July 4 – official opening ofEads Bridge (combined road and rail steel arch) over theMississippi River at St. Louis, Missouri, designed byJames B. Eads. It is the longest arch bridge in the world at this time, with an overall length of 6,442 feet (1,964 m); the first use of true steel as a primary structural material in a major bridge project;[13] the first built usingcantilever support methods exclusively; and the first major project to make use ofpneumatic caissons