A Signal Corps station and training camp was established near Darnestown in 1861. Signaling with flags, invented by army surgeon Albert J. Meyer and first used against the Navahos in border warfare before the Civil War, provided a rapid way to convey intelligence and battlefield orders. Red Hill in Georgetown Washington D. C. served as the main headquarters and training center for the Union Signal Corps. The Corps had 300 officers and 2,500 men and one of the highest casualty rates of any unit in the Union army. Information was relayed in a chain called "Signal Tree Lane" extending form Harper's Ferry up-river and all the way down-river to Georgetown. On a clear day signals could be seen for 10 miles, but with rain and fog, visibility was obscured, which sometimes made this an unreliable means of communication. A New York infantry officer recalled attending a school of instruction at Darnestown, where on the Magruder farm near Darnestown the "Chestnut-tree station" had platforms some forty or fifty feet above the ground, reached by rustic ladders, with an upper platform for a man to wave his flag and another beneath for an officer watching the communicating station through his glass. This was the first line of signal stations established on the Potomac. Other wartime communication at that time relied on scouts on horseback, word of mouth form local residents, observation balloons, and telegraph lines. From towers, building tops, tall trees, and platforms, the Signal Corps used flags and telescopes during the day and flares or torches at night to send coded messages, including the wig-wag system with white flags bearing red square centers.