Postbellum
After the war, Humphreys commanded the District of Pennsylvania. He became a permanent brigadier general and Chief of Engineers in 1866, a position he held until June 30, 1879, when he retired, serving during this period on lighthouse and other engineering boards. In retirement, Humphreys studied philosophy. He was one of the incorporators of the National Academy of Sciences. Humphreys's published works were highlighted by his 1867 Report on the Physics and Hydraulics of the Mississippi River, which gave him considerable prominence in the scientific community. He also wrote personal accounts of the war, published in 1883: From Gettysburg to the Rapidan and The Virginia Campaign of '64 and '65. He died in Washington, D.C., and is buried there in the Congressional Cemetery.
A military base was founded during World War I as Camp A. A. Humphreys, named for Andrew A. Humphreys. The post was renamed Fort Belvoir in the 1930s in recognition of the Belvoir plantation that once occupied the site, but the adjacent United States Army Corps of Engineers Humphreys Engineer Center retains part of the original namesake
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