the relentless infantry charges posed extreme danger to the union artillery batteries in the orchard and on the wheatfield road, and they were forced to withdraw under pressure. the six napoleons of capt. john bigelow's 9th massachusetts light artillery, on the left of the line, "retired by prolonge," a technique rarely used in which the cannon was dragged backwards as it fired rapidly, the movement aided by the gun's recoil. by the time they reached the trostle house, they were told to hold the position to cover the infantry retreat, but they were eventually overrun by troops of the 21st mississippi, who captured three of their guns.