Meriwether Lewis, an American explorer, born near Charlottesville, Va., Aug. 18, 1774, died near Nashville, Tenn., Oct, 11, 1809. He inherited a moderate fortune from his father, who died when he was a child. In 1794 he enrolled himself as a volunteer in the troops called out to quell the " whiskey insurrection " in western Pennsylvania, entered the regular service in 1795, became a captain in 1800, and between 1801 and 1803 was private secretary to President Jefferson. In the latter year he was recommended to congress by Jefferson to command the exploring expedition across the continent to the Pacific, In company with Capt. William Clarke, his associate in the conduct of the expedition, he set out in the summer of 1803, and encamped for the winter on the bank of the Mississippi, opposite the mouth of the Missouri. Their company was composed of nine young men from Kentucky, 14 soldiers, two Canadian boatmen, an interpreter, a hunter, and a negro servant of Capt. Clarke. In the spring of 1804 they began to ascend the Missouri. A second winter was passed among the Mandans, lat, 47° 21' N. On April 7,1805, they again moved forward, still ascending the Missouri, and reached the great falls by the middle of June. Above these, near the close of July, they attained the point where three nearly equal streams concur; to these were given the names of Jefferson, Madison, and Gallatin, then president, secretary of state, and secretary of the treasury of the United States. They ascended the Jefferson, the northernmost of the three, to its source.

Having in August procured horses and a guide from the Shoshone Indians, they travelled through the mountains until Sept. 22, when they entered the plains of the western slope. On Oct. 7 they embarked in canoes on the Kooskoosky, a left branch of the Columbia, and on Nov. 15 reached the mouth of that river, having travelled more than 4,000 m. from the confluence of the Mississippi and Missouri. They passed their third winter in an intrenched camp on the S. bank of the Columbia. On March 23, 1806, they began to re-ascend the Columbia on their homeward journey; and leaving their boats on May 2, they made a difficult journey on horseback across the mountains to the Missouri, upon which they reembarked Aug. 12, and reached St. Louis Sept. 23, after an absence of two years and four months. Congress made grants of land to the men of the expedition and to their chiefs; Lewis was made governor of Missouri territory, and Clarke general of its militia and Indian agent. In the comparative quiet of his new mode of life Lewis began to suffer from hypochondria, to which he had been subject from his youth. During one of these seasons of depression duty called him to Washington, and at a lodging place in Tennessee he put an end to his life.

A narrative of the expedition of Lewis and Clarke, from materials furnished by each of the explorers, was prepared by Nicholas Biddle and Paul Allen, with a memoir of Lewis by Jefferson (2 vols., Philadelphia, 1814; new ed., with additions by Archibald McVickar, 2 vols., New York, 1843).