JNew lonK, Jan. 20. The World says a letter from a corresDondent with the Army of the Potomac, states that the army is under marching orders, with three days' rations. Five pon toon bridgcs are to be thrown icross the RappahaUQOck some distanoe apart. Lee's army is composed of eight large divisions. Jackson commands the right, and Longstreet the left, oomprising five divisions. These aro apparontly stronger than ours, eomprising a greater number of brigades and regiments. . It is uncí'!'tain whether their total strength equals ours. A desperate struggle is evidently near. Stirring news raay bo expeoted shortly.