America Held Up Through New York, 2011, Jonathan Monk
"A center can be determined geometrically, mechanically or intuitively. Geometry uses ruler and compass as tools of intellectual construction. Mechanically one can determie the center of an object by weight. For example, one can make a flat figure out of cardboard or plywood and suspend it first by one and then by another point near its contour. The place where verticals dropped from the points of suspension cross is the center of gravity. One can obtain the same result by balancing a flat object on the tip of a finger."