SCIENCE OF COMMON THINGS. 183 



Sea water is rarely frozen. How the depth of water influences freezing. 



them ; other flakes of ice (impinging against them) are 

 arrested in like manner ; and the edges of the different 

 flakes overlapping each other, make the surface rough. 



13O5 Why do some parts of a river freeze less tlian others ? 



Because springs issue from the bottom, and (as they 

 bubble upwards) thaw the ice, or make it thin. 



13O0 When persons fall into a river in winter-time, why does the water 

 feel remarkably warm 1 



Because the frosty air is at least ten or twelve de- 

 grees colder than the water is. 



The water below the surface is at least 42, but the air 32, or even 

 less. ' 



1207 Why is sea water rarely frozen 1 



1. Because the mass of water is so great, that it re- 

 quires a very long time to cool the whole volume down 

 to forty degrees ; 



2. The ebb and flow of the sea interfere with the 

 cooling influence of the air ; and 



3. Salt water never freezes till the surface is cooled 

 down to twenty-seven degrees, or five degrees below the 

 freezing point of fresh water. 



13O3 Why do some lakes rarely (if ever) freeze? f 



1. Because they are very deep ; and 



2. Because their water is supplied by 



springs which 

 bubble from the bottom. 



ISO 9 How does the depth of water influence its freezing 1 



It is necessary that the whole volume of water should 

 be reduced to forty degrees before the surface will 

 begin to freeze ; and the deeper the water, the longer it 

 will be before the whole volume is thus reduced. 



1310 Why do springs at the bottom of a lake prevent its freezing ? 



Because they keep continually sending forth water 

 having a temperature above that of the mass of the 

 water, which prevents the lake from being reduced to 

 the necessary degree of coldness. 



1311 It is more chilly in a thaw than in a frost : explain the reason 

 of this. 



When froz3n water is thawed, it absorbs heat from 



