How did the narrator of "The Pit and the Pendulum" measure the circumference?

In
Edgar Allen Poe's short story "," the
unnamed narrator is sentenced to death by the
Spanish Inquisition. Upon
hearing the sentence, he faints and is carried away
unconscious.

He gradually comes to his senses and finds himself lying on his

back, unshackled. He opens his eyes, only to find himself in total darkness.
(Interestingly, he
never considers that he might be blind.) He thinks he
might have been put back in his former
dungeon cell, but the cell was never
entirely devoid of light.

He slowly
stands up and puts out
his arms in all directions but doesn't feel anything. He hesitates to
take
any steps away from where he's stranding, fearing that he might have been buried alive
in a
tomb.

In time, he works up the courage to take a step
forward, then another,
until his outstretched hands touched what he perceives
to be a wall, "seemingly of stone
masonryvery smooth, slimy, and cold." He
walks along the wall for a time, feeling his way
in the darkness. Lacking any
frame of reference, he's unable to determine the dimensions of the
space in
which he's been imprisoned.

He tears off a strip of the serge
robe
he's wearing in place of the clothes he was wearing when he was taken
into trial. He puts the
strip on the floor, perpendicular to the wall, so
he'll be able to know that he's made a circuit
of the space when he again
encounters the cloth. Then, he starts walking on the slippery floor,
counting
his steps. He counts fifty-two paces before he stumbles and falls to the
floor.
Physically and mentally exhausted, he falls asleep.

Upon
awaking, and stretching forth an arm, I
found beside me a loaf and a pitcher with water. I was
too much exhausted to
reflect upon this circumstance, but ate and drank with avidity.

He resumes his circuit of the wall from where he fell,
and
eventually he returns to the cloth he laid on the floor. He had counted
fifty-two paces before
he fell and forty-eight paces to the cloth since he
resumed his walk, for a total of one hundred
paces.

There were in all, then, a hundred paces; and,

admitting two paces to the yard, I presumed the dungeon to be fifty yards in
circuit.

Later, the narrator discovers
that his calculations about the size
and composition of the "vault," as he
calls it, were incorrect, and he also discovers
that even greater horrors lie
ahead for him.