124 MECHAJflCS. 



the act of cutting, and indicating tlie importance of giving special at- 

 tention to the cutting edge. 



THE MOULD-BOAED. 



A prominent difference between good and bad j)lows 

 results from, the form of the mould-board. To un- 

 derstand the best form, it must be observed that the slice 

 is first cut by the forward edge of the plow, 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 until completely inverted, may be represented by 

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



Fig. 129. 



holding one end, turning over the other, so as to bring 

 that also flat upon the table, as in Fig. 129. 



Now, if the sole object were merely to invert the sod, 

 the mould-board might have just such a shape as to fit 

 the furrow-slice while in the act Fig. 130. 



of turning over, or resemble pre- 

 cisely the twist of this leather strap. 

 All the parts of this screw will be 

 found to fit a straight-edge, if 

 measured across at right angles, as indicated by the dot- 

 ted lines in Fig. 130. 



But there are two objections to this form in practice. 

 The first is that the sod is laid over smoothly and un- 

 broken, and without being at all pulverized. On heavy and 

 hard soils this is a serious fault. The other objection is 

 that the sod is elevated as rapidly at the first movement, 

 when its Aveight is considerable, as just before falling, 

 when its pressure on the mould-board is slight. These diffi- 

 culties are in part removed by giving the mould-board a 



