HOW CREAM IS SEPARATED FROM MILK BY CENTRIFUGAL FORCE. The heavier things are, the harder they are thrown out by centrifugal force. Milk is heavier than cream, as you know from the fact that cream rises and floats on top of the milk. So when milk is put into a centrifugal separator, a machine that whirls it around very rapidly, the milk is thrown to the outside harder than the cream, and the cream therefore stays nearer the middle. As the bowl of the machine whirls faster, the milk is thrown so hard against the outside that it flattens out and rises up the sides of the bowl. Thus you have a large hollow cylinder of milk on the outside against the wall of the bowl, while the whirling cream forms a smaller cylinder inside the cylinder of milk. By putting a spout on the machine so that it reaches the inner cylinder, the cream can be drawn off, while a spout not put in so far will draw off the milk. WHY A SPINNING TOP STANDS ON ITS POINT. When a top spins, all the particles of wood of which the top is made are thrown out and away from the center of the top, or rather they _tend_ to go out and away. And the pull of these particles out from the center is stronger than the pull of gravitation on the edges of the top to make it tip over; so it stands upright while it spins. Spin a top and see how this is. _APPLICATION 21._ Explain how a motor cyclist can ride on an almost perpendicular wall in a circular race track. Explain how the earth keeps away from the sun, which is always powerfully pulling the earth toward it. INFERENCE EXERCISE Explain the following: 91. As you tighten a screw it becomes harder to turn. 92. There is a process for partly drying food by whirling it rapidly in a perforated cylinder. 93. It is easier to climb mountains in hobnailed shoes than in smooth-soled ones. 94. When you bore a hole with a brace and bit, the hand that turns the brace goes around a circle many times as large as the hole that is being bored. 95. The hands of some persons become red and slightly swollen if they swing them while taking a long walk. 96. A flywheel keeps an engine going between the strokes of the piston. 97. In dry parts of the country farmers break up the surface of the soil frequently, as less water comes up to the surface through pulverized soil than would come through the fine pores of caked earth. 98. After you have apparently cleaned a grease spot out of a suit it often reappears when you have worn the suit a few days. 99. Mud flies up from the back wheel of a boy's bicycle when he rides along a wet street. 100. A typewriter key goes down less than an inch, yet the type bar goes up nearly 5 inches. SECTION 13. _Action and reaction._ How can a bird fly? What makes it stay up in the air? What makes a gun kick? Why do you sink when you stop swimming?