In "The Pit and the Pendulum" how does the narrator determine the size of his prison? What error does he make?

The
prison where the narrator is placed is completely dark, leaving him unaware of the boundaries
and dimensions of the cell. Initially, the narrator walks forward with his arms outstretched
thinking about the numerous horror stories he has heard regarding the dungeons in Toledo. When
the narrator touches a wall, he begins to follow it and realizes that he has no understanding of
the dimensions of the prison because he may be walking in a circle. He then tears a long strand
of cloth from his robe and places it at a right angle against the wall so that he will feel the
cloth when he walks over it. While groping his way around the prison, the...