move up, they do so by pushing your body
down a little. When you swim, you push the water back and down with
your arms and legs, and this pushes your body forward and up. When a
bird flies up into the air, it pushes its body up by beating the air
down with its wings. When an airplane whirs along, its propeller fans
the air backward all the time. Street-car tracks are kept shiny by the
wheels, which slip a little as they tend to shove the track backward
in making the car move forward. Automobile tires wear out in much the
same way,--they slip and are worn by friction as they move the earth
back in pushing the automobile forward. In fact, if there are loose
pebbles or mud on the road, you can see the pebbles or mud fly back,
as the wheels of the automobile begin to turn rapidly and give their
backward push to the earth beneath.

[Illustration: FIG. 40. Action and reaction are equal; when he pushes
forward on the ropes, he pushes backward with equal force on the
seat.]

Here are a couple of experiments that will show you action and
reaction more clearly:

EXPERIMENT 26. Stand on a platform scale and weigh yourself.
When the beam is exactly balanced, move your hands upward and
notice whether you weigh more or less when they _start_ up.
Now move them downward; when they _start_ down, do you weigh
more or less? Toss a ball into the air, and watch your weight
while you are tossing it. Does your body tend to go up or down
while you are making the ball go up?

EXPERIMENT 27. Go out into the yard and sit in a rope swing.
Stop the swing entirely. Keep your feet off the ground all
through the experiment. Now try to work yourself up in the
swing; that is, make it swing by moving your legs and body and
arms, but not by touching the ground. (Try to make it swing
forward and backward only; when you try to swing sidewise, the
distance between the ropes spoils the experiment.) See if you
can figure out why the swing will not move back and forth.
Notice your bodily motions; notice that when half of your body
goes forward, half goes back; when you pull back with your
hands, you push your body forward. If you watch yourself
closely, you will see that every backward motion is exactly
balanced by a forward motion of some part of your body.

_APPLICATION 22._ Explain why you push forward against the
table to shove your chair back from it; why a bird