a heliograph and its components

Each of the watchtowers that march alongside the roads of the Guarded Land has a heliograph at its summit. This is a complex device consisting of a louvred panel of mirrored strips that can be angled and rotated to catch the rays of the sun so as to send messages to a neighbouring tower. Operated by the ammonites who staff each tower, messages can thus be jumped from one tower to the next, permitting back and forth communications between the Wise and the furthest parts of the Guarded Land. A message sent by a heliograph is called a heliogram

Heliography is very fast: a journey from Nothnarlan to the City at the Gates would take a traveller along the road more than forty days, and yet a message can be transmitted by heliograph from Nothnaralan to the Wise, in the heart of Osrakum, in around one third of a day