‘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.’

’Pardon me, my lord, but it seems to me that without the cistern there would be no need for the engine.  How should you want or how could you use the unhandsome thing?  Then how should the cistern be necessary to the engine?’

‘Handsome is that handsome does,’ returned his lordship.  ’Truly, cousin Dorothy, you speak well, but you must learn to hear better.  I did not say that the cistern existed for the sake of the engine, but for the sake of the perfecting of the engine.  Cousin Dorothy, I will give you the largest possible proof of my confidence in you, by not only explaining to you the working of my fire-engine, but acquainting you—­only you must not betray me!’

‘I, in my turn,’ said Dorothy, ’will give your lordship, if not the strongest, yet a very strong proof of my confidence:  I promise to keep your secret before knowing what it is.’

’Thanks, cousin.  Listen then:  That engine is a mingling of discovery and invention such as hath never had its equal since first the mechanical powers were brought to the light.  For this shall be as a soul to animate those, all and each—­lever, screw, pulley, wheel, and axle—­what you will.  No engine of mightiest force ever for defence or assault invented, let it be by Archimedes himself, but could by my fire-engine be rendered tenfold more