378 



SCIENCE OF AGRICULTURE. 



Part II. 



apt to set the points of their shares too low, so as by their inclined direction to occasion 

 a heavy pressure on the wheel, which must proceed horizontally:" the effect of this 

 struggle is an increased weight of draught, infinitely beyond what could be supposed : 

 for which reason, the wheel is to be considered as of no importance in setting a plough 

 for work ; but passing lightly over the surface, it will be of material aid in breaking 

 up old leys, or ground where flints, rocks, or roots of trees occur, and in correcting the 

 depression of the shares from any sudden obstruction, as well as in bringing it quickly 

 into work again, when thrown out towards the surface. (^Communications to the Boaird 

 of Agriculture, vol. ii. p. 419.) 



2507. The Scotch plough, with one or sometimes two wheels {jig. 288.), fixed near to 



the end of the beam, without any carriage, goes very light, and is very useful ; such alter- 

 ations as are necessary requiring very little time or trouble. Where two wheels are era- 

 ployed, the plough does very well without a holder on a good tilth or light sward, where 

 there are few stones, except at the setting in and turning out. Wheel ploughs should, 

 however, probably be seldom had recourse to by the experienced ploughman, though 

 they may be more convenient and more manageable for those who are not perfectly 

 informed in that important and useful art. 



2508. WUkie''s single horse wheel plough {fig. 289.) has the wheel {d) differently placed. ;lt is adapted 

 for the lightest description 



of soils, and in them only g~-^ . 289 



for the second or sub- 

 sequent stirrings. The 

 wheel, besides considera- 

 bly reducing the weight 

 of draught, is found to 



give a degree of steadi- "\^^>^^ ^ , .__^'^e--<1<'C^ ~~->^Sl^ 



ness seldom exceeded in 

 the use of the common 

 plough, excepting when 

 quite new, or recently re- 

 paired with a new sock 

 and sole-shoe. At that 

 period, when the back end 



of the sole is quite full and square, the common plough, (when well constructed), goes as well as can be 

 wished for ; but, by the great friction of the sole, the back end of it soon becomes convex ; and, consequently, 

 the plough loses the steady support of the extremity of the heel, or, in other words, in proportion as the 

 sole becomes more convex, the fulcrum of the lever is extended considerably forward so as to be too 

 near the centre of gravity. When that is the case, the least obstruction at the point of the share throws 

 the plough out of the ground. In order to remedy or counteract that tendency, the ploughman is obhgcd 

 to raise the point of draught at the end of the beam ; but this expedient, although it gives the plough 

 more hold by the point of the share, is attended with another inconvenience fully as bad as the former; 

 for, when the point of the share meets with an obstruction as before noticed, the heel of the plough is 

 raised, on account of the point of draught being fixed above the direct line of attraction. Thus, the com- 

 mon plough, when the sole becomes convex, is made to go very unsteady, and often requires the utmost 

 attention and exertions of the ploughman to direct it. What is stated above, however, can only apply 

 to the common plough when out of order by the sole becoming convex. 



2509. Placing the wheel. In order to understand in what manner the wheel ought to be placed so as to 

 reduce the friction, it may be necessary to remark, that one of the first properties of a plough is to be 

 constructed in such a manner as to swim fair on the sole. This depends principally on the form of the 

 sole, and position or inclination of the point of the sock, together with the iroint of draught at the end 

 of the beam (a). If these are properly adjusted, the pressure or friction of the sole will .be uniform 

 from the point of the share (i) to the back end of the heel (rf) ; or, in other words, the friction will be 

 balanced between these two points by means of the beam (a) acting as a lever, the heel {d) being the 

 fulcrum, and a point over the share (c) the centre of gravity. 



2510. The centre of gravity or of resistance will be extended nearer to the point of the share {b), in 

 proportion as the soil has acquired a greater degree of cohesion ; as in old pasture ground, or strong clays. 

 But wherever the point of resistance meets, it is evident that the point of draught at the end of the 

 beam must be placed, so as to balance the friction of the sole between its extreme points {b and d). 

 Viewing the machine, therefore, (with regard to the friction of the sole) merely as a sledge carrying a 

 considerable weight, by which it is pressed equally to the bottom of the furrow at the extreme points 



