New Ethiopian Serenaders now performing at Washington. : The Black Republican Opera Troop, now occupying the White House, respectfully inform the public that they have, at great expense to the nation, engaged the celebrated clown and acrobat, Abe Linkum, This artiste, so famous for his fete (a joke) between Harrisburg and Washington in March, is the source of much interest, though not a man of much principle. (another joke.) By permission of Gen. Beauregard he will in a few days conclude his entertainment with a flying leap from the dome of the Capitol to the Illinois State House. W.H. Seward, manager.