September 15—It seems that the Yanks gained possession of both gaps in South Mountain late yesterday evening. At one o’clock last night we were ordered to move to Martinsburg. We started immediately after we received the order. At Williamsport we forded the Potomac. It is about two feet deep, with a gentle current and smooth gravelly bottom. Williamsport is a town of about a thousand inhabitants, situated on the Maryland side of the Potomac, six miles from Hagerstown and twelve miles from Martinsburg. There is a natural embankment on the dike order about forty feet high between the town and the river, which entirely shuts from view the lower part of the town from the stream. We arrived at Martinsburg at dusk this evening and I left my gun there at a shop for repairs. We then moved about three miles down the Shepherdstown road and camped for the night.