sa MONON SUR Ae

mong themfelves, and very little againft any folid body over which they move. But the particles of earth have a confiderable friction among themfelves, and againft any body o- ver which they move; and therefore muft confiderably retard the motion of a body moving through them. Here, then, feems to be the circumftance that muft be decifive of the form of the plough moved by leaft force; for, it is plain, it muft be the one that has the leaft fri€tion, or that removes the particles of earth to an equal diftance with the leaft rubbing upon itfelf.

Now, the plough that does this, rauft be the one that expofes leaft furface to the foil; for, the greater the furface, the greater al- ways mutt be the quantity of particles that rub at the fame time, and confequently the greater the fum of the friction.

To be perfedtly fatisfied of the truth of this propofition, 1 made experiments by mo- ving half wedges of different lengths, but - of the fame thicknefs at the back, through {fand,. I caufed make four half wedges, whofe lengths were to one another as the

numbers 3, 45 5» 6. I computed pretty ac- curately