116 On Ayrlcu/tural Mechanics. 



weight of turf lying on it the shorter it is.* There is, then, a pro- 

 per dimension somewhere between the shortness thus shown to be best 

 and the length which according to the other view is best, in which, each 

 kind of force being taken into consideration, the sum of the forces re- 

 quired to overcome each is a minimum. In order to ascertain this we 

 require the knowledge of several facts — such as the nature of the twist 

 of the least friction, the friction of soils of dififerent kinds on iron, and 

 the force of what may be called anti-torsion in sods of different forms 

 and kinds of soil. If these were known, the question might be placed 

 in such a form as would admit of a mathematical solution. 



For all practical purposes, I conceive that a sufficiently accurate form 

 of the mould-board might be obtained by taking a model of the twisted 

 surface of a turf turned over forcibly into the position into which the 

 action of a good plough would turn it. If, for instance, a turf of pro- 

 per dimensions be cut, and left attached at its extremity only to the fast- 

 land — and if at a distance from this point of fixture, equal to the length 

 of the proposed mould-board, it be taken hold of and turned over, first 

 into a vertical position, and then into its ultimate position, 45° beyond 

 the vertical — it is conceived that the surface which it would present 

 would be that of a very excellent mould-board; one, at any rate, ap- 

 proximating to that of least resistance. It is evident that this form 

 would be very gradual in its rise, and nowhere abrupt in the twist 

 which it would give to the sod; and it is evident that it would nowhere 

 be full-breasted. 



Before leaving the subject of the plough, a word or two must be said 

 on the question of wheel and swing ploughs. 



It is evident that unless the wheel can be ])roved to be an advantage, 

 it is a disadvantage, as it increases the complexity of the instrument. 



The most that can be said for the wheel is, that on flat ground it 

 keeps the furrow-slice of uniform depth. Now any irregularity in this 

 matter must be owing to one of two causes — either to a faulty workman 

 or to an imperfect arrangement of the draught-chains on the bridle of 

 the plough : and neither of these thmgs can justly be brought forward 

 to jus.ify any alteration or addition to the i)lough, for both can be re- 

 medied without it. If this be the only defence of it where the ground 

 is dry and fiat, what can be said in its favour where it is irregular and 

 wet? The wheel cannot diminish the draught of the plough by lessen- 

 ing the pressure of the implement on the furrow, which has been shown 

 to be the source of the greater part of the friction. If there were one 

 wheel behind and another before the plough, they might have this eflTect; 

 but this would be too great a complication of the implement. 



However, in Stirlingshire, they have ploughs which rest not on the 

 sole-plate, but on a small w4ieel below the mould-board, which is said 

 to diminish the draught considerably. This, however, where the land 

 is wet, soon clogs, and it thus increases the draught. The remark made 

 by a practical man on the subject, when the Norfolk wheel-plough was 



* In the Report, however, of the Judges of Implements at Liverpool, 

 Appendix uf Part VII., it will be seen that, of the ploughs tried there, tnose 

 which had wheels were almost invariably the lightest m draught. — W. Miles. 



