The Lewis and Clark Expedition, also known as the Corps of Discovery Expedition, was the first American expedition to cross what is now the western portion of the United States, departing in May 1804, from near St. Louis on the Mississippi River, making their way westward through the continental divide to the Pacific coast. The expedition was commissioned by President Thomas Jefferson shortly after the Louisiana Purchase in 1803, consisting of a select group of U.S. Army volunteers under the command of Captain Meriwether Lewis and his close friend Second Lieutenant William Clark. Their perilous journey lasted from May 1804 to September 1806. The primary objective was to explore and map the newly acquired territory, find a practical route across the Western half of the continent, and establish an American presence in this territory before Britain and other European powers tried to claim it. The campaign's secondary objectives were scientific and economic: to study the area's plants, animal life, and geography, and establish trade with local Indian tribes. With maps, sketches, and journals in hand, the expedition returned to St. Louis to report their findings to Jefferson. Lewis's ship-master telescope he used. The pocket compass used by Clark. Elk Antlers. One of the few surviving specimens sent to Thomas Jefferson from the Lewis and Clark Expedition. A bear claw necklace acquired by Lewis and Clark from an Indian tribe. A formal Indian dress acquired by Lewis and Clark. Lewis' air gun that he shot off to signal he was entering a tribal camp. More artifacts Whaling Chiefs hat given to Lewis. Sextant used by Lewis to measure Longitude and Latitude. Camp Wood. The actual starting/sail off point of the whole expedition.