130 MECHANICS. 



CHAPTER VII. 



CONSTRUCTION AND USE OF FARM IMPLEMENTS AND MA- 

 CHINES. 



SECTION I. 



The application of meclianical principles in the 

 structure of the simpler parts of implements and ma- 

 chines has been treated of in a former part of this 

 work. It remains to examine more particularly those 

 machines chiefly important to the farmer, and to show 

 the application of these principles in their use and op- 

 eration. 



PLOWS AND PLOWING. 



One great difference between good and had plows is 

 in the form of the mould-hoard. To understand the 

 best form, it must be observed that the slice is first cut 

 by the forward edge, and then one side is gradually 

 raised until it is turned completely over, or bottom side 

 up. To do this, the mould-board must combine the 

 two properties of the wedge and the screw. 



The position of the furrow-slice, from the time it is 

 first cut till completely inverted, may be represented 

 by placing a leather strap flat upon a table, and then, 

 while holding one end, turning over the other, so as 

 to bring that also flat upon the table, as in Fi^. 103. 

 Now the mould-board should have just such a shape 

 as will fit the furrow-slice while in the act of turning 



