Minnie Smith, age 15, of Pittsburgh, Penna., for her question:

How can I prove the earth is round?

Suppose the earth were flat like a tabletop. Somewhere there would be an edge. Ships would sail over it and people could fall off. But there is no edge. The surface of the earth curves around and around to form a globe. If you travel far enough in one direction you come right back to where you started.

Wherever you may be, the earth slopes down and away to the horizon. On a flat plain or on the sea you can see the earth s curvature. You can prove it by watching distant objects disappear over the horizon.

The hull of a ship sinks below the horizon first, the smokestacks sink out of sight last. The ship is disappearing bit by bit over the curved surface of the earth, line of telegraph poles along a straight flit road behave in the same way. The bases sink from view as the row disappears over the distant horizon. The tops disappear last.