The attraction of gravity on any body tends to draw its particles toward one point, and hence, strictly speaking, the directions of these forces are not parallel. As the radius of the earth is very large, however, compared with the size of any object which is weighed, their divergence from parallel lines is, practically, not measurable.

The point of application of the resultant of all the parallel forces that make up the weight of a body is its center of gravity, center of mass, or center of inertia.

Demonstration. - Fit in a small wooden handle (or in a fixed support), two wires (Fig. 1): one, A, straight and the other, B, bent twice at right angles. In a piece of thin board C of any shape bore holes D and E in two corners. Suspend the board by one of these holes D from the wire B, and from A suspend a plumb line. See that D is exactly halved by the plumb  line when at rest, and mark a point F opposite the line. Suspend the board from the hole E, and mark the point G. Draw lines DF and EG, and their intersection O will determine the center of gravity. Test the accuracy of the work, by making a hole at O and rotating on the end of A.

Find, in the same way, the centers of gravity of a triangle, a square, a rectangle, and a circle.

In the above cases the center of gravity is midway between the two surfaces at the point O. It would still be at O, if the thickness of the board were infinitely reduced; hence we may speak of the center of gravity of a surface. The center of gravity of any body may be found by suspending it successively from two points on the body and finding the intersection of the lines of direction from those points of support to the center of the earth. This is because a body suspended from any point will hang with its center of gravity vertically below the point of suspension. The center of gravity is frequently outside the substance of the body, as in the case of a ring

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