Fig. 39.
100. The Centre of Gravity Seeks the Lowest Point.—The centre of gravity always takes the lowest place which the support of the body will allow. In a suspended body, therefore, it is always directly under the point of suspension. To get to the one side or the other of this position it must rise. This the attraction of gravity forbids, and if by any force it is made to rise, this attraction at once brings it back. This is manifest in the case of a suspended ball, Fig. 39. If the ball be moved to b, it will, on being let go, return to its first position, simply because its centre of gravity, in obedience to the earth's attraction, seeks the lowest place possible. From inertia (§ 49) it moves beyond this point, and continues to vibrate back and forth for some time; but when its motion is stopped, it hangs perpendicularly; that is, in such a way that its centre of gravity shall have the lowest possible position. I add a few other illustrations of the same point. When a rocking-horse is at rest, its centre of gravity is directly over the point at which it touches the floor, for thus it has its lowest possible place. If it be rocked, the centre of gravity is moved to a higher point, and for this reason it rocks back again. The same is seen in the swing, the cradle, the rocking-chair, etc. Most interesting illustrations of the same thing are found in the Laggan, or Loggan Stones as they are called, several of which are seen on the rugged parts of the British coast. An immense rock, which has been loosened by some convulsion, rests with a slightly-rounded base on another rock which is flat, and it is so nicely balanced that one person alone can produce a perceptible rocking motion in it. I saw, many years ago, a large rock near Salem, Massachusetts, thus situated. There is one also in Great Barrington, Massachusetts.
Fig. 40. Fig. 41.
Fig. 42.
Fig. 43.
101. Farther Illustrations.—It is because the centre of gravity always seeks the lowest place that an egg lies upon its side. When on its side, the centre of gravity is at its lowest point, as is manifest from comparing Fig. 40 with Fig. 41 (p. 71). Children often have a toy, called a witch, which illustrates the same thing in another way. It is a piece of light substance, as pith, with a shot fastened in one end. It always stands up on its loaded end, and can not be made to lie down on its side, because the centre of gravity would not then be at the lowest point. There is an amusing Chinese toy of the same kind. It is a figure of a fat old woman, Fig. 42, loaded with lead at the bottom, so that its centre of gravity is at a. If the figure be thrust over to one side, as shown by the dotted lines, the centre of gravity is raised, and the upright position is at once resumed. If the toy were not loaded, it would lie in the position represented in Fig. 43, just as the egg lies on its side.