It was a heavy oaken door two fingers thick, secured by two locks and four bolts, and sheathed on the inside with iron plates kept in place by nails at the top and bottom. It was through that door that the escape must be effected.
Benvenuto had noticed in the corridor, a few steps from the door, the stairway leading to the platform. At intervals of two hours he heard the footsteps of the relieving sentinel going up, then the steps of the other coming down; after which he would hear nothing more for another two hours.
The question for him to solve, then, was simply this: how to reach the other side of that door, which was secured by two locks and four bolts, and furthermore sheathed on the inside with iron plates kept in place by nails at the top and bottom. The solution of this problem was the task to which Benvenuto had devoted the month in question.
With his modelling tool, which was of iron, he removed, one by one, the heads of all the nails, save four above and four below, which he left until the last day: then, in order that his work might not be detected, he replaced the missing heads with exactly similar ones, modelled in clay and covered with iron filings, so that it was impossible for the keenest eye to distinguish the false from the true. As there were, at top and bottom together, some sixty nails, and as it took at least one hour, and sometimes two, to decapitate each nail, the magnitude of the task may be understood.
Every evening, when everybody had retired, and nothing could be heard save the footsteps of the sentinel walking back and forth over his head, he built a great fire on the hearth, and piled glowing embers against the iron plates on his door; the iron became red hot, and gradually transformed to charcoal the wood upon which it was applied; but no indication of the carbonizing process appeared on the other side of the door.
For a whole month Benvenuto devoted himself to this task, as we have said; but at the end of the month it was finished, and he only awaited a favorable opportunity to make his escape. He was compelled, however, to wait a few days, for the moon was near the full when the work was done.
There was nothing more to be done to the nails, so Benvenuto continued to char the door, and drive the governor to desperation. That very day the functionary entered his cell more preoccupied than ever.
"My dear prisoner," said the worthy man, whose mind constantly recurred to his fixed idea, "do you still propose to fly away? Come, tell me frankly."
"More than ever, my dear host," replied Benvenuto.
"Look you," said the governor, "you may say what you choose, but upon my word, I believe it's impossible."