Complex sentence: He superintended fortifications on the New England coast from 1841 until 1849, when he assumed command of the coast survey office in Washington, D. C., serving in that role until March 1853.
Split into simpler sentences: He superintended fortifications on the New England coast from 1841 until 1849. He was given command of the coast survey office in Washington, D. C., serving in that role until March 1853.