240. =Movable Pulley.=--You have a representation of a movable pulley in Fig. 176 (p. 186). It is evident here that the force of the weight is equally divided between the cords, A B, so that the cord B, extending over the fixed pulley, needs to have a weight, P, but half of the weight W to balance it. A movable pulley is sometimes called a "runner," and a fixed pulley is commonly connected with it, in order to give the desired direction to the force. Many pulleys are often connected together in various ways, as seen in Fig. 177. It is easy to estimate in such cases the relation of the power to the weight on the principles developed in relation to the lever. If, for example, in the system of pulleys on the left, the weight be 36 pounds, the two cords of the first pulley will each sustain a weight of 18 pounds, those of the next pulley each 9 pounds, and those of the next each 4½ pounds. The weight W then will be balanced by the weight P if it weigh 4½ pounds. [Illustration: Fig. 177.] [Illustration: Fig. 178.] 241. =Inclined Plane.=--The fourth mechanical power is the Inclined Plane. This being a very simple contrivance is much used, especially when heavy bodies are to be raised only a small height, as in getting large boxes and hogsheads into stores. The mechanical advantage of the inclined plane may be illustrated on Fig. 178. The line A _c_ represents an inclined plane. If a weight be drawn up this plane it is raised only the height B _c_. A smaller power is requisite to draw the weight up the plane than to raise it perpendicularly; and the power necessary will be the less the longer the plane. A power which would balance a weight on an inclined plane would be to the weight as the height of the plane to its length. Thus if A _c_ be twice as long as B _c_, a weight of four pounds on the plane may be balanced by a two-pound weight suspended by a cord passing from the weight over the summit of the plane. A flight of stairs is an inclined plane in regard to the principle on which the ascent is effected, the projections in it being for the purpose of affording a sure footing in making the ascent or the descent. So likewise hogsheads are let down the steps of a cellar-way by ropes, and it makes no difference in the principle of the operation whether the steps have or have not planks laid along them. It is supposed that the immense stones in the pyramids and other massive Egyptian structures were put into their position by means of the inclined plane. Roads, when they are not level, are inclined planes, and the steeper the inclination the more power is required to draw a load up the road. Great mistakes were formerly made in carrying roads too frequently over high hills. Besides failing to take advantage of the principles of the inclined plane, in many cases the horse in going over a hill passes over quite as much space as he would if the road were made to go round the base of the hill, and sometimes even more. If the