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greater power, and the fhorter the lefs. Their beft length and height will be thofe which allow them to be moft eafily and conveniently managed, or which oblige the perfon that holds them to bend his be- dy as little as poffible. In this view, their height muft be fuited to their fize and their length to his ftrength. If the foil bein a tolerable ftate of cultivation, and the ma- chine well conflruéted, they will not need to be very long; and the fhorter they are, provided the perfon that holds them have ftrength enough to manage them, the bet- ter; for he will be obliged to bend his bo- dy lefs to produce the fame effect.

As I have faid, the handles may be con- {idered as levers beginning from the center of refiftence, it will perhaps be afked, Whe- ther this is,in all cafes, ftrictly true; and how we fhall be able to decide, whether it is fo or not? To do this accurately, we fhould be determined exa@tly with regard to the fitua- tion of the center of refiftence. But the truth is, this determination cannot be made by any principles or operation previous to actual trial; becaufe it depends upon the friGtion and refiftence of the foil, which muft vary