Line of Draught of Plows. 171 



trees, hooks, and all that intervenes between the clevis and the 

 horse's shoulders. Let that distance be set off in the direction 

 bf, and, the average height of the horse's shoulders being four 

 feet two inches, let the point/ be fixed at that height, above the 

 })ase line, d e. Draw the line / c, which is the direction of the 

 line of draught, acting upon the centre of resistance, c; and if 

 the plow is in proper temper it will coincide with the clevis; e cf 

 being the angle of draught and equal to 20 degrees. It will be 

 easily perceived that, with the same horses and the same length 

 of yoke, the angle, e c f, is invariable; and if the plow has a 

 tendency to dip at the point of the share under this arrangement, 

 it indicates that the draught-bolt, b, is too high in the bridle. 

 Shifting the bolt one or two holes downwards will bring the plow 

 to swim evenly upon its sole. On the other hand, if the plow 

 has a tendency to raise at the point of the share, the indication 

 from this is that the evener is too low in the clevis, and the recti- 

 fication must be made by raising it one or two holes in the clevis. 



Suppose, again, that a pair of taller horses were yoked in the 

 plow, the traces, depth of furrow, and soil, and, by consequence, 

 the point of resistance, c, remaining the same, we should then 

 have the point / raised to /'. By drawing the line /' c we have 

 e cf as the angle of draught, which will now be 22 degrees; and 

 in this new arrangement the draught-bolt is found to be below the 

 line of draught,/"' c; and if the traces were applied at b, in the 

 direction of/' 6, the plow would have a tendency to rise at the 

 point of the share by the action of that law of forces which 

 oblige's the line of draught to coincide with the line which passes 

 through the centre of resistance; hence the draught-bolt, b, Avould 

 be found to rise to b\ which would raise the point of the share 

 out of its proper direction. To rectify this, then, the draught- 

 bolt must be raised in the bridle by a space equal to b b\ causing 

 it to coincide with the true line of draught, which would again 

 bring the plow to swim evenly on its sole. 



Regarding the relative forces required to overcome the resist- 

 ance of the plow when drawn at different angles of draught, we 

 have first to consider the nature of the form of those parts 

 through which the motive force is brought to bear upon the plow. 

 It has been shown that the tendency of the motive force acts in a 

 direct line from the shoulder of the animal of draught to the 

 centre of resistance; and, referring again to Fig. 81, were it not 

 for considerations of convenience, a straight bar or beam lying in 



