i6 



HEAT 



In the construction oflong wire fences provision must be made 

 for tightening the wire in summer, otherwise they sag. 



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 re- 

 gions where the changes 

 in temperature are sud- 

 den, severe, and frequent, 



FIG. 4. A cement walk broken by expansion , , 



due to sun heat. 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 change from the intense heat of the day to 

 the intense cold of night. The heat of 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 causing the rocks to crumble. 



The surface of the rock is the most 

 exposed part, and during the day the 

 surface, heated by the sun's rays, ex- 

 pands, becomes too large for the in- 

 terior, and crumbles and splits as a 

 result of the strain. With the sudden 

 fall of temperature in the late after- 

 noon 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 splits and crumbles (Fig. 5). 



On bare mountains, the heating and cooling effects of the 



FIG. 5. Summit of Mt. 

 Whitney. 



