求翻译：The Lewis and Clark expedition, sponsored by President Jefferson, was the most important official examination of the high plains and the Northwest before the War of 1812. The President's secretary, Captain Meriwether Lewis, had been instructed to "explore the Missouri River, and such principal streams of it as, by its course and communication with the waters of the Pacific Ocean …may offer her most direct and practicable water communication across the continent, for the purposes of commerce." Captain William Clark, the younger brother of famed George Rogers Clerk, was invited to share the command of the exploring party.是什么意思？

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