A Treatise on Painting/Chapter 64
Chap. LXIV.—Of balancing the Weight round the Centre of Gravity in Bodies.
A figure standing upon its feet without motion, will form an equipoise of all its members round the centre of its support.
If this figure without motion, and resting upon its feet, happens to move one of its arms forwards, it must necessarily throw as much of its weight on the opposite side, as is equal to that of the extended arm and the accidental weight. And the same I say of every part, which is brought out beyond its usual balance.