Just as the power of the inclined plane is proportional to the height and length of the plane, so is the power or force applied to the wedge proportional to its height and length. In this latter case, however, the length is the horizontal length or base ac (Fig. 28) and not the sloping face bg. By the principles of similar triangles, we can easily prove that when a force acts in a direction parallel to the base of a wedge, the wedge will lift a weight as many times greater than the force, as the base or length of the wedge is times as long as the vertical face or thickness This may be stated as a rule as follows:

FIG. 27. - Wedge.

To find the force required to lift a certain weight multiply the weight by the greatest thickness of the wedge and divide by the horizontal length.

On the inclined plane previously described the force acts in a direction parallel to the plane; that is, the cord attached to the ball pulls up the plane. In Fig. 28 a weight W is being lifted by driving two single wedges. To raise the weight we must strike or push on the face of either one of the wedges, as at F on the face ab. This force acting parallel to the base ac of the wedge causes a pressure P in a direction at right angles to the base.