McClellan at Fredericksburg.
Our latest advices leave no doubt of the fact that the young Napoleon of the Federal army has again ‘"changed his base,"’ and is now prepared with his grand army to co- operate with the forces of Pope along the line of the Rappahannock. The reports that have been industriously circulated that he was evacuating the Peninsula, are at length confirmed, and there seems to be no longer any doubt that he has swung round to Fredericksburg to act in conjunction with the Federal forces in that locality for the defence of Washington. Intelligence from Fredericksburg states that he arrived there on Friday, whether to assume command of the enemy's forces, or to play second to his rival Pope, is not known.