Several possibilities for this variant of “perches”: “A pole or similar marker set up in a river, harbor, etc., to aid navigation”; or “a measure of length for fences, walls, etc., standardized at 5.5 yards”; or “a measure of area equal to one square perch, 30.25 square yards or 1/160 of an acre”; or “a solid measure for stone, one linear perch in length and 1.5 feet high and 1 foot thick, having a volume of 24.75 cubic feet,” though this last is the only definition that does not predate Harriot. The context suggests that the linear measurement is the most likely.