The _pendulum_ is considered the nearest approach to perpetual motion. This is so well known that no description is needed, but we may say a few words concerning it. By the diagram, we see that if we lift the ball to _b_, and let it fall, it will descend to _l_, and pass it to _a_ opposite, nearly as far from _l_ as _b_ is from it. So the oscillations will continue, each beat being less and less, till rest is reached by the action of gravity (page 23). Were it not for friction and the pressure of the air, the oscillations would continue for ever; as it is, it declines by shorter swings till it remains in equilibrium. [Illustration: Fig. 35.—The pendulum.] The seconds’ pendulum oscillates sixty times an hour, and must be of a certain length in certain places. In London it is 39·1393 inches, and furnishes a certain standard of length, and by an Act of Parliament the yard is divided into 36 parts, and 39·1393 such parts make the seconds’ pendulum in the latitude of London (_in vacuo_) in a temperature of 62°. [Illustration: Fig. 36.—Centrifugal Force.] But the same pendulum will not perform the same number of oscillations in one minute in all parts of the globe. At the equator they will be less, and at the pole more. Thus it was discovered that, as the movements of the pendulum are dependent upon the force of gravity, and as this force decreases the farther we get from the centre of the earth, the equator must be farther from the earth’s centre than the poles, and therefore the poles must be depressed. The decline of the pendulum at the equator is also, in a measure, due to Centrifugal Force. _Centrifugal Force_, which means “flying from the centre,” is the force which causes an object to describe a circle with uniform velocity, and fly away from the centre; the force that counteracts it is called the _centripetal_ force. A very simple experiment will illustrate it. [Illustration: Fig. 37.—Another illustration of centrifugal force.] To represent its action, we shall have recourse to an ordinary glass tumbler placed on a round piece of cardboard, held firmly in place by cords. Some water is poured in the glass, and we then show that it can be swung to and fro and round without the water being spilt, even when the glass is upside down (fig. 36). Another experiment on the same subject is as shown in the above illustration, by which a napkin ring can be kept in revolution around the forefinger, and by a continued force the ring may be even held suspended at the tip of the finger, apparently in the air, without support (fig. 37). FOOTNOTES: [8] The experiment we have just described is a very old one. M. V. Sircoulon has told us that it was described at length in the works of Rabelais. The following remarks are in “Pantagruel,” book II., chap. xvii.