MACHINES 189 



155. The Lever and Its Adaptations. The lever is a rigid 

 bar, straight or curved, which turns on an axis called a, fulcrum. 

 The divisions of the lever are called arms. If the arms of a 

 lever are equal, a force applied to one arm will exactly balance 

 an equal weight at the other. The balances in the chemical 

 laboratory are of this kind. If the arms are unequal, how- 

 ever, it will be necessary to place a larger weight on the short 

 arm to balance a given weight on the long arm. The old- 

 fashioned steelyards and all platform scales are of this type. 

 A general law of mechanics is that the power multiplied by 

 its distance from the fulcrum is equal to the weight mul- 

 tiplied by its distance from the same point. Thus, 10 pounds 

 on the arm of a lever 5 inches long will just balance a weight 

 of 5 pounds on an arm 10 inches long. There are three classes 

 of levers. In levers of the first class, the fulcrum is between 

 the power and the weight, as in the balanced scales of the 

 merchant. In levers of the second class, the weight is between 

 the power and the fulcrum, as when we move an object by 

 lifting on a bar the other end of which rests on the ground 

 beneath it. In levers of the third class, the weight is at one 

 end, the fulcrum at the other, and the power applied in the 

 middle, as in the treadle of most foot-power machines. The 

 wheel and axle may be considered as an adaptation of the lever 

 what might be called a lever arm revolving about a fulcrum. 

 The mechanical advantage in this type of machine is found in 

 the ratio that the circumference, the diameter, or the radius 

 of the wheel bears to the circumference, diameter, or radius 

 of the axle. Thus, if the diameter of a wheel is twice that of 

 the axle, the force applied to the wheel will lift twice as much 

 as if .applied to the axle. The pulley may be considered 

 another adaptation of the lever, the shaft on which the 

 pulley is fixed being the fulcrum. The rope running over the 

 pulley acts as a balance lever, since any force pulling down on 

 one end will exactly balance an equal weight on the other. If 



