Advantages and Disadvantages 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. 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.
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 débris. 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.
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.
On bare mountains, the heating and cooling effects of the sun are very striking; the surface of many a mountain peak is covered with cracked rock so insecure that a touch or step will dislodge the fragments and start them down the mountain slope. The lower levels of mountains are frequently buried several feet under débris which has been formed in this way from higher peaks, and which has slowly accumulated at the lower levels.
FIG. - As the air in A
is heated, it expands and forces the drop of ink up the tube.
FIG. A cement walk broken by expansion due to sun heat.
FIG. - Splitting and crumbling of rock caused by alternating heat and cold.
FIG. - Debris formed from crumbled rock.