...Kirby Smith, they retreating toward Cumberland Gap.--The monitor Nahant was successfully launched from Harrison Loring's yard, in South-Boston, Mass., at eleven A. M. to-day.--The Twelfth regiment of Vermont volunteers left Brattleboro for Washington City. A skirmish took place in the vicinity of Sibley's Landing, Mo., between a detachment of the Fifth Missouri cavalry and the combined rebel forces...