EFFECTS OF HEAT 



73 



advantage, for the force exerted by a body in expanding 

 or contracting is very great. To make a wagon tire fit 

 tightly, a blacksmith often puts it on after heating it; 

 upon cooling, it contracts and fits the wheel closely. 

 Similarly the parts of boilers, bridges, and other steel 



FIG. 57 



structures are fastened with rivets which are put in 

 while red-hot; these cool and contract, drawing the 

 parts tightly together. 



83. Exceptions to the Rule. A few substances do 

 not obey the general rule for expansion and contrac- 

 tion. Of these, water is a common example. We have 

 seen ice floating upon water, which shows that it is 

 lighter than the liquid ; but as ice is only frozen water, 

 we know that it must have expanded upon cooling. 



