Statistics for occurrence #1 of “Fort Whipple” in chapter 6, page 83 of The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 5: Forts and Artillery.:

... artillery. This photograph shows the flat nature of the open country about Washington. There were no natural fortifications around the city. Artificial works were necessary throughout. Fort Whipple lay to the south of Fort Corcoran, one of the three earliest forts constructed. It was built later, during one of the recurrent panics at the rumor that the Confederates were about to descend upon ...
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Fort Whipple (Arizona, United States) 67 8 1 0 0 user votes

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