“To an observer at A the tower is quite invisible because the curvature of the ground hides the view. To the observer at B the upper half of the tower is visible, but the lower half is still hidden. Finally, when the observer is at C he can see the whole tower. It would not be thus if the earth were flat. From any distance the whole of a tower would be visible. Afar off, no doubt, it would be seen with less clearness than near to, on account of the distance; but it could be seen more or less well from top to bottom.” Here is another drawing of Uncle Paul’s, representing two spectators, A and B, who, placed at very different distances, nevertheless see the tower from top to bottom on a flat surface. He resumed his talk. [Illustration: A Tower] “On dry land it is rare to find a surface that in extent and regularity is adapted to the observation I have just told you about. Nearly always hills, ridges, or screens of verdure intercept the view and prevent one’s seeing the gradual appearance, from summit to base, of the tower or steeple that one is approaching. On the sea no obstacle bars the view unless it be the convexity of the waters, which follow the general curvature of the earth. It is, accordingly, there especially that it is easy to study the phenomena produced by the rounded form of the earth. “When a ship coming from the open sea approaches the coast, the first points of the shore visible to those on board are the highest points, like the crests of mountains. Later the tops of high towers come into sight; still later the edge of the shore itself. In the same way an observer who witnesses from the shore the arrival of a vessel begins by perceiving the tops of the masts, then the topsails, then the sails next below, and finally the hull of the vessel. If the vessel were departing from the shore, the observer would see it gradually disappear and apparently plunge under the water, all in inverse order; that is to say, the hull would be first hidden from view, then the low sails, then the high ones, and finally the top of the mainmast, which would be the last to disappear. Four strokes of the pencil will make you understand it.” [Illustration: A Ship at Sea] “How large is the earth?” was the next question from Jules.