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How Heliographs worked to send military messages
A heliogragh reflects the sun using mirrors to send signals to a distant
line-of-sight receiving station.
A military heliograph crew would climb to the peak of a high mountain
to set up a highly-polished mirror. Their goal was to reflect the sun
toward another military crew on a distant high mountain top. Each crew
mounted a mirror so it could "nod" on pivots, then connected the back of
that mirror to a Morse key. A trained heliograph operator using the key
could control whether the mirror pointed precisely toward the distant
receiving station--or "nodded" slightly off that precise point. The
resulting flashes of light could be short or a bit longer, representing
the dots and dashes of Morse code.
The crew at the second distant high mountain included a trained signal
man who watched through a telescope and saw the flashes of reflected
sunlight. He also knew Morse code, and deciphered the message.
It worked much like signalling by using flags, or for that matter, by
using puffs of smoke. Using flags, as often done aboard boats at sea,
one sailor on a boat could send messages to another sailor watching
from another boat. The difference was that a heliograph was used to
send flashes of reflected sunlight to carry messages.
Heliographs worked best in locations with many sunny days and where the
land surface had unblocked high mountains, such as the deserts of New Mexico
and the southwest. An advantage of heliograph communication for military
use is that your enemy can't intercept your messages, as they might if
you used the telegraph. And your enemy, in the Southwest in the 1880's,
Apache Indians, couldn't cut your lines as was done to telegraph wires.
A possible disadvantage is that both the sending crew and the receiving
crew could be exposed to your enemy. For the Army in the southwest, this
meant including up to five guards to accompany every heliograph crew.