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U.S. Army Quartermaster's Department, Alexandria, Virginia. View full size. Albumen print from a photograph by Andrew J. Russell taken during the Civil War (1861-1865). Note the elaborate signal mast on the roof.
This structure had been the Alexandria station of the Alexandria, Loudon and Hampshire RR to Leesburg. At the end of the 19th century the AL&H became the electrified Washington and Old Dominion RR, abandoned in 1968.
See other views of this building taken about the same time in Ames Williams' book "Washington and Old Dominion RR, 1847-1968".
...A dentist, Mahlon Loomis, supposedly "demonstrated" a method of wireless communication about a year after the war. This was largely forgotten.
http://en.wikipedia.org/wiki/Mahlon_Loomis
Marconi's work came later in the century, to better acclaim.
[The thing on the roof is, as noted in the caption, a signal mast. - Dave]
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