36 A N BS sa 2S

that of animal force muft unavoidably be, would very foon bring it out of the ground altogether.

To prevent this laft confequence, the plough is fo formed as to have a conftant tendency to penetrate deeper into the earth, which it acquires chiefly by the inclination of the coulter; and from this circumftance arifes the neceflity of an upward direCtion of the force, to balance the downward ten- dency of the machine, and the convenient application of animal force in order to pull ifs:

The proper line, then, of force that moves. the plough, fhould begin from its center of refiftence, with fuch a direction upwards, as juft to balance the tendency of the machine downward; for in this cafe only can the motion be performed ftraight forward.

Now, this line, it is evident, in coming upward from the center of‘refiftence, be- hoved to pafs through the firm ground be-

fore the plough*, and cut a paflage for it- felf

* Suppofe C, fig. 8. the center of refiftence,

CD the line of draught; the real line, if there is only