An example of lour proportionals may also be found in the relation existing between the arms of a lever and the weights suspended at their ends. A familiar example of a lever is seen in the common steelyard used by merchants in weighing goods. This is a bar, A B, of steel, arranged as in Fig. 272, with hooks and links, and a suspended platform to carry R, the article to be weighed; and with a weight P, suspended by a link at B, from the bar A B, along which the weight P is movable.

The entire load is sustained by links attached to the fulcrum, or point of suspension C. The apparatus is in equilibrium without R and P. In weighing any article, R, the weight P is moved along the bar B C until the weight just balances the load, or until the bar A B will remain in a horizontal position. If the weight P be too far from the fulcrum C the end of the bar B will fall, but if it be too near it will rise.