232. =Lever of the Third Kind.=--In the third kind of lever the power is between the fulcrum and the weight, as seen in Fig. 164. In the first two kinds of lever the power may be less than the weight, but in this the power must always be greater than the weight. This lever has, then, no mechanical advantage, as that expression is commonly used. Applying the same rule here as in the other levers, see what is the result. If the weight, as in Fig. 164, be 9 times as far from the fulcrum as the power is, it will require a power equal to a weight of 648 pounds to sustain a weight of 72 pounds, for 9×72=1×648. [Illustration: Fig. 165.] 233. =Examples.=--When a man puts his foot against the end of a ladder, and raises it by taking hold of one of the rounds, the ladder is a lever of this kind. It is evident that he spends his force upon it at a great mechanical disadvantage, for the power is applied much nearer to the fulcrum than the weight of the ladder, taken as a whole, is. If you push a door to by placing your hand very near the hinges, you do not shut it as easily as when you take hold of it at its edge. In the first case it is a lever of the third kind, and the hand moves through a small space, and therefore must exert a considerable force; while in the latter case the door is a lever of the second kind, and the hand, moving through a greater space, puts forth less force. When we use a pair of tongs we use a pair of levers of the third kind. They are an instrument in which convenience rather than power is needed. We can not grasp any thing very firmly with them because the power is so much nearer to the fulcrum than the weight to be lifted. For this reason a pinch with the ends of the tongs is nothing compared with one in the hinge. The most beautiful example of this lever we have in the moving apparatus of animals. Take, for example, the principal muscle which bends the elbow, as represented in Fig. 165 (p. 182). This comes down from the shoulder in front of the bone of the arm, and is inserted just below the elbow-joint into one of the bones of the forearm. It pulls upon the forearm very near the fulcrum, which is the elbow-joint, and so acts at a great mechanical disadvantage. The object of this arrangement is to secure quickness of movement, which is here, as in almost all muscular motions, of more importance than great strength. When great weights are lifted the fact that the muscles act at such mechanical disadvantage makes the exhibition of power wonderful. [Illustration: Fig. 166.] [Illustration: Fig. 167.]