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The Story Book of Science by  Jean Henri Fabre


 

 

CHAPTER LII

THE EARTH

[238]

"T
HE 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."


[Illustration]

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 [239] any distance the whole of a tower would be visible. Afar off, no doubt, it would he 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]

"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 per- [240] ceiving 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]

"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?"

[241] "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 hardly put one foot before the other. It would take me, then, from seven to eight years to go round the world, walking every day as far as my strength would let me."

"Your calculation is right."

"The earth then is a very large ball?"

"Yes, my friend, very large. Another example will help you to understand it. Let us represent the terrestrial globe by a ball of greater diameter than a man's height—by a ball two meters in diameter; then, in correct proportion, represent in relief on its surface some of the principal mountains. The highest mountain in the world is Gaurisankar, a part of the Himalaya chain, in central Asia. Its peaks rise to a height of 8840 meters. Rarely are the clouds high enough to crown its crest, and its base covers the extent of an empire. Alas! what does man become, materially, in face of such a prodigious colossus! Well, let us raise the giant on our large ball representing the earth; do you know what will be needed to represent it? A tiny little grain of sand which would be lost between your fingers, a grain of sand that would stand out in relief only a millimeter and a third! The gigantic mountain that overwhelmed us with its immensity is nothing when compared with the earth. The highest mountain in [242] Europe, Mont Blanc, whose height is 4810 meters, would be represented by a grain of sand half as large as the other."

"When you told us of the roundness of the earth," put in Claire, "I thought of the enormous mountains and deep valleys, and asked myself how, with all these great irregularities, the earth could nevertheless be round. I see now that these irregularities are a mere nothing in comparison with the immensity of the terrestrial ball."

"An orange is round in spite of the wrinkles in its skin. It is the same with the earth: it is round in spite of the irregularities of its surface; it is an enormous ball sprinkled with grains of dust and sand proportioned to its size, and these are mountains."

"What a big ball!" exclaimed Emile.

"To measure the circumference of the earth is not an easy thing, you may be sure; and yet they have done more than that: they have weighed the immense ball as if it were possible to put it in a scale-pan with kilograms for counterweights. Science, my dear children, has resources demonstrating in all its grandeur the power of the human mind. The immense ball has been weighed. How it was done cannot be explained to you to-day. No scales were used, but it was accomplished by the power of thought with which God has endowed us, to solve, to His glory, the sublime enigma of the universe; by the force of reason, for which the burden of the earth is not too heavy. This burden is expressed by the fig- [243] ure 6 followed by twenty-one zeros, or by 6 sextillions of kilograms."

"That number means nothing to me; it is too large," Jules declared.

"That is the trouble with all large numbers. Let us get around the difficulty. Suppose the earth placed on a car and drawn on a surface like that of our roads. For such a load, what should the team be? Let us put in front a million horses; and in front of that row a second million; then a third row, still of a million; a hundredth, finally a thousandth. We shall thus have a team of a thousand millions of horses, more than could be fed in all the pastures of the world. And now start; apply the whip. Nothing would move, my children; the power would be insufficient. To start the colossal mass, it would need the united efforts of a hundred millions of such teams!"

"I don't grasp it any better," said Jules.

"Nor I, it is so enormous," assented his uncle.

"Yes, enormous, Uncle."

"So that the mind gets lost in it," said Claire.

"That is what I wanted to make you acknowledge," concluded Uncle Paul.


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