I bicycled to Edwards Ferry on the C&O Canal the other day to discover that this is where Union troops launched their hydrogen balloons to observe Confederate troop movements across the river near Leesburg.
The hydrogen was made from box-like contraptions that mixed sulphuric acid with iron filings to make the lighter-than-air gas. The balloons were made of silk, varnish, and cotton cord. If the whole thing sounds dangerous as hell, I think you might be right!
Edwards Ferry was also the site of two 1,400-foot long pontoon bridges used to carry the 75,000 man Union Army of the Potomac, commanded by General Joseph Hooker, from Maryland into Virginia and Pennsylvania just prior to the Battle of Gettysburg.