I stumbled upon one while hiking the Swedish coast north of Stockholm [https://www.atlasobscura.com/places/den-optiska-telegrafen-the-optical-telegraph https://

I stumbled upon one while hiking the Swedish coast north of Stockholm [https://www.atlasobscura.com/places/den-optiska-telegrafen-the-optical-telegraph https://www.atlasobscura.com/places/den-optiska-telegrafen-t...]. We took a minute or so to guess what it could be before reading the explanation plate. It impressively allowed communication through the baltic towards Åland. In parallel, regular "snail mail" was brought from mainland Sweden to Åland along a similar route, but through a peasant relay. Local farmers were obliged by the state to move bags of mail to the next farmers. And the coastal ones were taking turns to sail the 44 km to Åland all year round.