In 1833 he was appointed teacher of mathematics in the Navy, and made a cruise to South America, which occupied about two and a half years of time. While absent, a law was passed creating the office of professor of mathematics in the Navy, for which Fremont upon his return was examined, and appointed. Without entering upon the duties of the place, he declined the position, and accepted the post of surveyor and railroad engineer upon the railway line between Charleston and Augusta. In 1838 and 1839 he was associated with M. Nicollet, a Frenchman and a member of the Academy of Science, in an exploring expedition over the Northwestern prairie and along the valley of the Mississippi. During his absence, he was appointed by President Van Buren a second lieutenant in the corps of topographical engineers. Upon his return from the Upper Mississippi, and for the period of a year, he was engaged with Nicollet and Mr. Hassler, then the head of the Coast Survey, in the arrangement of the scientific materials that had been collected during the expedition, and in the preparation of a map and a report. In 1842 he was directed by Colonel Abert, the chief of the topographical corps, to make an exploration of the Northwestern frontier of the State of Missouri to the Rocky Mountains, and with special reference to an examination of what was known as South Pass in those mountains. This expedition was on a small scale, consisting of twenty-one men only, most of whom were of French extraction. In this expedition he traced the waters of the Platte to the South Pass, which he reached the 8th of August. It was stated by Dr. Linn, then a Senator from the State of Missouri, that "over the whole course of the road barometrical observations were made by Mr. Fremont to ascertain the elevations both of the plains and of the mountains, astronomical observations were made to ascertain latitudes and longitudes, the face of the country was marked as arable or sterile, the facility of traveling and the practicability of routes noted, the grand features of nature described and some represented in drawings, military positions indicated, and a large contribution to geology and botany was made in varieties of plants, flowers, shrubs, trees and grasses, and rocks and earths, which were enumerated." The second expedition of May, 1843, was upon a larger scale, and it was not completed until the month of July, 1844. He was directed to extend his survey across the continent, on the line of travel between the State of Missouri and the tide-water region of the Columbia. In its execution, much more ground was covered than had been contemplated in the order. Fremont was the first person that visited the basin of the Great Salt Lake who was able to furnish a scientific and accurate description of the region. Von Humboldt, in his work entitled "Aspects of Nature" (pp. 32-34) has given a summary of the results reached by Fremont in his first and second expeditions, as follows: