110 On Agricultural Mechanics. 



alternately raised or depressed ; while the one is in a working 

 position, the other is carried above. The rod b, extendin^: to 

 the coulter at/ in moving the mould-board moves also the coulter 

 one inch at the point, so as to give it the proper position with 

 the point of the sock at g. 



Having thus seen the immense variety that exists among 

 ploughs, it may be well, before inquiring into what ought to be 

 the form of the plough, to inquire what it is required to do. 



The objects in ploughing are three ; and the mode of plough- 

 ing must of course be varied to suit the object. 



The furrow is opened up, and the furrow-slice is turned, 



1. That the after-action of the harrow in covering the seed 

 may be facilitated, and 



2. That a new surface of mould may be exposed, or 



3. That anything laid on the surface may be buried. 



Now, it is evident that these being the objects of ploughing, 

 they cannot vary so much as to require that immense variety of 

 implements which exists. The position in which the furrow-slice 

 will lie does not depend on the plough, except in so far as its form 

 and dimensions are regulated by that implement. When once 

 the form of a series of slices which are to rest on one another is 

 fixed on, then their position is also determined. The form of 

 the furrow- slice then, as it regulates its after position, must be 

 regulated according to the work to be done. 



First, then, with regard to the first object of ploughing, the 

 preparation of the ground for the after- action of the harrow. It is 

 demonstrable that when the angle of inclination of the furrow-slice 

 is 45°, there is a greater quantity of earth raised above the level 

 in the form of ribs than under any other circumstance. Now, as 

 the action of the harrow will tear this lip or series of lips down, 

 the more earth there is contained in them the greater will be the 

 depth of loose mould thus formed. Now it is also demonstrable 

 that this position of the furrow- slice can only be obtained when, the 

 breadth of the furrow- slice being 10 inches, the depth is about 7. 

 Here, then, we have at once the form and position of the sod, by 

 the adoption of which we shall derive the greatest advantage from 

 a subsequent harrowing. 



The second object of ploughing being to expose a fresh sur- 

 face of mould to the action of the air, it is evidently important to 

 know under what circumstances the greatest amount of surface 

 will be exposed. It will be at once seen from the subjoined 

 solution of the question,* that this is the case when, as before, 

 the width being 10 inches, the depth of the sod is 7 inches. It 

 will then lie at an angle of 45°, and the width of the portion on 



* See Appendix. 



