s who fly high suffer keenly from the cold. There are
several reasons for this coldness of the high air. This is just _one_
of them.

_APPLICATION 26._ Explain why air usually cools when it rises;
why high mountain tops are always covered with snow.

INFERENCE EXERCISE

Explain the following:

131. You should not fill a teakettle brim full of cold water
when you are going to put it on the stove.

132. It is harder to erase an ink mark than a pencil mark.

133. Bearings of good watches, where there is constant rubbing
on the parts, are made of very hard jewels.

134. You feel lighter for an instant when you are in an
elevator which starts down suddenly.

135. When men lay cement sidewalks, they almost always make
cracks across them every few feet.

136. To cool hot coffee one sometimes blows on it.

137. It is much easier to turn the latch of a door with the
knob than with the spindle when the knob is off.

138. Patent-leather shoes do not soil as easily as plain
leather shoes.

139. We use rubber bands to hold things together tightly.

140. As air goes up it usually cools.

SECTION 17. _Freezing and melting._

When water freezes in a pipe, why does the pipe burst?

What is liquid air?

Why does not the wire in an electric lamp melt when it is red
hot?

Suppose we looked at a piece of ice through the imaginary microscope
that shows us the molecules. The ice molecules would be different from
the iron molecules in size, but they would be vibrating back and forth
in exactly the same way, only with less motion. It is because they
have less motion that we say the ice is colder than the iron. Then let
us suppose that the sun was shining on the ice while we watched the
ice molecules.

First we should see movements of the ice molecules become gradually
more rapid, just as the iron molecules did when the iron was warmed.
Then, as they moved faster and faster, they would begin to bump into
each other and go around every which way, each molecule bumping first
into one neighbor, then into another, and bouncing back in a new
direction after each collision. This is what causes the ice to melt.
When its molecules no longer go back and forth in the same path all
the time, the ice no longer keeps its shape, and we call it water--a
liquid.

Almost all solid substances will melt when they are heated. Or, to
put it the