a hollow bellowing, a pent-up rumbling. Seized by a vague terror, she clung to the parapet and trembled. But even the great wall beneath her, solid as the earth itself, seemed to tremble under her feet, as with some inward commotion or dismay. The next moment the water in the moat appeared to rush swiftly upwards, in wild uproar, fiercely confused, and covered with foam and spray. To her bewildered eyes, it seemed to heap itself up, wave upon furious wave, to reach the spot where she stood, greedy to engulf her. For an instant she fancied the storming billows pouring over the edge of the battlement, and started back in such momentary agony as we suffer in dreams. Then, by a sudden rectification of her vision, she perceived that what she saw was in reality a multitude of fountain jets rushing high towards their parent-cistern, but far-failing ere they reached it. The roar of their onset was mingled with the despairing tumult of their defeat, and both with the deep tumble and wallowing splash of the water from the fire-engine, which grew louder and louder as the surface of the water in the reservoir sank. The uproar ceased as suddenly as it had commenced, but the moat mirrored a thousand moons in the agitated waters which had overwhelmed its mantle of weeds. 'You see now,' said lord Herbert, rejoining her while still she gazed, 'how necessary the cistern is to the keep? Without it, the few poor springs in the moat would but sustain it as you saw it. From here I can fill it to the brim.' 'I see,' answered Dorothy. 'But would not a simple overflow serve, carried from the well through the wall?' 'It would, were there no other advantages with which this mode harmonised. I must mention one thing more--which I was almost forgetting, and which I cannot well show you to-night--namely, that I can use this water not only as a means of defence in the moat, but as an engine of offence also against any one setting unlawful or hostile foot upon the stone bridge over it. I can, when I please, turn that bridge, the same by which you cross to come here, into a rushing aqueduct, and with a torrent of water sweep from it a whole company of invaders.' 'But would they not have only to wait until the cistern was empty?' 'As soon and so long as the bridge is clear, the outflow ceases. One sweep, and my water-broom would stop, and the rubbish lie sprawling under the arch, or half-way over the court. And more still,' he added with emphasis: 'I COULD make it boiling!' 'But your lordship would not?' faltered Dorothy. 'That might depend,' he answered with a smile. Then changing his tone in absolute and impressive seriousness, 'But this is all nothing but child's play,' he said, 'compared with what is involved in the matter of this reservoir. The real origin of it was its needfulness to the perfecting of my fire-engine.'