It was therefore constructed of blocks of hard stone.
It seemed to Francis that this was sandstone, and to test its quality, he sat down in the corner where the guard had, the night before, placed his supply of food and water.

First he moistened a portion of the wall, then he took up a link of his chain, and rubbed for some time against it.

At last, to his satisfaction, a bright patch showed that the stone was capable of wearing away iron.

But in vain did he try to twist his legs so as to rub the rivet against the wall, and he gave up the attempt as impossible.
It was clear, then, that he must have a bit of the stone to rub with.
He at once began to dig with the dagger in the earth at the foot of the wall, to see if he could find any such pieces.

For a long time he came across no chips, even of the smallest size.