The Federal at Tybee.
--A reconnaissance made a few days since by our troops, reveals the fact that the federal have erected a battery on the Martello tower on Tybee Island, which will throw shot over Fort Pulaski.--They have also erected a battery on Decent Creek, South east of Fort Pulaski, near Lenaretto Creek, on an old hulk, which is thought will also threw shot on the fort.--Nothing further was discovered which it would be prudent to record.-- Savannah News, 19th. inst.