Ill STORY OF THE PlOW. 



51 



They are the plow-stufF and hummer nut-kc}', represented in Fig. 

 50 and Fiir. 51. 



J^/(/. 50. 



Comparing the sections of this plow, 

 Plate III, figs. 117 and 118, it will be seen 

 I^ig. 6/. that the lines are sharply convex, which 



"will increase its pulverizing power, as will be shown hereafter, and 

 it will also be seen that the zero line o o coincides for half its length 

 with the face of the furrow slice in its vertical position, and there- 

 fore has less tendency to rub the angle at y, than Small's plow. 

 It is two inches narrower from the beam to the rear upper angle 

 of the mould-board; the left handle stands nine inches farther to 

 the left. The edge of the feather rises from the point to the rear 

 angle of the feather. The Mid Lothian Plow partakes of the 

 characteristics of the Lanarkshire and East Lothian phjws. The 

 curves of its mould-board, as seen in Plate III, figs. 113 and 114, 

 show a tendency to convexity in the extreme lower part, while 

 the greater portion of the upper part are straight, like Small's. 

 The zero line makes a smaller angle with the vertical furrow- 

 slice, and has therefore less tendency to disturb it. Its share 

 rises from the point to the rear corner of the feather, like that of 

 the Lanarkshire, and it wall of course leave the bottom of the 

 furrow as represented in Fig. 98. 



Mr. Stephens has invented a mathematical method of forming 

 a mould-boaid, which, though substantially on the plan of Mr. 

 Jefferson's, has some modifications which are decided improve- 

 ments. Instead of using a straight diagonal, like Mr. Jefferson, 

 he uses the arc of a circle, and, by lateral additions to his block, 

 he is enabled to make such an overhang to the wing of the mould- 

 board as to make it fall into its place with certainty, and, at the 

 same time, to give an easy and gradual entrance to the breast of 

 the plow^ We give his method in his own words: 



"From a careful study of the foregoing diagrams (Plates III 

 and IV), and from a comparison of numerous implements and 

 their practical effects, together with a consideration of the dynam- 

 ical principles on wdiich the plow operates, I have been led to 

 adopt a theoretical form of mould-board, which seems to fulfill 

 all the conditions rcfjuired in the investigation, and which is capa- 

 ble, ])y very simple modifications, of adaptation to the circuni- 



