TEE SLING, BOW, AND OTHER WEAPONS 133 



It is this centrifugal force that causes a flywheel or rapidly re- 

 volving grindstone to break and fly in pieces, sometimes doing 

 much damage. This same force is used in the cream separator and 

 the centrifugal laundry wringer. In the latter the clothes are 

 put in a rotating drum with perforated sides, out of which the 

 water is thrown as the drum whirls. In the cream separator, 

 water, casein, and other heavy parts of the milk are thrown out 

 from the rapidly rotating bowl while the light cream remains at 

 the center. 



The common top is an admirable illustration of this same law 

 of inertia. When the top is set spinning each particle of it 

 travels in its own path and resists any force that acts to move it 

 out of that path, so that while it would not for a moment stand 

 straight up on its peg if it were not spinning but would promptly 

 topple over, when it is set going it resists the pull of gravity and 

 stands erect as it spins. When the top is spinning on your hand 

 you may incline your hand but the top remains upright. The 

 skilful lad even lets the top spin down a string stretched from 

 hand to hand, one end lower than the other, and the top main- 

 tains a fixed inclination as it slides along instead of falling off, for 

 its inertia resists the pull of the earth. 



A passenger boat is just being put on the route from New 

 York to England that has in its hold a great metal disk weighing 

 100 tons that is set on an axle in a frame so it may be rotated 

 with great speed. It is expected that this great rotating disk 

 will resist the force of the waves that makes a ship roll and keep 

 it steady, a gyroscope stabilizer. If the device is as successful 

 as its designer expects, many passengers will be delighted to 

 have the good ship spin its top all the way over. 



The bow and arrow are very old weapons. Crude, chipped- 

 stone arrowheads are found very deep in piled-up strata of soil, 

 clays, sand, and gravel that must have taken many thousands of 

 years to accumulate. In the same beds in which the arrowheads 

 are found, there have been discovered in Europe parts of the 

 skeletons of very primitive men and of ancient animals that man 



