225. =Scales and Steelyards.=--In the common scale-beam we have a lever, the two arms of which are equal, and therefore equal weights suspended at the ends balance. If they be not exactly equal, a heavier weight will be necessary on the shorter arm than on the longer. The inequality will injure the buyer if the prop be too near the scale in which the weights are placed, and the seller if it be too near that which holds the article to be sold. Any difference can be easily detected by changing the places of the article and the weights. Whenever cheating is practiced by the "false balance," it is of course done in a small way, to avoid any observation by the eye of the inequality of the two arms of the scale-beam, and the weight of the scale hanging from the shorter arm is made a little greater than that of the other, so that they may balance. Scales may be rendered very accurate by making the fulcrum or pivot of hardened steel, and of a wedge shape, with a sharp edge, in order to avoid friction as much as possible. The steelyard differs from the scale-beam in having the arms of different lengths. The principles on which this instrument is constructed were developed in what I said of Fig. 158. When either with the balance or the steelyard two weights balance each other the centre of the weights and the apparatus taken together is just over the fulcrum, § 195. We see in this the reason that it is necessary to have the prop near the large weight when we wish to balance it by a small one. 226. =Other Examples.=--Scissors are double levers of the first kind. The fulcrum is the rivet, the weight or the resistance to be overcome is the article to be cut, and the power is applied to the long arms of the levers by the fingers. With large shears hard substances can be cut. Even plates of iron are cut like paper by shears which are worked by a steam-engine.--Pincers are double levers. The hinge, or rivet, is the fulcrum.--The common hammer, as used in drawing nails, is a good example of the power of this kind of lever. Though crooked, it acts in the same way with a straight lever. The fulcrum is the point on the board where the hammer rests, and this is between the resistance to be moved, the nail, and the power, that is, the hand which grasps the handle. [Illustration: Fig. 159.]