When a ship sails off toward the horizon, it doesn't just get smaller and smaller until it's not visible anymore. Instead, the hull seems to sink below the horizon first, then the mast. When ships return from sea, the sequence is reversed: First the mast, then the hull, seem to rise over the horizon.
The ship-and-horizon observation is so self-evident that 1881's "Zetetic Astronomy," the first modern flat-Earth text, devotes a chapterto "debunking" it. The explanation relies on assuming that the sequential disappearance is simply an illusion brought on by perspective. This debunking does not make much sense, however, as there's nothing about perspective (which just says that things are smaller over longer distances) that should make the bottom of an object disappear before the top. If you'd like to prove to yourself that perspective isn't the reason for boats disappearing hull-first and returning mast-first, bring a telescope or binoculars on your trip to the harbor. Even with vision enhancement, the ship will still dip below the curve of the Earth.
