Using the "round Earth" theory, setting an object on the earth would be like setting grains of sand on a beach ball. Certainly a few grains would stay - right around the top, the surface is nearly horizontal - but when you stray too far from the absolute top of the ball, the grains of sand start sliding off and falling onto the ground. The Earth, if round, should behave in exactly the same fashion. Because the top is a very localized region on a sphere, if the Earth were in fact round, there would be only a very small area of land that would be at all inhabitable. Stray to the outside fringes of the "safe zone", and you start walking at a tilt. The further out you go, the more you slant, until your very survival is determined by the tread on your boots. Reach a certain point, and you slide off the face of the planet entirely. Obviously, something is wrong.