Opposed to the downward force is the upward force. If an object does not fall, it must be supported by a force in the upward direction; the hand must exert a force perpendicular to the mass which it carries; the body must hold itself erect in order to bear its own weight. Just so, an architectural member, if it is not to collapse, must raise itself upward. Upward forces are revealed by the vertical lines of a building--the prevailing lines of columns, piers, shafts, pinnacles, towers, spires. We interpret vertical lines as moving upward, partly because the eye moves upward in scanning them, partly because we ourselves move in lines of this general direction in going from the bottom to the top of a building. Even when we are at the top of a building we apprehend its vertical lines as rising rather than as descending, because we ourselves had to rise in order to get there. Converging lines, as of towers and spires, we also interpret in the same way as going to the point of meeting above. Acting in conjunction with the downward force is an outward one. The lower parts of a construction tend to spread out as they give way under the weight of the superincumbent masses; if they are very much broader than the latter, they give the impression of great weight carried. As a result, a horizontal line is introduced, and the longer it is in comparison with the vertical line of height, the heavier the effect. Compare, for example, the impression made by a tall and thin triangular shape, with a low and broad one; and compare also the relative lengths of the horizontal and the vertical lines. The former shape seems simply to rise, while the latter lifts. We seem to observe the working of this outward force, as Lipps has remarked, in the spreading out of the trunks of trees at the base and in the feet of animals; and we feel it in ourselves whenever we spread our limbs apart to brace ourselves to withstand a load. Whenever the outward force is resisted, it gives evidence of the existence of a force operating in the opposed direction--inward. Without this force, the lower parts of a construction would lack all solidity and spread like a molten mass on the ground. This is especially striking where the material, instead of spreading outward and downward, seems to press itself inward and upward. Compare, for example, a shape whose base-line is smaller than the line of its top with one in which the reverse holds true. The former gives the impression of lightness and agility, with a prevailing upward trend, the other an impression of weight and heaviness, with a prevailing trend towards the ground. Obviously, the outward and the inward forces are correlative and complementary: we have already observed that a construction would collapse without the inward; we can now see that it would disappear entirely without the outward. Obviously, also, the inward and upward go together, and the downward and outward.