A simple form of inclined plane is pictured in Figure 17, which shows a weight W being rolled up an incline. The effort required to carry it to the top of the incline depends, of course, upon the steepness of the incline. The drawing shows a rise of 3 feet on a slope 5 feet long, and the weight of the wheel is, say 20 pounds. To find the effort required, the weight is multiplied by the rise (20 × 3 = 60) and divided by the length of the slope (60/5 = 12) and we find that it takes only 12 pounds to roll the 20-pound wheel to the top of the incline. This holds true when the pull is parallel to the inclined face. If the pull is parallel to the base of the incline, as in Figure 18, we must divide by the length of the base instead of the length of the incline (60/4 = 15) and we find that it takes 15 pounds of effort to pull the weight up the incline. If the pull is exerted at an angle both to the base and the inclined face, we have a problem that is slightly more complicated and we need not go into it here because it involves a bit of trigonometry. In all cases, however, it may be noted that the amount of rope that is taken in, in hauling the weight up the incline, bears a definite relation to the amount of effort required to raise the weight. In Figure 17, 5 feet of rope must be pulled in, in order to raise the weight 3 feet, so that ⅗ of 20 or 12 pounds is all that is required to pull up the weight, while in Figure 18, 4 feet of rope is hauled in for a lift of 3 feet, so that ¾ of 20 or 15 pounds is required to pull up the weight. In this respect the inclined plane is exactly like the lever or the pulley, for the _effort_ multiplied by the _distance_ through which it is exerted is always exactly equal to the _weight_ multiplied by the _distance_ through which it moves. Thus in Figure 17, the effort 12 pounds multiplied by the distance 5 = the weight 20 pounds times the distance 3, and in Figure 18, effort 15 x distance 4 = weight 20 x distance 3. Of course, we are ignoring the weight of the rope and the friction which, in actual practice, are important factors to be reckoned with. [Illustration: FIG. 19.--ENDLESS SCREW OR WORM GEAR] So far we have considered a fixed inclined plane, but when the inclined plane is moved between the weight and a fixed base it is known as a wedge, and in this case, too, the effort required to move the wedge multiplied by the distance the wedge moves is equal to the weight multiplied by the distance it is lifted.