He had left her side and gone a few paces away, but kept on speaking.

’One strange thing I can tell you about them, cousin—­the roots of that whitest of flowers make a fine black dye!  What apophthegm founded upon that, thinkest thou, my father would drop for Dr Bayly?’

‘You perplex me much, my lord,’ said Dorothy.  ’I cannot at all perceive your lordship’s drift.’

’Lay a hand on each side of the battlement where you now stand; lean through it and look down.  Hold fast and fear nothing.’  Dorothy did as she was desired, and thus supported gazed upon the moat below, where it lay a mere ditch at the foot of the lofty wall.

‘My lord, I see nothing,’ she said, turning to him, as she thought; but he had vanished.

Again she looked at the moat, and then her eyes wandered away over the castle.  The two courts and their many roofs, even those of all the towers, except only the lofty watch-tower on the western side, lay bare beneath her, in bright moonlight, flecked and blotted with shadows, all wondrous in shape and black as Erebus.

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.’