Could it be that a key was necessary to its proper solution?
I began to think so, and, moreover, that Mrs.Packard had made use of some such help as I watched her puzzling in the window over these symbols.

I recalled her movements, the length of time which elapsed before the cry of miserable understanding escaped her lips, the fact that her dress was torn apart at the throat when she came out, and decided that she had not only drawn some paper from her bosom helpful to the elucidation of these symbols, but that this paper was the one which had been the object of her frantic search the night I watched her shadow on the wall.
So convinced was I by these thoughts that any further attempt to solve the cryptogram without such aid as I have mentioned would end by leaving me where I was at present,--that is, in the fog,--that I allowed the lateness of the hour to influence me; and, putting aside my papers, I went to bed.

If I had sat over them another hour, should I have been more fortunate?
Make the attempt yourself and see..