138 THE WATERS OF THE EARTH 



such tremendous force that it exerts a pressure of more than 

 100 tons to the square foot. No wonder it bursts water 

 pipes, splits rocks and concrete sidewalks, and heaves the 

 foundations of buildings that have not been laid below 

 " frost line." After ice has once formed, it again begins 

 to contract as the temperature is lowered, but it never 

 reaches the density of water. i 



It can easily be seen why any river or lake or other 

 body of water freezes from the top down. Since water at 

 the freezing point is less dense and therefore 

 lighter than slightly warmer water, it remains 

 at the surface, where it freezes. Ice is even 

 BOMB BURST lighter than water at the freezing point, and 

 BY FREEZING so ft floats. As soon as ice has formed over 

 the surface, it acts as a blanket, allowing the 

 heat to escape only very slowly from the water underneath. 

 Thus the ice increases in thickness so slowly that spring 

 comes before a deep body of water can freeze to the bottom ; 

 and so fish and other forms of water life never become 

 chilled below freezing nor suffer serious inconvenience. 



Ability of Water to Absorb Heat. We have already 

 learned that it takes more heat to raise a given mass of 

 water one degree of temperature than to cause a like in- 

 crease in temperature in an equal mass of almost any other 

 substance. This was shown in Experiment 29. When 

 water cools, it gives out the heat it took up when its tempera- 

 ture was raised. A pound of water in cooling one degree 

 gives out about as much heat as a pound of iron in cooling 

 nine degrees. It is for this reason that hot-water furnaces 

 are so efficient, that hot-water bags are used to keep people 

 warm, and that farmers sometimes in winter carry down 



