(C5~ A letter, under date of March 2oth from Gustavus Raus, of the 3'0th Regimest, says that tha army under Gen. Buell is muring south from Nashville; that tire 30;h is in advance, and were a± that time 40 miles from Nashville, at Columbia, building a bridge across the river at that point, which would be j completed in a few days, nod the ar my would mve on to Decatur, Alabama, to co operate with Gen. Gran! against the combined forces of Beauregard, Bragg, and Evans. Perhaps the greatest battle ofthe war will be fought i:i the vicin ifcy of Corinth. Mississippi; for both sides aro unking stupendous preparations. If it takes plac?, it will be the hist general engagement. Federal victory at that point will leave the whole south powerless to the Gull of Mexico.The 30th is in good health; have the march the post of honor, and in the fight the boys trill give a good account of themselves.