Ploughs and Ploughing. 



293 



length and direction the actual "useful" draught, and the 

 perpendicular AD the direction and amount of the "lifting" 

 power of the draught, the wasted power which has to be 

 counteracted by giving the point of the share or the sole-plate 

 or slade a slight dip downwards. 



In the language of mechanics, the line AC is the " resultant" 

 which can be divided into its "component forces" x^B and 

 AD, and by trigonometrical formulae these can be Calculated 

 for any angle. 



If we take the pull of the two horses at the maximum figure 

 of 6 cwt. (672 lb.), and at the angle of 20 degrees, then : — 



AB = AC cos 20 

 = 672 x -93969 

 = 631-47 11). 



= actual draught of plough on level, in lb. 



Again, 



AD - BC = AC sin 20 



= 672 x '34202 

 = 229*83 lb. 



= lifting pull on plough in lb. 



This applied in practice means that the smaller the angle 

 CAB the less will be the draught ; a steam plough, for 

 instance, which is pulled horizontally, requires less power per 

 cubic foot of earth moved than an ordinary horse-drawn one. 

 On the other hand, by lengthening the plough-chains so as to 

 reduce the angle of draught the draught itself is reduced, but. 

 as above stated, practical experience has shown that about 10 

 feet from bridle-hook to hame-hook is the most convenient 

 length. When the ground is very hard, however, as after 

 harvest, and it is difficult to get the plough to cut into it, by 

 lengthening the chain between the bridle and whipple- trees 

 the direct pull is increased and the lifting pull lessened. 

 Putting a chisel point on the share also helps its penetrating 

 power, especially on stony land. 



It will be noticed that the line AC is approximately at right 

 angles to what will be the line of the hames, i.e., the slope of the 

 shoulder-blade of the horse — an arrangement to be aimed at. 

 The shoulder-slope differs between different horses — being 

 usually more upright in heavy work horses, for instance, than 

 among driving or " hunter cattle " — while the angle of draught 

 itself varies as between a plough or other tillage implement and 



