Summary. It was Nan Pringle, and the reason was the will signed that same morning — the one that left the whole of the Cruikshank estate to Effie Strange and struck Nan out of it entirely, after eleven years in which she had kept the house and the Colonel's accounts for her keep and nothing more. Here is how I came to it.

The Reckoning. The method was plain enough once the hour was fixed. Someone drew the screws of the fourth stair-rod from the top sometime between four o'clock, when Mrs Gow swept the cellar stair and found every rod tight, and six o'clock, when the Colonel came up from broaching his pipe. In that window the back passage stood empty of staff, which is to say it stood open to anyone of the household with lamps or linen to see to — an errand that takes a person up and down that passage a dozen times of an afternoon and draws no eye. The screwdriver went back into the umbrella stand, wiped; one screw was lost into a gardening apron hanging where aprons hang. The rod itself was left sitting loose in its tread, so that the runner would slide under the first foot set upon it with any weight. No hand needed to be near the stair when it happened. The Colonel did the rest himself, sending Effie down for the second decanter at nine o'clock, as he has sent someone down for a second decanter most evenings of his life. It is not a cunning device. It is a patient one, built to wait on the house's own habits, which is why it answers to no alibi for the evening at all — only for the two hours in the afternoon when the rod was loosened, and those two hours clear nearly everyone else in the case rather than Nan.

The Colonel himself was in the cellar from four until six with the cooper and Zane, which puts him inside the only window that signifies and in company the whole of it besides; and it was his own new will the fall undid, which is not a thing a man arranges against himself. Aziz was at Hartfield House until a quarter to eight and never within reach of the stair, and the ring he wore that Effie had once priced aloud was never taken nor reached for. Quill was at a confinement in Fisher's Row from three until seven, the midwife swearing to it, and came direct to the Manor after. Cousin R sat in the library from four, Mrs Gow bringing his tea and removing it at a quarter past five, the length of the house from the back passage, and he wanted nothing of Effie that he had not already for free. Lavinia did not come through the door until seven, having sat in her own rooms at five with her landlady's tray before her, and the four pounds between them was no motive at all once two of it was handed back at the threshold, laughing. Each of these is a fact, not an impression, and each closes its door quietly.

What is left, once the others are set down, is the person who stood to lose everything that morning and nothing the morning before. Under the old will the house and the bulk of the estate came to Nan in time, and she could wait for that, having waited eleven years already on less. Under the new one she had nothing — no settlement, no annuity, no family to go to, and a woman in the house who meant, by her own happy announcement at table, to sell the land and the plate and winter abroad the moment it was hers. A servant may absorb a great deal of unfairness over eleven years if there is an end to it in sight. Take away the end and you have taken away the only thing that made the unfairness bearable. I had thought at first the lost screw in the gardening apron a small matter, hardly worth the noting, until I recalled that the apron is Nan's own, kept on her peg in that passage and nowhere else — which puts the tool in her hand at the hour it mattered, and nowhere else does.

She has given her account, and it does not trouble to deny the facts of the stair, only the weight of them. I have charged her accordingly. It is the third this twelvemonth, by my count, and I confess I had hoped, this time, to be sent somewhere other than Cruikshank Manor.

Nan kept the screws and returned the tool
While Effie went down, as a dutiful fool
The stair gave a shiver, the candle went out
And nobody mentioned what falling's about
We'll vow to be watchful, and careful, and wise
Then leave every loose little danger to rise