History of the Plow. 47 



of the plow; ill is a board about three feet square, provided with 

 broad feet behind, which can be fastened securely to the table, 

 10 X m ?/, by means of thunil) screws. The two parallelograms, 

 ah cd and cdcf, are connected by pins, as shown in the figuio, 

 so that all the joints can work freely; the lower end of the bar, 

 ef, has a stout Avire, /, inserted into it for a tracer. A sheet of 

 paper, i' h', is tacked to the board, i /t, at the four corners, and a 

 tracing pencil is inserted in the socket, (/. The face of the board 

 is now set on the line q r, the face of the square, v r, is set on the 

 line 2, the face,^? u, lying on the table parallel to in I. The tracer, 

 j, is then brought up to the top of the mould-board and is carried 

 along to the bottom, touching the surface at every point in its 

 passage, being directed by the face, v r, the line of motion being 

 recorded accurately by the pencil, </, on the paper, i' //. The face 

 of the board is now set on the line o jh and the face of the square, 

 V r, is placed on the line 2, and the line 2 is traced on the paper 

 as before. In this way the 1)oard is successively set backward on 

 all the lines ruled on the table, takino- care that its lower riorht- 

 hand corner is kept on the line m I until all the lines of the mould- 

 board are transferred to the paper. A little attention to the action 

 of the jointed parallelograms, abed and cdef, will show that 

 the path of the pencil, g, must be exactly parallel to that of the 

 tracer, j. If the point of the tracer passes down a vertical line, 

 the pencil will trace a vertical line on the paper. If it passes 

 along a horizontal line, or an angle of 45 deg., or over a convex 

 or concave line, precisely similar lines will be traced on the paper, 

 and it is by this method that the lines of the figures on Plate IV 

 have been traced. It may be, perhaps, proper to say that the 

 bars, ah, c d and ef, are each three feet long, one and one-half 

 inches wide, and three-quarters of an inch thick. The four bars, 

 a c, h d, c e and df, are of similar breadth and thickness, and are 

 all made of hard wood, and are jointed together on brass studs, 

 care being taken that the perforations are all made exactly equi- 

 distant from each other. 



Fig. Ill of Plate III is a geometrical elevation of Small's or 

 the East Lothian plow in a plane parallel to the land side of the 

 mould-board, I d being its base line. The perpendicular lines of 

 division, commencing from the line o o, or zero, and extending 

 right and left, are the lines of section. Those to the right or fore 

 end of the mould-board, marked a a, bb, etc. The curved line, 

 xi/m, represents the \)a(h described on the face of the mould-board 



