For observers near sea level the difference between this " geometrical horizon " ( which assumes a perfectly flat, infinite ground plane ) and the " true horizon " ( which assumes a spherical Earth surface ) is imperceptible to the naked eye ( but for someone on a 1000-meter hill looking out to sea the true horizon will be about a degree below a horizontal line ).
12.
I could not distinctly assert that we really saw the Mediterranean, for the distance is so enormous that it becomes impossible to distinguish the horizontal line which separates the sky from the water : but judging from the perfect clearness with which we saw the very end of the Maritime Alps, and from the fact that the Apennines were not high enough to appear noticeable in the view, it is undoubtedly certain that our true horizon must have included part of the Gulf of Genoa, though the similarity of tint at so immense a distance made it impossible to say which was sky and which was water.
