FARM MACHINERY 187 



the fundamental devices for modifying forces and motions. 

 They are six in number, and are the lever, the wheel and axle, 

 the inclined plane, the screw, +he wedge, and the pulley. 



Essentials of a Machine. Any machine to be satisfac- 

 tory must fulfil at least four requirements. First, it must do 

 the work required of it satisfactorily; for instance, a har- 

 vester must cut the standing grain and bind it into bundles 

 with never-failing accuracy. Second, the machine must do 

 its work efficiently; that is, it must require little power to 

 drive it, as in the case of horse-drawn machines, where the 

 draft must be low. Third, the parts of the machine must be 

 strong enough to resist breakage. Fourth, the machine 

 must be so designed as to be durable, or able to resist wear ; 

 such parts that are subject to wear should be capable of 

 adjustment or replacement. 



The first two of these requirements demand proper con- 

 struction on the part of the machine and skillful adjustment 

 and management on the part of the operator. The number 

 of farm machines now manufactured is very large, and in 

 most cases there are several types and sizes of a machine for 

 each kind of work. Each machine will do its best work and 

 render the best service when used under the conditions for 

 which it is made to work. The part of this text devoted to 

 farm machinery is planned, in the main, to give instruc- 

 tion in the selection, adjustment, and operation of the various 

 farm machines required in general farm practice. In addi- 

 tion, there will be a discussion of the principles involved in 

 the strength and durability of a machine. 



Friction. As a machine operates, there must be at cer- 

 tain points a sliding of one surface over another. It matters 

 not how carefully the "surfaces may be prepared there is 

 always some resistance to the sliding, which resistance is 

 known as friction. The magnitude of this resistance in 



