Gen. McClellan, as our dispatches show, is driving everything before him in his impetuous advance upon Richmond. After a hard battle, on the 6th, he drove the rebel army out of Williamsburg in utter route and confusion, and now has possession of that strongly fortified place. The forces of Magruder are precipitately retreating before him; but the activity with which our army is pursuing them gives us reason to believe that McClellan will bag them before they can leave the Peninsula. Gen. Franklin, who is above with 20,000 men, will in due time strike them a telling blow on their right flank. "Onward to Richmond!" is now the cry in real earnest.