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 Thus, we know to-day that, by the laws of a universal magnetism whose cause is still unknown, two bodies (no obstacle intervening) tend to unite by an accelerated impelling force which we call gravitation. It is gravitation which causes unsupported bodies to fall to the ground, which gives them weight, and which fastens us to the earth on which we live. Ignorance of this cause was the sole obstacle which prevented the ancients from believing in the antipodes. “Can you not see,” said St. Augustine after Lactantius, “that, if there were men under our feet, their heads would point downward, and that they would fall into the sky?” The bishop of Hippo, who thought the earth flat because it appeared so to the eye, supposed in consequence that, if we should connect by straight lines the zenith with the nadir in different places, these lines would be parallel with each other; and in the direction of these lines he traced every movement from above to below. Thence he naturally concluded that the stars were rolling torches set in the vault of the sky; that, if left to themselves, they would fall to the earth in a shower of fire; that the earth was one vast plain, forming the lower portion of the world, &c. If he had been asked by what the world itself was sustained, he would have answered that he did not know, but that to God nothing is impossible. Such were the ideas of St. Augustine in regard to space and movement, ideas fixed within him by a prejudice derived from an appearance, and which had become with him a general and categorical rule of judgment. Of the reason why bodies fall his mind knew nothing; he could only say that a body falls because it falls.

With us the idea of a fall is more complex: to the general ideas of space and movement which it implies, we add that of attraction or direction towards a centre, which gives us the higher idea of cause. But if physics has fully corrected our judgment