It was far too narrow to squeeze through, and was crossed by three sets of bars.

The chance of widening the narrow loophole and removing the bars without detection was extreme; besides, Rupert had a strong idea that the loophole looked into the courtyard.
Finally he came to the conclusion, that if an escape was to be made it must be by raising a flag of the floor, tunnelling between his room and that underneath it, and working out through the solid wall.

It would be a tremendous work, for the loophole showed him that the wall must be ten feet thick; still, as he said to himself, it will be at least something to do and to think about, and even if it takes five years and comes to nothing, it will have been useful.
Thus resolved, Rupert went to work, and laboured steadily.

His exercise with the chair and table succeeded admirably, and after six months he was able to perform feats of strength with them that surprised himself.

With his scheme for escape he was less fortunate.