"My own examination was somewhat more particular, and was so for the reason I have just given-because here it was, I knew, that all apparent impossibilities «must» be proved to be not such in reality.

"I proceeded to think thus-»á posteriori.» The murderers «did» escape from one of these windows. This being so, they could not have re-fastened 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 knew, 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 re-closed 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 neighbour. I now looked at the nail. It was as stout as the other, and apparently fitted in in the same manner-driven in nearly up to the head.