Summary. It was Nan Pringle, and she did it to keep what she had already spent eleven years earning. Here is how I came to it.

The Reckoning. The rod told its own tale before anyone spoke a word against her. The tradesmen had set and tested every rod on the Friday — the foreman's docket said as much, dated and initialled, a dull document that happens to be an honest one — so the seventh rod was sound three days before the fall. Someone loosened it after. The driver that did the work was clean while its fellows in the roll carried a fortnight's dust; it had been used once, lately, by a hand that knew where to find it and troubled to put it back. That narrows the field to whoever passed the housemaid's cupboard at the stair-head on the night in question, and fetched something from it, and came down again by a stair that let her avoid the hall. Only one person in the house did all three: Nan, on her way for the Colonel's drops.

The lamp confirms it rather than merely repeating it. Aziz orders the household's lamps and did not move that one; he was, besides, in view of the kitchen and then the hall from dinner to half past ten, occupied with a homecoming dinner that had gone off. Wat was in the gun-room with Xerxes from half past nine, over lading bills a man cannot forge in his sleep, and had no interest in Effie's death in any case — she owed him eleven pounds he could only collect from her alive and solvent. Gaunt sat at cards from ten with Lady Lavinia and Cousin R and never left the drawing-room; his quarrel with Effie was over her soul's correction, which a rigged stair does nothing to accomplish, and he had already accepted Xerxes's refusal to give him the ordering of her penance without complaint. Cousin R likewise never went up, and stood to gain nothing by the change of will — he had spent neither the Colonel's confidence nor Nan's long service, only collected them, as is his habit. That leaves the one person who went up, came down dark, and remarked aloud that a passage wanted a lamp — which is the sort of thing a person says when she wishes to be remembered as having noticed a want of light, rather than as having caused one.

The motive lies where the dinner-table announcement left it. Eleven years of unpaid keeping, of mended sashes and set clocks and a life arranged around an old man's comfort, all resting on an expectation the Colonel had altered four times before and altered again that afternoon, in Charlotte Street, in Effie's favour. Effie meant to spend it in a season — Nice, a mare, a spirit-cabinet — and Nan meant to go on being useful and unpaid until the Colonel's habits swung back her way, as they had twice before. This time she judged, rightly or not, that they would not swing back in time. She did not loosen that rod to inherit. She loosened it because she could not afford to be dispossessed twice more before the Colonel died. It is a mean sort of murder, the meanest kind there is — not for gain, but against ruin — and I have seen its like before in this house, and expect, the Lord willing I am elsewhere by then, to see it again. I make it the third such matter this twelvemonth, by my count. I have laid the charge against Nan Pringle, and the file is closed.

When all's said and counted, the deed it was done,
and Nan the answer, when answer was none.
Poor Effie lies quiet, a candle, a knife,
the tidiest exit from Cairnholm and life.
The lesson is here, should we trouble to learn —
but the cards, being cards, will be dealt a fresh turn.