12 FARM MACHINERY 



ceeds the amount received from it. Some work is used 

 in overcoming the friction of the machine. The ratio be- 

 tween the amount of work received from a machine and 

 the amount put into it is called the efficiency of the 

 machine. 



22. Simple machines are the elements to which all ma- 

 chinery may be reduced. A machine like a harvester, 

 with systems of sprockets, gears, and cranks, consists 

 only of modifications of the elements of machines. These 

 elements are six in number and are called (i) the lever, 

 (2) the wheel and axle, (3) the inclined plane, (4) the 

 screw, (5) the wedge, and (6) the pulley. These six may 

 be conceived to be reduced to only two the lever and 

 the inclined plane. 



23. The law of mechanics holds that the power multi- 

 plied by the distance through which it moves is equal to 

 the weight multiplied by the distance through which it 

 moves. Thus, a power of I pound moving 10 feet equals 

 10 pounds moving I foot. This is true in theory, but in 

 practice a certain amount must be added to overcome 

 friction. 



24. The lever, the simplest of all machines, is a bar 

 or rigid arm turning about a pivot called the fulcrum. 

 The object to be moved is commonly designated as the 

 weight, and the arm on which it is placed is called the 

 weight arm. The force used is designated as the power, 

 and the arm on which it acts is called the power arm. 

 Levers are divided into three classes ; for an explanation 

 of the classes refer to any text on physics.* The law of 

 mechanics may be applied to all levers in this manner. 

 The power multiplied by the power arm equals the weight 

 multiplied by the weight arm. 



*"General Physics." By C. S. Hastings and F. E. Beach and 

 others. 



