“I proceeded to think thus — à posteriori.  The murderers did escape from one of these windows.  This being so, they could not have refastened the sashes from the inside, as they were found fastened; — the consideration which put a stop, through its obviousness, to the scrutiny of the police in this quarter.  Yet the sashes were fastened.  They must, then, have the power of fastening themselves.  There was no escape from this conclusion.  I stepped to the unobstructed casement, withdrew the nail with some difficulty and attempted to raise the sash.  It resisted all my efforts, as I had anticipated.  A concealed spring must, I now know, exist; and this corroboration of my idea convinced me that my premises at least, were correct, however mysterious still appeared the circumstances attending the nails.  A careful search soon brought to light the hidden spring.  I pressed it, and, satisfied with the discovery, forbore to upraise the sash.

“I now replaced the nail and regarded it attentively.  A person passing out through this window might have reclosed it, and the spring would have caught — but the nail could not have been replaced.  The conclusion was plain, and again narrowed in the field of my investigations.  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 us 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.