168 Report on Trials of Plows. 



ments, that in average soils and conditions that if the land side 

 was made with seven degrees of obliquity to the perpendicular, 

 or eighty-three degrees to the plane of the sole, the axis of the 

 beam would lie in the plane passing through the centre of resist- 

 ance. When the axis of the beam lies in this direction the plow 

 moves forward without any tendency to deviate from the line of 

 motion either to the right hand or the left. I'his arrangement is 

 illustrated in Fig, 80, in which a vertical section of the plow is 

 represented. The dotted line, C D, being the per- 

 ^ pendicular to the plane of the sole, S; the angle 



IT C D B is 7 deg.; L is the standard, and the lower 



I / part of it is the land side; M, section of mould- 



1 board; B, section of the beam. 



h\ There is also a horizontal as well as a perpen- 



1 \ dicular plane of resistance. When the draught 



I X'v^'^ coincides with this plane there is no tendency in 

 N\^^ the plow to go deeper or shallower; if the draught 



^"s -^^ is above it the point of the plow is drawn down- 



I^iff. 80. ward with a force proportioned to the height of the 

 point of draught above this plane; if placed below it, the point 

 of the plow will in like manner rise out of the ground, and these 

 tendencies can only be counteracted by the plowman pressing on 

 the handles of the plow in the former case, or lifting them b}^ 

 main force in the latter case. 



Again, we have a third plane of resistance, which is vertical and 

 transverse. The plow enters the unbroken earth by a single 

 point, the transverse sections increasing gradually in area until 

 they attain to their maximum dimensions; but as the point enters 

 and breaks the solid earth, the force required does not increase 

 nearly as fast as the sectional area increases, hence the position 

 of this plane raises from one to five inches behind the point of 

 the plow, according to the consistency of the soil and the shape 

 of the plow. 



The point of resistance of the plow is therefore situated at the 

 intersection of these three planes. If the line of draught passes 

 through this point, the plow will, in the language of the plow- 

 man, "swim free;" and it is therefore a point of great practical 

 importance to ascertain the locality of the point of resistance, 

 and to have a ready means of adjustment by which the line of 

 draught may be made to pass through it. 



