Far south in the Atlantic there is an island that at first sight from the deck of a ship seems little more than a great rock. In shape it is oblong, with perpendicular sides several hundred feet high. It is called St. Helena because the Portuguese, who discovered it in 1502, came upon it on the birthday of St. Helena, Constantine's mother. To describe it as the geographies might, we may say that it lies in latitude 15 55' South, and in longitude 5...