The Lewis and Clark expedition of 1804–1806 across America from Pittsburg to the Pacific and back was the third recorded transcontinental journey. President Jefferson had negotiated the Louisiana Purchase of over two million square kilometres from the French in 1803, and the aim of the expedition was to investigate the territory involved. He commissioned a Corps of Discovery as a scientific and military expedition to survey the acquisition, appointing his aide Meriwether Lewis (1774–1809) to lead it. It was hoped to discover that the Missouri and other rivers could be used for transcontinental communication and transport, and to assess the natural resources of the area. Some of the party returned east with specimens, reports and a map, while the remainder reached the Pacific in December 1805. Volume 3 covers the return journey, during which Lewis and Clark separated to survey more of the tributaries of the Missouri.