...lan resolved to make a powerful effort. He left Washington, it is said, with a force of 80,000 men. From the correspondents of the Yankee papers we heard of him at Rockville and other places on the National road, some time last week, from which we conclude that his army marched upon that road in the direction of Fredericktown. The road passes through Fredericktown, but whether McClellan kept it that f...