Another form of visual signalling was the flare, a brief burst of colour or white stars sent into the air from a flare gun, or Very pistol, named for its American inventor. Such flares were often used — fired straight into the air — as a distress signal, but they could also be used to send messages, using a pre-established pattern of flares.
Of course, the watcher had to remember the sequence correctly.
Bairnsfather, Bruce. Still More Fragments from France. Vol 3. nd.31.