`That is one of the points upon which your own researches have shed a light. Your interview with the lady has cleared the situation very much. I did not know about a projected divorce between herself and her husband. In that case, regarding Stapleton as an unmarried man, she counted no doubt upon becoming his wife.'

`And when she is undeceived?'

`Why, then we may find the lady of service. It must be our first duty to see her - both of us - to-morrow. Don't you think, Watson, that you are away from your charge rather long? Your place should be at Baskerville Hall.'

The last red streaks had faded away in the west and night had settled upon the moor. A few faint stars were gleaming in a violet sky.

`One last question, Holmes,' I said as I rose. `Surely there is no need of secrecy between you and me. What is the meaning of it all? What is he after?'

Holmes's voice sank as he answered:

`It is murder, Watson - refined, cold-blooded, deliberate murder. Do not ask me for particulars. My nets are closing upon him, even as his are upon Sir Henry, and with your help he is already almost at my mercy. There is but one danger which can threaten us. It is that he should strike before we are ready to do so. Another day - two at the most - and I have my case complete, but until then guard your charge as closely as ever a fond mother watched her ailing child. Your mission to-day has justified itself, and yet I could almost wish that you had not left his side. Hark!'

A terrible scream - a prolonged yell of horror and anguish burst out of the silence of the moor. That frightful cry turned the blood to ice in my veins.

`Oh, my God!' I gasped. `What is it? What does it mean?'

Holmes had sprung to his feet, and I saw his dark, athletic outline at the door of the hut, his shoulders stooping, his head thrust forward, his face peering into the darkness.

`Hush!' he whispered. `Hush!'

The cry had been loud on account of its vehemence, but it had pealed out from somewhere far off on the shadowy plain. Now it burst upon our ears, nearer, louder, more urgent than before.

`Where is it?' Holmes whispered; and I knew from the thrill of his voice that he, the man of iron, was shaken to the soul. `Where is it, Watson?'

`There, I think.' I pointed into the darkness.

`No, there!'

Again the agonized cry swept through the silent night, louder and much nearer than ever. And a new sound mingled with it, a deep, muttered rumble, musical and yet menacing, rising and falling like the low, constant murmur of the sea.

`The hound!' cried Holmes. `Come, Watson, come! Great heavens, if we are too late!'

He had started running swiftly over the moor, and I had followed at his heels. But now from somewhere among the broken ground immediately in front of us there came one last despairing yell, and then a dull, heavy thud. We halted and listened. Not another sound broke the heavy silence of the windless night.