“Despite our precautions,” I said, “they will almost certainly know that a watch is being kept.”

“They may or they may not,” replied Bristol.  “Either way I’m disposed to think there will be another attempt.  Their mysterious method is so rapid that they can afford to take chances.”

This was not my first night vigil since I had become in a sense the custodian of the relic, but it was quite the most dreary.  Amid the tomb-like objects about us we seemed two puny mortals toying with stupendous things.  We could not smoke and must converse only in whispers; and so the night wore on until I began to think that our watch would be dully uneventful.

“Our big chance,” whispered Mostyn, “is in the fact that any day may change the conditions.  They can’t afford to wait.”

He ceased abruptly, grasping my arm.  From somewhere, somewhere outside the building, we all three had heard a soft whistle.  A moment of tense listening followed.

“If only we could have had the place surrounded,” whispered Bristol—­ “but it was impossible, of course.”

A faint grating noise echoed through the lofty Burton Room.  Bristol slipped past me in the semi-gloom, and gently opened the communicating door a few inches.

A-tiptoe, I joined him, and craning across his shoulder saw a strange and wonderful thing.

The newly glazed east window again was shattered with a booming crash!  The yellow blind was thrust aside.  A long something reached out toward the broken case.  There was a sort of fumbling sound, and paralyzed with the wonder of it—­for the window, remember, was thirty feet from the ground—­I stood frozen to my post.

Not so Bristol.  As the weird tentacle (or more exactly it reminded me of a gigantic crab’s claw) touched the case, the Inspector leapt forward.  A white beam from his electric torch cut through to the broken cabinet.

The thing was withdrawn . . . and with it went the slipper of the Prophet.

“Raise the blinds!” cried Bristol.  “Mr. Cavanagh!  Mr. Mostyn!  We must not let them give us the slip!”

I got up the blind of the nearer window as Bristol raised the other.  Not a living thing was in sight from either!

Mostyn was beside me, his hand resting on my shoulder.  I noted how he trembled.  Bristol turned and looked back at us.  The light from his pocket torch flashed upon the curator’s face; and I have never seen such an expression of horrified amazement as that which it wore.  Faintly, I could hear the constable racing up the steps from the hall.

Ideas of the supernatural came to us all, I know; when, with a scuffling sound not unlike that of a rat in a ceiling, something moved above us!

“Damn my thick head!” roared Bristol, furiously.  “He’s on the roof!  It’s flat as a floor and there’s enough ivy alongside the water-spout on your house adjoining, Mr. Mostyn, to afford foothold to an invading army!”