PLOWS AND PLOWING. 



131 



over, else it will wear unequally, become clogged with 

 soil where the earth rubs slightly, and require greater 

 strength in the team. By examining, it will be found 

 that, although the strap {Fig. 103) twists like a screw. 



Fig. 103. 



yet all parts wiU be straight if measured across at right 

 angles, as shown by the dotted lines. Therefore, by 

 applying this principle, the farmer can judge of one im- 

 portant quality in selecting a plow. If, for example, 

 he finds that a straight-edged stick will be flat upon 

 Fig. 104. the face at right angles to the 



line of motion, as shown by the 

 dotted lines in Fig. 104, the 

 mould-board will be so far 

 right ; but if the straight edge 

 must be placed in other posi- 

 tions, as in Fig. 105, it is de- 

 fective in form. A mould- 

 board may be much modified, 

 with this principle pre^^erved in every instance ; that is, 

 it may be short, so as to raise the earth abruptly, or it 

 may be long, so as to raise it gradually; it may be 

 adapted to a deep furrow, lifting the furrow-slice to a 

 considerable height, or to a shallow one, tin-owing it 

 quickly over. These modifications are required for dif- 

 ferent soils and for different purposes. When, for ex- 

 ample, it is desired to break up the slice and pulverize 

 a heavy soil, the twist must be short and abrupt ; when 

 a sod in hght soil is to be inverted smoothly, the mould- 

 board must be longer, and the twist more gradual. In 



