The full sap begins at two in the morning
At two o’clock, the water having gone down enough to make it possible, Captain Walker started the full sap with a sapping brigade of ten men from his own company of New York Volunteer Engineers. He had sixty feet done by eight and was relieved. From here on the Federals advanced on Battery Wagner behind a rolling wicker screen, a few feet an hour.
Major Brooks’ journal, OR I, 28, Pt. 1