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ARLY the next morning Xodar and I commenced work upon our plans for escape. First I had him sketch upon the stone floor of our cell as accurate a map of the south polar regions as was possible with the crude instruments at our disposal—a buckle from my harness, and the sharp edge of the wondrous gem I had taken from Sator Throg.

From this I computed the general direction of Helium and the distance at which it lay from the opening which led to Omean.

Then I had him draw a map of Omean, indicating plainly the position of Shador and of the opening in the dome which led to the outer world.

These I studied until they were indelibly imprinted in my memory. From Xodar I learned the duties and customs of the guards who patrolled Shador. It seemed that during the hours set aside for sleep only one man was on duty at a time. He paced a beat that passed around the prison, at a distance of about a hundred feet from the building.

The pace of the sentries, Xodar said, was very slow, requiring nearly ten minutes to make a single round. This meant that for practically five minutes