This detachment of Hill with a strong force to a point so far away as to forbid his direct coöperation with the rest of the army during the battle, seriously diminished Lee's effective strength, but the advantage of position which he possessed and the advantage of fighting behind breastworks which his enemy must assail from the open, compensated him somewhat. Burnside's first difficult task was to get his army across the river. This he could do only by the use of frail pontoon bridges of which he purposed to lay five--three in front of the city and two below. The pontoon trains were slow in arriving, and when they came it was for a time impossible to put the pontoons into position, owing to the destructive fire of the sleepless sharpshooters Lee had posted along the banks to interfere with the work. In consequence of this and other difficulties it was not until the tenth of December that a crossing was made which Burnside had confidently expected to make more than a fortnight earlier. In the meanwhile Lee had busied himself night and day in strengthening his position in every possible way, and he was soon fully prepared for the contest. Burnside's first assault was made about ten o'clock on the morning of December 13. It was made in two columns, striking simultaneously, the one against Lee's right and the other against his left. The assault upon his right was at first attended with a partial success, but Jackson hurried troops to the breach and quickly hurled the assailants back in confusion, pursuing them nearly to the river's bank where a heavy artillery fire checked his progress. The Federal assault on that part of the Confederate line was not renewed during the day. From official reports and otherwise, it appears that Burnside at first intended to direct his main attack upon this right wing of Lee's army. The hills there were lower and far less defensible than were those on the Confederate left, and offered, certainly, a much more tempting opportunity to the Federal commander. It was without doubt the weakest point in Lee's line--the point which the Federals might have assailed with greatest hope of success or at least of inflicting the heaviest loss upon their foes. But, for some reason which has never been clearly explained, Burnside changed his plan of battle almost at the last moment, and directed his heaviest columns against Marye's Heights, the well-nigh impregnable stronghold of Lee's left wing. Here Lee had his batteries and a host of infantry strongly posted in formidable earthworks on top of the hills, in a position of great advantage.