Is the see-saw, as shown in Fig. 43, an example of the lever? Yes; the see-saw is an example of the lever. [Illustration: Fig. 43.] What are the parts of the plank on each side of the fence-rail called? They are called the arms of the lever. What is the fulcrum in the see-saw? The fence-rail on which it rests, is the fulcrum. What is the power in the see-saw? The power is the boy on one end of the lever. What is the weight in the see-saw? The weight is the boy on the other end of the lever. How do children play at see-saw? By pushing against the earth with their feet, the arms of the lever are alternately forced to rise and fall in the air, thus making a see-saw. Where must the fulcrum be when the two boys are of the same weight? The fulcrum must be under the middle of the lever. Where must the fulcrum be when one boy is heavier than the other? The fulcrum must then be nearest to the heavier boy, so that they may balance. Does it require much force to play at see-saw? No; when the see-saw is balanced, a very little force will cause it to move up and down. Of what use is a pair of scales? A pair of scales is very useful in weighing different substances. [Illustration: Fig. 44.] Are the scales, as shown in Fig. 44, an example of the lever? The scales are an example of the lever. Where is the fulcrum of the scales? The fulcrum is the point on which the lever rests. What is the weight? The weight is the lead put in one scale. What is the power? The power is the substance put in the other scale, so as to balance the weight. When the lead put in one scale weighs exactly a pound, how much sugar must be put in the other scale to balance the lead? Exactly one pound of sugar must be put in to balance the lead. For what is a balance mostly used? A balance is mostly used for weighing heavy bodies. Is the balance a lever? Yes; the balance is a lever. Which is the fulcrum in the balance? The fulcrum is the hook that holds the balance up. [Illustration: Fig. 45.] What is the weight? The weight is the large body hanging from the short arm of the lever. What is the power? The power is the smaller body hanging from the long arm of the lever. In Fig. 45, the distance from the fulcrum to the power is ten times the distance from the fulcrum to the weight; now, if the small body weighs one pound, how much must the large body weigh, so as to balance it? The large body must weigh ten pounds. When the small body weighs ten pounds, how much must the large body weigh, so as to balance it? The large body must weigh ten times as much, or one hundred pounds. If the smaller body be moved half-way to the fulcrum, how much weight will then balance it? It will then take only half the weight, or fifty pounds to balance it. What other familiar example of this kind of lever may be mentioned? The pump-handle is a lever of this kind. [Illustration: Fig. 46.] What is the power?