Napoleon was just then planning military operations on a great scale against England, and he was in sore need of "the sinews of war"; so he was glad to sell to this country this immense area for fifteen million dollars. The addition of the Louisiana territory more than doubled the area of the United States. =288. The Wonders revealed by the Lewis and Clarke Exploring Expedition.=--The next year President Jefferson thought it best to learn all about this wonderful addition to our territory, and so sent an expedition, under Captains Lewis and Clarke, to explore it. They started from St. Louis in May, 1804. What a remarkable journey it was!--more than two thousand miles up the Missouri River to its source, then across the Rocky Mountains, next down the Columbia River to the Pacific. They were the first white men who ever explored this vast domain, having traveled about six thousand miles. On their return in September, 1806, they were welcomed with unbounded joy. The stories of their perilous adventures sounded like a fairy romance, and the book of their travels was read everywhere. The explorers brought back word that the Indians had immense quantities of valuable furs. Soon throngs of American hunters and trappers began to roam over the vast plains and through the forests. All the way from the Missouri River to the Pacific a chain of trading posts, and stores for exchanging goods for furs were established. The wagons of the fur-traders and the winding caravans of emigrants that went under their protection soon made a pretty fair road. This was known as the Oregon trail, and in time it became the principal northern highway for Pacific travel. =289. How Dr. Whitman saved Oregon to the Union.=--In 1836 a group of young missionaries, two of them with their brides, went from New England to Oregon, taking with them a wagon all the way from the Missouri to their new homes on the Columbia River. One of these was a doctor by the name of Marcus Whitman, whose labors and counsel became of great value to the company. After they had been settled in Oregon some years Dr. Whitman discovered, one day in October, 1842, that the British were sending large bands of settlers down into Oregon, apparently to crowd American emigrants out of that rich country and to take complete possession. "The country is ours! The United States is too late. England will have Oregon and you cannot help it," exclaimed an English subject to him. "I will see," was the doctor's quiet reply. The moment Dr. Whitman heard this he became alarmed at the danger. If the President at Washington could only be informed of the facts, the threatened loss might be averted. The National Capital was three thousand or more miles away; and yet to delay a year or two might mean the seizure of all this rich country by the British.