es of Expansion and Contraction. We owe
the snug fit of metal tires and bands to the expansion and contraction
resulting from heating and cooling. The tire of a wagon wheel is made
slightly smaller than the wheel which it is to protect; it is then
put into a very hot fire and heated until it has expanded sufficiently
to slip on the wheel. As the tire cools it contracts and fits the
wheel closely.

In a railroad, spaces are usually left between consecutive rails in
order to allow for expansion during the summer.

The unsightly cracks and humps in cement floors are sometimes due to
the expansion resulting from heat (Fig. 5). Cracking from this cause
can frequently be avoided by cutting the soft cement into squares, the
spaces between them giving opportunity for expansion just as do the
spaces between the rails of railroads.

[Illustration: FIG. 5: A cement walk broken by expansion due to sun
heat.]

In the construction of long wire fences provision must be made for
tightening the wire in summer, otherwise great sagging would occur.

Heat plays an important part in the splitting of rocks and in the
formation of debris. Rocks in exposed places are greatly affected by
changes in temperature, and in regions where the changes in
temperature are sudden, severe, and frequent, the rocks are not able
to withstand the strain of expansion and contraction, and as a result
crack and split. In the Sahara Desert much crumbling of the rock into
sand has been caused by the intense heat of the day followed by the
sharp frost of night. The heat of the day causes the rocks to expand,
and the cold of night causes them to contract, and these two forces
constantly at work loosen the grains of the rock and force them out of
place, thus producing crumbling.

[Illustration: FIG. 6.--Splitting and crumbling of rock caused by
alternating heat and cold.]

The surface of the rock is the most exposed part, and during the day
the surface, heated by the sun's rays, expands and becomes too large
for the interior, and crumbling and splitting result from the strain.
With the sudden fall of temperature in the late afternoon and night,
the surface of the rock becomes greatly chilled and colder than the
rock beneath; the surface rock therefore contracts and shrinks more
than the underlying rock, and again crumbling results (Fig. 6).

[Illustration: FIG. 7.--Debris formed from crumbled rock.]

On bare mountains, the heating and cooling effects of the