Of what use is the inclined plane? It aids very much in raising weights. Are all planes inclined alike? No; some planes are much steeper than others. [Illustration: Fig. 57.] In Fig. 57, where is the power? The power is at P. Where is the weight? The weight is at W. Upon what does the power and the weight depend? They depend upon the height and the length of the inclined plane. When an inclined plane is ten feet long and only one foot high, how much will one pound at P balance? One pound at P will balance ten pounds at W. When an inclined plane ten feet long is two feet high, how much will one pound at P balance? One pound at P will then balance only five pounds at W; because the plane is five times as long as it is high. Where else may the power be exerted? The power may be exerted behind the weight, as in Fig. 58. [Illustration: Fig. 58.] Will any more power be needed behind the weight than before it? No; the power required is the same in both cases. Would it require as much power to roll barrels into a wagon as to lift them in? No; they can be rolled into a wagon much more easily than lifted in. Can they be taken out of the wagon any more easily on an inclined plane than without one? Yes; they can be taken out more easily on an inclined plane. Why are drays, used in cities, made in the form of the inclined plane? So that barrels and hogsheads may the more easily be rolled on them. Are our stairs in the form of the inclined plane? Yes; they are inclined planes with steps cut in them to make the ascent easy. Why are house-roofs made in the form of the inclined plane? So that the water falling on them may run off readily. What familiar example of the inclined plane may be mentioned? The hills, all over the country, are so many inclined planes. Of what use are the hills? The hills carry off the waste water from the earth, so that it is kept dry enough for plants to grow. What other example of the inclined plane may be mentioned? The beds of the creeks and rivers are so many inclined planes down which their waters flow. Wedge. What is a wedge? A _wedge_ is a double inclined plane, as is represented in Fig. 59. [Illustration: Fig. 59.] Of what use is the wedge? The wedge is used in splitting logs of wood and blocks of stone. For what other purposes is the wedge used? The wedge is used to fasten handles in hammers, hatchets, axes, and hoes. How is power applied to the wedge? Power is applied to the wedge by means of sudden blows from a hammer. What prevents the wedge from flying back after the blow? The friction between the wedge and the other body, keeps it from flying back. What instruments are made in the form of a wedge? Knife-blades, hatchets, axes, razors, and cutting instruments of nearly all kinds, are in the form of the wedge. Screw. What is a screw?