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It has been objected against our theory that the weight of the earth is known and that it is much heavier than would be the case if it were hollow as we have claimed. At first glance that might sound like a plausible objection, but a moment's thought will dispel it. After all, weight is a purely relative matter. The same object weighs more at the polar orifice than it does at the equator because the pull of gravity upon it differs. Weight, the reader will remember is due to the attraction of the force of gravity and it varies according to the distance there may be between the object and the center of the force of gravity which is pulling on it. And as the force with which the earth attracts objects is the original unit upon which the whole idea of weight is based, it follows that we cannot use that unit to measure itself--we have to take it for granted. What scientists actually did when they "weighed the earth" was to assume, first of all that it was a more or less homogenous sphere. They then observed the force with which it exerted attractions on various bodies. But the amount of attraction which the earth exerts, say, on a pendulum, bringing it to rest in a certain time, is a fact which is not altered by showing that
the earth is hollow. All we can say is that a lesser mass of earth than we had supposed is after all able to affect the pendulum. Or we can put it this way. We had supposed in the past that it took the gravitational force of a solid or semi-solid sphere the size of the earth to hold us on its surface against the centrifugal force which would tend to throw us off the surface of the earth owing to its fast rate of revolution. Nov some people argue as if the truth of our theory would reduce the gravity of the earth so that that centrifugal force would throw us all off the earth. But that is not so. All we have to do is to admit that the pulling force of the earth is greater than we had thought, for now it appears that a globe which is not solid but hollow--however, having a very thick shell--is sufficient to hold us by gravity against the push of the centrifugal force.
And one correspondent has even asked why the inhabitants, human or animal, of the interior would not be pulled into the central sun by the force of its gravitational pull. The answer is of course, easy. In the first place the central sun is a very small body and its gravitational pull is therefore counteracted by the gravitational pull of the shell of the earth. And again, in the interior of the earth the centrifugal force tends to hold things down to the ground, because it is always directed from the center out at
right angles, and its direction is therefore the same whether one is on the outside or the inside of the shell of the earth. When we are on the outside we are held down against its pull by the combined gravitational pull of the whole earth--shell and interior sun. If we were on the inside of the earth we should be held by the balance of the forces, the interior sun exerting a force of gravity on us which, if unopposed would pull us into itself, the gravitational pull of the earth's shell pulling us the other way, and the centrifugal force of the whirling earth adding to that pull by pressing us against the inner surface. For that reason if a man on the interior of the earth dug a hole--which would point the opposite way to a hole which we would dig on our surface, he would be likely to fall up, if he were careless, as we are to fall down a hole if we do not take precautions.
Next: Chapter XXII. How Our Theory Has Been Received