[393] Why he who wishes to drive in by pulling the iron into the ground raises the opposite leg, bent.
He who by pulling wishes to drive or draw the bar into the ground raises the leg opposite to the arm that pulls, and bends it at the knee; and he does this in order to balance himself over the foot that rests on the ground, without which bending or twisting of the leg it could not be done, nor could he pull, if such a leg did not extend itself.

[394] The ponderation of bodies that do not move.
The ponderations, or balances, of men are divided into two parts, that is, simple and compound. Simple balance is that which is made by a man upon his immovable feet, upon which the man, opening the arms with various distances from his middle, or bending while standing over one or both feet, does so in such a way that the centre of his gravity is always on a perpendicular line above the centre of the foot that rests on the ground; and if he rests upon both feet equally, then the man's weight will have its centre perpendicular in the middle of the line that measures the space interposed between the centres of those feet.
The compound balance is understood to be that which a man makes who supports upon himself a weight by various motions; as in representing Hercules who crushes Antaeus, who, suspending him from the earth between his breast and arms, requires that you make his figure as far behind the central line of his feet as Antaeus has the centre of his gravity in front of those same feet.