This year this nation observes the 200th anniversary of its birth under what is now the world's oldest Constitution. And this spring, John Seelye looks back to an early opus in praise of our Republic and of its discovery in a New World by Columbus, a discovery whose cinquecentenary we are approaching. The epic is Joel Barlow's The Columbiad. It appeared in 1807, the same year Barlow's friend, Robert Fulton, launched the Clermont, the world's first successful steamboat. With the latter America began a transportation and technological revolution destined to link its major centers in the East and the Midwest and finally the Pacific Coast by rails, rivers, or canals. "Nothing similar," Mr. Seelye observes, "followed upon the launching of The Columbiad, an ambitious work on which Barlow had labored for years only to watch it sink below the waters of public acknowledgement with barely a ripple." Yet, Mr. Seelye contends, The Columbiad should not be consigned to a full fathom five of obscurity. "There is therefore," he writes, "something down below worth saving ... buried in the sheer bulk of Barlow's poem and in the collective works of his fellow poets of the Revolutionary and Republican era we can detect elements of a unique if not entirely new poetic genre, a form that was not only suited to the regnant spirit of the Republic during the last years of the 18th century but which recommended itself to Whitman's far greater talent a half-century later."