This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The center of gravity of a figure or body is that point upon which the figure or body will balance in whatever position it may be placed, provided it is acted upon by no other force than gravity.
If a plane figure is alike or symmetrical on both sides of a center line, the latter line is termed an axis of symmetry, and the center of gravity lies in this line. If the figure is symmetrical about any other axis, the intersection of the two axes will be the center of gravity of the section. Thus, the center of gravity of a square, rectangle, or other parallelogram is at the intersection of the diagonals; of a circle or ellipse, at the center of figure; etc. The center of gravity of a triangle is found at the intersection of lines drawn from the middle of each side to the opposite apex; or, it is 2/3 the distance from any apex to the middle of the opposite side. For any section, the center of gravity may be found by the principles explained in the following article on Neutral Axis. It may be determined approximately, but simply, by drawing to scale upon cardboard the outline of the section; then, by cutting out the figure, and balancing it in different directions on a knife edge, the center of gravity will be at the intersection of the lines on which the section balances.