Before revision: Semi-automatic operation of foghorns was achieved by using a clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them.See for example Fox, F. Siren, Point of Ayre Lighthouse , accessed 08-09-2008 History Early fog signals
After revision: Fog horn made with a marine sheell, with a hole on its narrowest side.Semi-automatic operation of foghorns was achieved by using a clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them.See for example Fox, F. Siren, Point of Ayre Lighthouse , accessed 08-09-2008 History Early fog signals