She knew that any sound would bring aid and rescue at once.

There would be plenty of time to scream, to pull the bell or to do whatever was necessary later.

And something, she could not tell what, something she could not recognize, impelled her to take the course she did; to wait, armed.
But the wait began to tell on her sensibilities.

The sound of somebody or something moving mysteriously to-and-fro behind the curtains over against the wall at the other end of the room began to work on her nerves.

It takes an iron steadiness, a passive capacity for endurance which is quite different from woman's more or less emotional courage, to wait under circumstances like that.
Just when she had reached the limit of her endurance and was persuaded that she could stand no more, her attention was attracted by a slight click as of a lock or catch, a movement as of something heavy, as of a drawer or door, and then the footsteps turned and came toward the window.