Great Sen Fight at Newport News. lt;]7/itf Cumberland and Congress sunk by the rebel Floating Battery Merrhwic. — Engagement Ret ween the Jfonitor and Merrjmac.— The Jlfcrrimac Compelled to Retire,Fortress Moxrok, March 9.Tlic long expected Confederate steamer Merrimac made her appearance yesterday P. M., and with the assistance of two gunboats which came out with her from Norfolk, made an attack upon Newport News, and the naval vessels stationed at that place.The Merrirnae was first seen from the ramparts of Fortress Monroe, on her way to Newport News, at about a quarter before one o’clock. Two rebel gnu boats followed her. They all carried a rebel flag at the stern, and had a French flag at the masthead.The Merrimac had a flag at her bows, which was described by some as a Commodore's blue flag, and by others as a black flag. The sides, bow and stern of the Merrimac were covered with sloping iron plates extending about two feet below the water line and meeting above like the roof of a house. On her bows on the water line are two sharp points resembling plows, about six or seven feet apart. The number of guns is staled at twelve, but she might not have bad so many. At her bows were seen two guns protruding• from clintical norlhnles.