234. =Compound Levers.=--When several levers are connected together we call the whole apparatus a compound lever. Let each of the levers in Fig. 166 be 3 inches long, the long arms being 2 inches, and the short ones 1 inch. One pound at A will, according to the rule, balance 2 at B, and 2 at B will balance 4 at C, and 4 at C will balance 8 at D. Therefore 1 pound at A will balance 8 pounds at D. And you see that an equilibrium is effected when the power is to the weight as the product of all the short arms is to the product of all the long arms. The compound lever is used in weighing heavy loads--as hay, coal, etc. You have a representation of the arrangement in Fig. 167. The load, W, stands on a platform, A B, which rests upon two levers, E D and E C. The long arms of these levers are E G and E F, and the short arms are G D and F C. The ends of the long arms press upon the fulcrum of the lever, H I. The pressure is now transmitted from the end of the long arm by the rod, I K, to a small lever, K L, where a small weight or power, P, balances the weight of the heavy load, W. The two objects secured by this arrangement are accuracy and the occupation of a small space. [Illustration: Fig. 168] 235. =Wheel and Axle.=--The mechanical power next in simplicity to the lever is the Wheel and Axle. The most familiar applications of this power we see in drawing water and in raising heavy articles in stores. The principle of this power is the same as that of the lever, as may be shown in Fig. 168, which represents a section of the wheel and axle. The power, P, hangs by a cord which goes round the wheel, and the weight, W, by a cord around the axle. We may consider the power as pulling on a lever represented by A B, the long arm of which is A C, and the short arm B C. You see that the wheel and axle, then, may be viewed as a constant succession of levers, and it is therefore sometimes called the perpetual lever. And the same rule of equilibrium applies here as in the simple lever. [Illustration: Fig. 169.] 236. =Windlass.=--In the common windlass the power is applied to a winch or crank, D C B, Fig. 169, instead of a wheel. In estimating the power of this arrangement B C must be considered the long arm of the lever, and half of the diameter of the axle, B A, as its short arm. [Illustration: Fig. 170.] [Illustration: Fig. 171.]