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question. "Why did not man discover by looking around him, that he was living on the surface of what is, practically speaking, an immense sphere (to be exact spheroid)? And why did man for centuries think that the earth was flat? Simply because the sphere was so large that he could not see the curvature but thought it was a flat surface, and that he should be able to move all over the surface of it appeared so natural that, when scientists first told him it was a sphere he began to wonder why he did not fall off, or at least, if he lived in the Northern Hemisphere, he wondered why the Australians did not fall off - for he had no conception of the law of gravity. "Now, in the case of the polar explorers the same thing is true. They sail up to the outer edge of the immense polar opening, but that opening is so vast, considering that the crust of the earth over which it curves is eight hundred miles thick, that the downward curvature of its edge is not perceptible to them, and its diameter is so great - about 1,400 miles - that its other side is not visible to them. So, if an explorer went far enough he could sail right over that edge, down over the seas of the inner world and out through the Antarctic orifice, and all that would show him what he had done would be that as soon as he got inside he would see a smaller sun than he was accustomed to - only to him it might look larger owing to its closeness - and he would not be able to take any observations by the stars because there would be neither stars nor even a night in which to see them. "But, says the reader, would not the force of gravity pull the explorer who got inside the orifice away from the surface into the central sun; for does not gravity pull everything to the center of the earth? "The answer to this is, that in gravitational pull it is not the geometrical position that counts. Center, in the geometrical sense of the word, does not apply. It is the mass that attracts. And if the great mass of the earth is in its thick shell, it is the mass of that shell that will attract, and not a mere geometrical point which is not in the shell at all, but 2900 miles away from it, as is the approximate distance between the central sun and the inner surface of the earth. As a matter of fact it is the equal distribution of the force of gravity all through the shell that keeps the sun suspended in the spot which is equidistant from every part of the shell. When we are on the outside of the shell it is the mass of the shell that attracts us to its surface. When we go over to the inside of the shell that same force will still keep our feet solidly planted on the inner surface. "We shall see all that when we explore the Arctic in earnest, as we shall easily be able to do with the aid of airships. And when once we have seen it we shall wonder why it was that for so long we were blind to evidence which, as is shown in this book, has been before men's eyes for practically a whole century and over." Twenty-seven years after Gardner wrote this, Admiral Byrd did exactly what he hoped would be done. He flew by airplane into the north polar opening for 1700