Vol. XIII. Xo. 324. 



THE AGEICULTUE.\L XEWS. 



315 



STUDENTS' CORNER. 



DYNAMICS OF A PLOUGH.-^ 



I'AKT 11. 



The ((uestion to Le considered is the be.st position and 

 direction to ii]jply the force to overronie these three several 

 re.si.stances. To do so it is necessary to reduce them to one 

 single resistance, and to fi.\ approximately the position and 

 direction of this. Under the laws of dynamics (force in 

 motion) any number of forces or resistances (components) 

 acting on a solid body and ditiering in magnitude and direc- 

 tion may be resolved into a single force or resistance, the 

 position, magnitude, and direction of which are fixed by the 

 magnitude and direction of of the several components. 



ir ,' 



Fic. 7. B= Force acting in echelon, parallel to beam. 



In Stephen's Book of Farm linijlements, jjulilishcd in 

 18.5s, dealing with this subject, the point of resultant resis- 

 tance — that is, the final resistance arrived at on combining 

 the several resistances in operation — of a plough is called the 

 'centre of gravity'. As, however, gravity is not the principal 

 cause of the resistance of a jilough, I prefer to call the point 

 in question the 'resultant centre of resistance and point of 

 balance'. By this is meant the imaginary point in 

 the body of the plough from which a single force, 

 acting horizontally to the said point and parallel to 

 the land side, will overcome the resistance with a min- 

 imum of loss. If the force Vie applied obliquely to the said 

 point the direction of the plough will not be altered, but the 

 Jilough will advance in eche]<in to and parallel with the said 

 force. The word 'echelon' may be better understood by 



recalling the drawing of a barge in a canal by a horse walking 

 on the bank. This point, then, from the estimate already 

 inade of the components of resistance, I place approximately 

 in thejjlough we are considering at \'> inches to the rear of 

 the point of share 2 inches up from the base line of plough 

 (which is a straight line from the point of share to tread of 

 ba<-k wheel) and 3 inches to the right of the land side of the 

 plough, looking from the rear of the plough. To ascertain 

 the precise position is not essential to the argument. 



It should here be noted that this imaginary point when 

 a plough is at work is not a fixed point, but is constantly 

 on the move from side to side and up and down, due to the 

 \ariation in intensity of the three components caused liy the 

 vaiying composition of the land being ploughed; but under 

 ordinary circumstances, it will be fairly steady within an 

 inch or two of the position stated. It is not possible t(5 

 •ipply the force required to overcome the resultant resistance 

 at the Ijest theoretical position — namely, in a line horizontal 

 with the said point and parallel with the land side of the 

 plough— owing, firstly, to this line passing through the 

 earth in front of the plough, as shown in figure 4, and, 

 secondly, to the fact that the hame-hooks of a horse are 

 about 4 feet above the surface of the ground. 



I'or obxi.jus reasons, the power can neitiier be applied on 

 the line AC nor the line AB. It therefore becomes necessary 

 to equip the plough-body with a beam raised high enough t(j 

 clear both the furrow in front of the share and the ordinary 

 surface growth on the land, and carried back from the share 

 far enough to provide room for the front part of the mould- 

 board to join the share in a regular curve. These considera- 

 tions fix the length and shape of leg and beam, which together 

 form a straight shanked hook, the horses being attached to 

 the end of the shank, the share and the mould-board being 

 joined to the returned curved end of the said hook. As the 

 force ha.s to be attached to the front end of the beam, and as 

 force acts in a straight line, the power of the horses attached 

 to the front of the beam crosses the gap between the point of 

 the beam and the centre of resistance in a straight line. The 

 leg and the beam nmst lie made strong enough to resist the 

 strain thereby set up in them, tending to straighten out the 

 hook formetl by the leg and beam. 



^^ 



Fn:. 8. .\. (V'ntreof liesistancc and Point of Balance. 



B. Best Theoretical l^isition for Line of 

 Draught. 



C. Hame-hook of ordinary Draught Horse 

 (4 feet above surface and 14 feet from A). 



AC. Straight line from Hame-hook to centre 

 of liesistance. 



♦ The 

 ^0, 1914. 



Joxnuil of Agricnltme of New Zealand for .Tune 



Fii;. 9. A. Centre of Kesistance. 



C. Hame-hook, 4 feet atiove Surface and 14 



feet from A. 

 AC. Line of Force, and passing througli 



Bridle-irin at 1). 



The beam must not only be high enough to clear the 

 furrow and strong enough to stand the strain, but must be 

 h.ng enough to bring the line of draught in a straight line 

 from the hame-hooks to the centre of resistance, as shown 

 in Fig. 5. 



