The heliograph had some powerful advantages. It allowed long
distance communication without a fixed infrastructure, though it
could also be linked to make a fixed network extending over
hundreds of miles, as in the fort-to-fort network used in the
Geronimo campaign. It was highly portable, required no power
source, and was relatively secure since it was invisible to
those not near the axis of operation. However, anyone in the
beam with the correct knowledge could intercept signals without
being detected. In the Boer war, where both sides used
heliographs, tubes were sometimes used to decrease the
dispersion of the beam. The distance that heliograph signals
could be seen depended on the clarity of the sky and the size of
the mirrors used. A clear line of sight was required, and since
the earth's surface is curved, the highest convenient points
were used. Under ordinary conditions, a flash could be seen 30
miles (48 km) with the naked eye, and much farther with a
telescope. The maximum range was considered to b