The Heliograph in the Apache Wars

"The mountains and the sun...were made his allies, the eyes of his command, and the carriers of swift messages. By a system of heliograph signals, communications were sent with almost incredible swiftness; in one instance a message traveled seven hundred miles in four hours. The messages, flashed by mirrors from peak to peak of the mountains, disheartened the Indians as they crept stealthily or rode swiftly through the valleys, assuring them that all their arts and craft had not availed to conceal their trails, that troops were pursuing them and others awaiting them. The telescopes of the Signal Corps, who garrisoned the rudely built but impregnable works on the mountains, permitted no movement by day, no cloud of dust even in the valleys below to escape attention. Little wonder that the Indians thought that the powers of the unseen world were confederated against them."

So wrote Major George W. Baird as he described Miles's tactics against Geronimo in Century magazine, July, 1891.