If we take a thin bar of iron and place it on a table, it will remain there. Remove the support, and the bar will fall to the ground. All bodies act in the same way. The earth attracts them, and this force is called gravitation. If a bar of iron is laid across a support, one particular point will be found at which it will balance, and remain at rest; that point is called the center of gravity of the iron bar, because it is the point at which the entire weight of the body may be considered as centered; if the bar is of the same thickness throughout its length, it will be exactly in the center. If the support is changed to any other point, the bar will fall to the ground; or if a weight of 1 lb. be fixed on one end, and a weight of 4 lbs. on the other end, then the center of gravity will be 1 ft. from the 4-lb. weight. The center of gravity is also called the center of inertia or the center of mass. It is the point in a body about which the mass is evenly disposed and if pivoted at that point, the body ought to be balanced.