After they had drifted for four days they sighted a submarine and used the reflective surface of a tobacco tin as a heliograph to attract her attention.
Although the original heliograph was designed for geodetic survey, it was used extensively for military purposes during the late 19th and early 20th century.
The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century.