(150.) An enquiry is here suggested: does the direction of the equivalent force thus determined depend on the position of the body with respect to the surface of the earth, and how is the direction of the equivalent force affected by a change in that position? This question may be at once solved if the body be suspended by different points, and the directions which the suspending thread takes in each case relatively to the figure and dimensions of the body examined. The body being suspended in this manner from any point, let a small hole be bored through it, in the exact direction of the thread, so that if the thread were continued below the point where it is attached to the body, it would pass through this hole. The body being successively suspended by several different points on its surface, let as many small holes be bored through it in the same manner. If the body be then cut through, so as to discover the directions which the several holes have taken, they will be all found to cross each other at one point within the body; or the same fact may be discovered thus: a thin wire, which nearly fills the holes being passed through any one of them, it will be found to intercept the passage of a similar wire through any other. This singular fact teaches us, what indeed can be proved by mathematical reasoning without experiment, that there is _one_ point in every body through which the single force, which is equivalent to the gravitation of all its particles, must pass, in whatever position the body be placed. This point is called _the centre of gravity_. (151.) In whatever situation a body may be placed, the centre of gravity will have a tendency to descend in the direction of a line perpendicular to the horizon, and which is called the _line of direction_ of the weight. If the body be altogether free and unrestricted by any resistance or impediment, the centre of gravity will actually descend in this direction, and all the other points of the body will move with the same velocity in parallel directions, so that during its fall the position of the parts of the body, with respect to the ground, will be unaltered. But if the body, as is most usual, be subject to some resistance or restraint, it will either remain unmoved, its weight being expended in exciting pressure on the restraining points or surfaces, or it will move in a direction and with a velocity depending on the circumstances which restrain it.