The assassins _must_ have escaped
      through the other window. Supposing, then, the springs upon each
      sash to be the same, as was probable, there _must_ be found a
      difference between the nails, or at least between the modes of
      their fixture. Getting upon the sacking of the bedstead, I looked
      over the head-board minutely at the second casement. Passing my
      hand down behind the board, I readily discovered and pressed the
      spring, which was, as I had supposed, identical in character with
      its neighbor. I now looked at the nail. It was as stout as the
      other, and apparently fitted in the same manner—driven in nearly
      up to the head. “You will say that I was puzzled; but, if you think so, you must
      have misunderstood the nature of the inductions. To use a
      sporting phrase, I had not been once ‘at fault.’ The scent had
      never for an instant been lost. There was no flaw in any link of
      the chain. I had traced the secret to its ultimate result,—and
      that result was _the nail._ It had, I say, in every respect, the
      appearance of its fellow in the other window; but this fact was
      an absolute nullity (conclusive as it might seem to be) when
      compared with the consideration that here, at this point,
      terminated the clew. ‘There _must_ be something wrong,’ I said,
      ‘about the nail.’ I touched it; and the head, with about a
      quarter of an inch of the shank, came off in my fingers. The rest
      of the shank was in the gimlet-hole where it had been broken off. The fracture was an old one (for its edges were incrusted with
      rust), and had apparently been accomplished by the blow of a
      hammer, which had partially imbedded, in the top of the bottom
      sash, the head portion of the nail. I now carefully replaced this
      head portion in the indentation whence I had taken it, and the
      resemblance to a perfect nail was complete—the fissure was
      invisible. Pressing the spring, I gently raised the sash for a
      few inches; the head went up with it, remaining firm in its bed. I closed the window, and the semblance of the whole nail was
      again perfect. “The riddle, so far, was now unriddled. The assassin had escaped
      through the window which looked upon the bed. Dropping of its own
      accord upon his exit (or perhaps purposely closed), it had become
      fastened by the spring; and it was the retention of this spring
      which had been mistaken by the police for that of the
      nail,—farther inquiry being thus considered unnecessary. “The next question is that of the mode of descent. Upon this
      point I had been satisfied in my walk with you around the
      building. About five feet and a half from the casement in
      question there runs a lightning-rod. From this rod it would have
      been impossible for any one to reach the window itself, to say
      nothing of entering it. I observed, however, that the shutters of
      the fourth story were of the peculiar kind called by Parisian
      carpenters _ferrades_—a kind rarely employed at the present day,
      but frequently seen upon very old mansions at Lyons and Bordeaux. They are in the form of an ordinary door (a single, not a folding
      door), except that the lower half is latticed or worked in open
      trellis—thus affording an excellent hold for the hands. In the
      present instance these shutters are fully three feet and a half
      broad. When we saw them from the rear of the house, they were
      both about half open—that is to say, they stood off at right
      angles from the wall. It is probable that the police, as well as
      myself, examined the back of the tenement; but, if so, in looking
      at these _ferrades_ in the line of their breadth (as they must
      have done), they did not perceive this great breadth itself, or,
      at all events, failed to take it into due consideration. In fact,
      having once satisfied themselves that no egress could have been
      made in this quarter, they would naturally bestow here a very
      cursory examination. It was clear to me, however, that the
      shutter belonging to the window at the head of the bed, would, if
      swung fully back to the wall, reach to within two feet of the
      lightning-rod. It was also evident that, by exertion of a very
      unusual degree of activity and courage, an entrance into the
      window, from the rod, might have been thus effected. By reaching
      to the distance of two feet and a half (we now suppose the
      shutter open to its whole extent) a robber might have taken a
      firm grasp upon the trellis-work. Letting go, then, his hold upon
      the rod, placing his feet securely against the wall, and
      springing boldly from it, he might have swung the shutter so as
      to close it, and, if we imagine the window open at the time,
      might even have swung himself into the room. “I wish you to bear especially in mind that I have spoken of a
      _very_ unusual degree of activity as requisite to success in so
      hazardous and so difficult a feat. It is my design to show you,
      first, that the thing might possibly have been accomplished:—but,
      secondly and _chiefly_, I wish to impress upon your understanding
      the _very extraordinary_—the almost præternatural character of
      that agility which could have accomplished it. “You will say, no doubt, using the language of the law, that ‘to
      make out my case,’ I should rather undervalue, than insist upon a
      full estimation of the activity required in this matter. This may
      be the practice in law, but it is not the usage of reason. My
      ultimate object is only the truth. My immediate purpose is to
      lead you to place in juxtaposition, that _very unusual_ activity
      of which I have just spoken with that _very peculiar_ shrill (or
      harsh) and _unequal_ voice, about whose nationality no two
      persons could be found to agree, and in whose utterance no
      syllabification could be detected.”

      At these words a vague and half-formed conception of the meaning
      of Dupin flitted over my mind. I seemed to be upon the verge of
      comprehension without power to comprehend—as men, at times, find
      themselves upon the brink of remembrance without being able, in
      the end, to remember. My friend went on with his discourse. “You will see,” he said, “that I have shifted the question from
      the mode of egress to that of ingress. It was my design to convey
      the idea that both were effected in the same manner, at the same
      point. Let us now revert to the interior of the room. Let us
      survey the appearances here. The drawers of the bureau, it is
      said, had been rifled, although many articles of apparel still
      remained within them. The conclusion here is absurd. It is a mere
      guess—a very silly one—and no more.