20 A N Bo Stas Ary

moving the wedge by ftrokes, at intervals,

compared with the continued action that pro- duces the motion of the plough, is no ob- jection againft the analogy between them, as the total fum of the force requifite to pro- duce the fame effect muft in both cafes be equal.

The determination, then, of the form of the plough moved by leaft force, feems re- ducible to this fingle queftion. Suppofing any number of wedges of equal backs, but of different lengths, what is that particular length which meets with leaft refiftance, in moving through fucha loofe fubftance as earth, the fize and cohefion of whofe parti- cles are nearly equal?

I have fuppofed the backs of the wedges of equal thicknefs, becaufe the greateft breadth of the ploughs compared together muft be equal, as fuch only can be ima~ gined to do the fame quantity of work in the fame time. I have likewife fuppofed, that the fize and cohefion of the particles of earth are nearly-equal, both becaufe in drawing general conclufions all particular accidental circumftances muft be abftracted

from, and becaufe ploughed earth in fome meafure