As the saps in front of Bainbridge's and Duryea's batteries drew every day nearer to the bastion and the priest-cap, the working parties were harassed and began to be greatly delayed by the unceasing fire of the Confederate sharp-shooters. Moreover, in spite of the vigilance of the sharp-shooters in the trenches, their adversaries had so much the advantage of ground that they were able to render the passage of certain exposed points of the approaches slow and hazardous. At first, cotton bales were used to protect the head of the sap, but these the adventurous enemy set alight with blazing arrows or by sallies of small parties under cover of darkness. In the short night it was impossible to raise a pile of sand-bags high enough to overlook the breastworks. Toward the end of June this was changed in a single night by the skill and ingenuity of Colonel Edward Prince, of the 7th Illinois cavalry. Happening to be at headquarters when the trouble was being talked about, he heard an officer suggest making use of the empty hogsheads at the sugar-house; how to get them to the trenches was the next question. This he promptly offered to solve if simply ordered to do it and left to himself. Cavalry had never been of any use in a siege, he said; it was time for a change. The order was instantly given. Prince swung himself into the saddle and rode away. Before daylight his men had carried through the woods and over the hills to the mouth of the sap, opposite the southern angle of the priest-cap, enough sugar hogsheads to make two tiers. The heads had been knocked in, a long pole thrust through each hogshead, and thus slung, it was easy for two mounted troopers to carry it between them. Quietly rolled into position by the working parties and rapidly filled with earth, a rude platform erected behind for the sharp-shooter to mount upon, with a few sand-bags thrown on top to protect his head,--this was the beginning of the great trench cavalier, whose frowning crest the astonished Confederates awoke the next morning to find towering high above their heads. Afterwards enlarged and strengthened, it finally dominated the whole line of defence not only in its immediate front, but for a long distance on either side.