18 fe ES 72°"

of the fhare lies in the plane of the left fide of the plough, becaufe the greateft refiftance muft be on that fide; and from the point it has a fharp edge projecting towards the right, called the fin or feather of the fhare, which penetrates below the earth of the fur- row, and cuts it out.

I have been fo minute in the defcription of this inftrument for two reafons. The firft is, that its manner of operation, and the effect of every part of its conftruction, may be perfectly underftood, fo that any perfon may comprehend what form is neceflary to lay the foil in the fituation he fhall judge moft proper, and be enabled to give directi- ons for making it accordingly.

The other reafon is, to lay a foundation for the application of fome theory, by fhow- ing that the form and operation of the plough are reducible to mechanical princi- ples, and that from them conclufions may fairly be drawn towards determining its moft advantageous figure.

The beft plough is certainly that which, all other circumftances being equal, is pulled by leaft force. But, if it fhall happen, which

So Ee

SS