ON THE PLOUGH. 37

felf as it moved along, a thing at firft fight utterly impracticable. But the fame pur- pofeisanfwered by the beam, as if what has been obferved were really poffible; and this is the circumftance that determines all at once both its ufeand length. For the beam muft be fo placed, and be of»fuch a length, as to coincide with a point of this line of direc- tion, at a convenient height above the fur- face of the ground, and by taking it up here, fuperfede the neceffity of its being continu- ed farther down; fothat the effect is per- feétly the fame’ as if the force acted on a line coming direétly from the center of refift-

ence. The beam muft be fituated at a conveni- ent

enly one force applied, but the mean line, if two, found by bife€ting the angle which the direCtions of the two forces make with one another; without a team, this line CD mufthave been fixed at C, be- low the furface of the earth KH; fo that the fur- face of the ground would have interfered with the direétion of it. This is prevented by the point of the beam that takes up this line at E, as near the fur- face of the earth as poflible. So that the point of the beam lies in the line of force pafling to the center of refiftence.