214. =Illustrations of Centrifugal Force.=--When a wet mop is whirled the water flies off in every direction by its centrifugal force. On the same principle a dog, coming out of the water, shakes off the water by a semi-rotary motion.--When a suspended bucket of water is turned swiftly around the water rises high on its sides, and leaves a hollow in the middle. It is the tendency to fly away from the centre of motion that causes this.--Large wheels, revolving with great velocity, have been broken by the centrifugal force of its particles, and hence the necessity of having such wheels made very strong. The immense grindstones used in gun-factories have sometimes been broken through in the middle, or have flown into pieces from the same cause.--A man riding horseback on turning a sharp corner inclines his body toward the corner, to avoid being thrown off by the centrifugal force. So, in the feats of the circus, a man standing on a horse running at full speed around the ring inclines his body strongly inward, as you see in Fig. 148 (p. 167). The horse also instinctively inclines in the same direction for the same reason. If the rider finds himself in danger of falling, by making the horse go a little faster, thus adding to the centrifugal force, the difficulty is relieved.--The centrifugal force is made use of in milling. The grain is admitted between two circular stones by a hole in the centre of the upper one, and as the stone revolves it constantly moves toward the circumference, and there escapes as flour. [Illustration: Fig. 149.] 215. =Bends in Rivers.=--We see the operation of the centrifugal force in the bends of rivers. When a bend has once commenced in a river it is apt to increase, for as the water sweeps along the outer bank of the bend it presses strongly against it, just as the water in the whirled bucket, § 214, presses against its sides, by its centrifugal tendency, or, in other words, its tendency to assume a straight motion. Of course the result is a wearing away of this outer bank, and in proportion to the looseness of the material of which it is composed and the velocity of the river's current. And when one bend is formed another is apt to form below, but in an opposite direction. The water, by sweeping along the bend _a_, Fig. 149, is directed by it toward the opposite bank at _b_, and makes a bend there also. [Illustration: Fig, 150.]