But rotation is not the only influence that tends to pull a planet out of shape. The attraction which the earth exerts upon the moon is stronger on the near side and weaker on the far side of our satellite than at its center, and this difference of attraction tends to warp the moon, as is illustrated in Fig. 23 where _1_, _2_, and _3_ represent pieces of iron of equal mass placed in line on a table near a horseshoe magnet, _H_. Each piece of iron is attracted by the magnet and is held back by a weight to which it is fastened by means of a cord running over a pulley, _P_, at the edge of the table. These weights are all to be supposed equally heavy and each of them pulls upon its piece of iron with a force just sufficient to balance the attraction of the magnet for the middle piece, No. _2_. It is clear that under this arrangement No. _2_ will move neither to the right nor to the left, since the forces exerted upon it by the magnet and the weight just balance each other. Upon No. _1_, however, the magnet pulls harder than upon No. _2_, because it is nearer and its pull therefore more than balances the force exerted by the weight, so that No. _1_ will be pulled away from No. _2_ and will stretch the elastic cords, which are represented by the lines joining _1_ and _2_, until their tension, together with the force exerted by the weight, just balances the attraction of the magnet. For No. _3_, the force exerted by the magnet is less than that of the weight, and it will also be pulled away from No. _2_ until its elastic cords are stretched to the proper tension. The net result is that the three blocks which, without the magnet's influence, would be held close together by the elastic cords, are pulled apart by this outside force as far as the resistance of the cords will permit.
[Illustration: FIG. 23.--Tide-raising forces.]
An entirely analogous set of forces produces a similar effect upon the shape of the moon. The elastic cords of Fig. 23 stand for the attraction of gravitation by which all the parts of the moon are bound together. The magnet represents the earth pulling with unequal force upon different parts of the moon. The weights are the inertia of the moon in its orbital motion which, as we have seen in a previous section, upon the whole just balances the earth's attraction and keeps the moon from falling into it. The effect of these forces is to stretch out the moon along a line pointing toward the earth, just as the blocks were stretched out along the line of the magnet, and to make this diameter of the moon slightly but permanently longer than the others.