lls 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

CONSERVATION OF ENERGY

SECTION 10. _Levers._

_How a big weight can be lifted with a little force; how one thing
moving slowly a short distance can make another mo