Five years after graduation he had worked up to a first lieutenancy, and two years more found him a captain. In 1835 he was appointed on a commission to fix the boundary line between Michigan and Ohio. A few months later he was detailed to make an important study of the Mississippi River and Valley with a view to determining how to prevent the annual overflows with their consequent damage to property. His researches were chiefly along the upper river at Illinois. It is said that while there he was struck with the enormous potential energy of the current, and reported that if a dam were constructed at a certain place, a great storehouse of power would be possible. This was long before the day of the dynamo, by which such power could be harnessed. Many years later, however, his dream came true, at the place he had indicated,--the great power dam nearly a mile long blocking the "Father of Waters" for the first time in his tumultuous career, at Keokuk, Iowa. Farther down stream, above St. Louis, he began a system of river improvements which aroused no little opposition among property owners. The dispute that arose was one of the first things which brought the name of Robert E. Lee to public attention. But despite the short-sighted protests of some citizens of St. Louis, Lee went quietly ahead and carried the work through to the permanent betterment of the city. "I was sent here to do certain work, and I shall do it," was his terse comment. When he had completed his work on the Mississippi, he was sent to New York to complete the harbor defenses at Fort Hamilton--down at the gateway of the city. He had been made captain of engineers by this time, and was looked upon as one of the ablest men in his line of work, in the army. It was not long before his mettle was to be tested in actual warfare. The trouble with Mexico which had been smouldering for several years at length burst into flame. After the first victories along the border under General Zach. Taylor, a campaign from the sea was undertaken, under General Winfield Scott, who landed at Vera Cruz. The purpose was to march overland to the capital, reducing the country as they went; and to make this possible the army engineers were in demand. They answered the call gladly, for the spirit of adventure ran high, and every army officer welcomed the chance to see active service. In the corps of engineers we find several names destined to become famous--Lee, Beauregard, McClellan, Foster, Tower, Stevens, Totten, and others; while Grant was attached to the commissary of the same army. It was in effect a training school for the great drama of a few short years later.