When early in the game I find that I'm on a small island with room for just 5 cities (my current game), it becomes a priority to discover Map Making and build a pair of triremes. Map Making also makes available the possibility to build the Lighthouse - one of the two cheapest wonders of the game (the other being the Colossus). Although the benefit of possessing the Lighthouse expires after the discovery of Magnetism, I believe it can be a valuable wonder, especially before the discovery of Navigation. In fact, an extra movement point for your triremes can make the difference and help finding new land. On its turn, Magellan's Expedition is a must have wonder, because its benefit never expires and sea units become more and more important in the later stages of the game whether you play peaceful or you're engaged in wars. Magellan's Expedition becomes available with the discovery of Navigation. This means that both wonders (Magellan's Expedition and Lighthouse) should remain active until the discovery of Magnetism when a civilization possesses both of them. However, this is not the case. When you already possess the Lighthouse, building Magellan's Expedition won't give you an extra movement point for sea units. When a civilization possesses the Lighthouse, the benefit of Magellan's Expedition only becomes real after discovering Magnetism. Nevertheless, it is important not to miss Magellan's Expedition for a rival civilization. I don't think that this should happen. In that period of time between the discovery of Navigation (and the building of Magellan's Expedition) and the discovery of Magnetism, sea units should be given two extra movement points. I think that there's nothing on Civilopedia or the game manual to warn players about this.