Before daybreak on the morning of the 11th, in a thick fog, the work was begun; but the bridges had not spanned more than half the distance when the sun had risen and the fog lifted sufficiently to reveal what was going on. A detachment of Mississippi riflemen had been posted in cellars, behind stone walls, and at every point where a man could be sheltered on the south bank; and now the incessant crack of their weapons was heard, picking off the men that were laying the bridges. One after another of the blue-coats reeled with a bullet in his brain, fell into the water, and was carried down by the current, till the losses were so serious that it was impossible to continue the work. At the lower bridges the sharp-shooters, who there had no shelter but rifle-pits in the open field, were dislodged after a time, and by noon those bridges were completed. But along the front of the town they had better shelter, the National guns could not be depressed enough to shell them, and the work on the three upper bridges came to a standstill. Burnside tried bombarding the town, threw seventy tons of iron into it, and set it on fire; but still the sharp-shooters clung to their hiding places, and when the engineers tried to renew their task on the bridges, under cover of the bombardment, they were destroyed by the same murderous fire. At last General Hunt, chief of artillery, suggested a solution of the difficulty. Three regiments that volunteered for the service--the Seventh Michigan, and the Nineteenth and Twentieth Massachusetts--crossed the river in pontoon boats, under the fire of the sharp-shooters, landed quickly, and drove them out of their fastness, capturing a hundred of them, while the remainder escaped to the hills. The bridges were then completed, and the crossing was begun; but it was evening of the 12th before the entire army was on the Fredericksburg side of the river. On the morning of the 13th Burnside was ready to attack, and Lee was more than ready to be attacked. He had concentrated his whole army on the fortified heights, Longstreet's corps forming his left wing and Jackson's his right, with every gun in position, and every man ready and knowing what to expect. The weak point of the line, if it had any, was on the right, where the ground was not so high, and there was plenty of room for the deployment of the attacking force. Here Franklin commanded, with about half of the National army; and here, according to Burnside's first plan, the principal assault was to be made. But there appears to have been a sudden and unaccountable change in the plan; and when the hour for action arrived Franklin was ordered to send forward a division or two, and hold the remainder of his force ready for "a rapid movement down the old Richmond road," while Sumner on the right was ordered to send out two divisions to seize the heights back of the city. Exactly what Burnside expected to do next, if these movements had been successful, nobody appears to know.