12 HEAT 



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 some- 

 times 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 be- 



FlG. 5. A cement walk broken by expansion due r 



to sun heat. tween them giving op- 



portunity 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 sag- 

 ging 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. 



The surface of the rock is the most exposed part, and 

 during the day the surface, heated by the sun's rays, expands 



