ONeTEER PLOUGH. 2.5

curately the furface of each of them, and found, that the weights that moved them were very nearly proportional to their fur- faces*.

Thefe experiments are a proof, not only that fri€tion is in proportion to the furface of the plough, but alfo eftablith the preceed- ing theory; namely, that, abftracting from this circumftance, the refiftence made to all wedges of the fame thicknefs, whatever their lengths be, is the fame, in producing equal effects.

Now, of all wedges of equal backs, the fhorter have always lefs furface. Hence follows this general conclufion: Of all

ploughs of the fame breadth, thofe that D have

* I put a quantity of fand upon a large table, and then moved the wedges through it by a weight fut- pended on a thread that run over a pully fixed on the edge of the. table. I taok all the care I could to bring the fand into the fame{tate every time the wedges were pulled through it; and repeated the experiment many times, with very nearly the fame refult.