Mapping the Louisiana Purchase.

No one knew precisely how large Louisiana was or what it contained, so President Thomas Jefferson secured $2,500 from Congress in early 1803 to explore the territory. He appointed Meriwether Lewis, his private secretary and a former soldier, to command the expedition. To prepare for the mission, Lewis immersed himself in books on zoology, astronomy, and botany. For the expedition's coleader Lewis picked William Clark, another former soldier and an experienced mapmaker who had earned a reputation for canny negotiations with Indians.

Reaching the Pacific.

In the spring 1804 Lewis, Clark and forty-one men started up the Missouri River from the village of St. Louis in two dugout canoes and a fifty-five-foot keelboat. During the summer and fall the team traveled one thousand six hundred miles through country inhabited by the Missouri, Pawnee, Crow, Sioux, and Mandan peoples. That...