The early voyages into the deep waters of the Atlantic rank among the greatest feats of exploration. In tiny, fragile vessels the Irish monks searched for desolate places in the ocean in which to pursue their vocation; their successors, the Vikings, with their superb ship-building skills, created fast, sea-worthy craft which took them far out into the unknown, until they finally reached Greenland and America. G.J. Marcus looks at the history of these expeditions not only as a historian, but also as a practical sailor. Besides the problem of what these early explorers actually achieved, he...