It stood the whole length of the window." There was a lull of the wind.

The hail was not falling so thickly--moreover, it now fell, what there was of it, straight, and yet a strange clattering sound came upon the glass of that long window.

It could not be a delusion--she is awake, and she hears it.

What can produce it?
Another flash of lightning--another shriek--there could be now no delusion.
A tall figure is standing on the ledge immediately outside the long window.

It is its finger-nails upon the glass that produces the sound so like the hail, now that the hail has ceased.