“THE earth is round, as proved by the following facts. When, in order to reach the town he is journeying toward, a traveler crosses a level plain where nothing intercepts his view, from a certain distance the highest points of the town, the summits of towers and steeples, are seen first. From a lesser distance the spires of the steeples become entirely visible, then the roofs of buildings themselves; so that the view embraces a great number of objects, beginning with the highest and ending with the lowest, as the distance diminishes. The curvature of the ground is the cause of it.”

Uncle Paul took a pencil and traced on paper the picture that you see here; then he continued:

“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.

“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.”

“How large is the earth?” was the next question from Jules.

“The earth is forty million meters in circumference or 10,000 leagues, for a league measures four kilometers. To encircle a round table, you take hold of hands, three, four, or five of us. To encircle in the same manner the vast bosom of the earth, it would take a chain of people about equal to the whole population of France. A traveler able to walk day after day at the rate of ten leagues a day, which no one could do, would take three years to girdle the globe, supposing it to be all land and no sea. But, where are the hamstrings that could resist three years of such continual fatigue, when a walk of ten leagues generally exhausts our strength and makes it impossible for us to begin again the next morning?”

“The longest walk I ever took was to the pine woods, where we went to look for the nest of the processionary caterpillars, the day of the thunderstorm. How many leagues did we go?”

“About four, two to go and two to come back.”

“Only four leagues! All the same I was played out. At the end I could ............