The following is an extract from a letter received in Philadelphia, dated Beaufort, 8. C., June 28: “ We are preparing an expedition from this place, under Geu. Saxton, consisting of the Fifty Sixth New \ork, the First and Second South Carolina, and another colored regiment, (Twenty Sixth, Col. Sill man), in all four regiments. The destination of it is supposed to be a raid on the mainland below Savannah. The troops are ordered to take sixteen days’ rations. They were to start on the JOth on transports from Beaufort.