Complex sentence: He then moved to Geneva College, where he taught as a professor of mathematics and natural philosophy until he left in 1848 to head the Free Academy of New York, where he continued until retirement in 1869.
Split into simpler sentences: He then moved to Geneva College, where he taught as a professor of mathematics and natural philosophy. He left in 1848 to head the Free Academy of New York, where he continued until retirement in 1869.