In the 1840s, the U.S. Coast Survey began the first accurate mapping of the nation’s coasts under its superintendent, Professor Alexander D. Bache, a brilliant scientist who was Benjamin Franklin’s great-grandson and Jefferson Davis’s West Point roommate. Seeking to overcome the longstanding problem that measuring tools expanded and contracted with temperature changes, Bache invented the bar of invariable length by placing an iron rod beside a brass rod within a tin tube and connecting them at one end with a compensating lever so their different expansion rates canceled each other out, producing reliable measurements. Bache later helped establish the Smithsonian Institution and the National Academy of Science.