Afterwards he went to the door to examine the fastenings.

They consisted of two enormous iron bolts sliding into holes made in the wall; and as the corridor outside was too narrow to admit of any battering arrangement or even to permit an axe to be swung, nothing could burst the door open--unless gunpowder.

But while he was still making sure that the lower bolt was pushed well home, he received the impression of somebody's presence in the room.

It was so strong that he spun round quicker than lightning.
There was no one.

Who could there be?
And yet.