WHY WE OIL MACHINERY. We can decrease friction by keeping objects from pressing tightly against each other, and by making their surfaces smooth. The most common way of making surfaces smooth is by oiling or greasing them. A film of oil or grease makes things so smooth and slippery that there is very little friction. That is why all kinds of machinery will run so smoothly if they are kept oiled. And since the oil decreases friction, it decreases the wear caused by friction. So well-oiled machines last much longer than machines that are not sufficiently oiled. [Illustration: FIG. 25. The friction of the stone heats the nail and wears it away.] WHY BALL BEARINGS ARE USED. There is much less friction when a round object rolls over a surface than when two surfaces slide over one another, unless the sliding surfaces are very smooth; think how much easier it is to pull a wagon forward than it would be to take hold of the wheels and pull the wagon sidewise. So when you want the least possible friction in a machine you use ball bearings. The bearings are located in the hub of a wheel. Then, instead of the axle rubbing against the hub, the bearings roll inside of the hub. This causes very little friction; and the friction is made still less by keeping the bearings oiled. _APPLICATION 14._ Suppose you were making a bicycle,--in which of the following places would you want to increase the friction, and in which would you want to decrease it? Handle grips, axles, pedals, tires, pedal cranks, the sockets in which the handle bar turns, the nuts that hold the parts together. _APPLICATION 15._ A small boy decided to surprise his mother by oiling her sewing-machine. He put oil in the following places: On the treadle, on the large wheel over which the belt runs, on the axle of the same wheel, on the groove in the little wheel up above where the belt runs, on the joint where the needle runs up and down, on the little rough place under the needle that pushes the cloth forward. Which of these did he do well to oil and which should he have let alone? INFERENCE EXERCISE Explain the following: 61. Rivers flow north as well as south, although we usually speak of north as "up north." 62. Tartar and bits of food stick to your teeth. 63. Brushing your teeth with tooth powder cleans them. 64. When a chair has gliders (smooth metal caps) on its feet, it slides easily across the floor. 65. When you wet your finger, you can turn a page more easily. 66. A lamp wick draws oil up from the lower part of a lamp to the burner. 67. The sidewalks on steep hills are made of rough cement. 68. Certain fish can rise in the water by expanding their air bladders, although this does not make them weigh any less. 69. When your hands are cold, you rub them together to warm them. 70. It is dangerous to stand up in a rowboat or canoe. CHAPTER THREE