"Rosanna has done one of two things.

She has either gone direct to Frizinghall (before I can get there), or she has gone first to visit her hiding-place at the Shivering Sand.

The first thing to find out is, which of the servants saw the last of her before she left the house." On instituting this inquiry, it turned out that the last person who had set eyes on Rosanna was Nancy, the kitchenmaid.
Nancy had seen her slip out with a letter in her hand, and stop the butcher's man who had just been delivering some meat at the back door.
Nancy had heard her ask the man to post the letter when he got back to Frizinghall.

The man had looked at the address, and had said it was a roundabout way of delivering a letter directed to Cobb's Hole, to post it at Frizinghall--and that, moreover, on a Saturday, which would prevent the letter from getting to its destination until Monday morning, Rosanna had answered that the delivery of the letter being delayed till Monday was of no importance.

The only thing she wished to be sure of was that the man would do what she told him.