Were the earth round...
Using a compass at the south pole, or south of the equator, the needle would DIP DOWN, attempting to point to the north. The needle would actually want to point
through the planet.
In a spherical compass the inner circle freely floats around in ALL directions—it can even float upside down (especially if you remove the bottom weight).
If earth were a sphere, holding such a compass at the south pole or south of the equator, north would always point
to the earth, because north would be on the other side of the planet.
http://img.dxcdn.com/productimages/sku_1336_1.jpgYet as we know, all compass needles point straight ahead, and do not dip, so long as the compass is held level to the ground.
—Rx