SOME EFFECTS OF HEAT 145 



mains at the top and, if its temperature continues to fall 

 to C., it changes to ice. 



You see, therefore, that practically all of the water 

 in a lake or pond must reach a temperature of 4 C. before 

 any ice is formed. Consequently, it is easy to see why 

 deep bodies of water take so much longer to freeze over 

 than do shallow ones. Also it is easy to see why lakes and 

 ponds do not freeze solid to the bottom, and why fish and 

 other forms of life get along quite comfortably 

 under the ice. The ice forms a protecting 

 barrier through which the heat of the water 

 below it penetrates only very slightly. So 

 the water under the ice remains at about 

 4 C.; this temperature permits life to continue 

 which would cease to exist if the freezing- 

 point were reached. 



You have noted that changes in the tem- 

 perature of the water cause movements or cur- . 



FIG. 57- Con- 

 rents, as a result of which any reduction of vection cur- 

 temperature tends to become evenly distrib- 

 uted throughout the whole mass. (Would an increase of 

 temperature at the surface of a body of water have the 

 same effect?) Evidently, here is a means whereby heat 

 is distributed or transported from one place to another. 

 You have learned that heat migrates through solids by 

 means of conduction. You now note that it may migrate 

 through liquids (and the same is true for gases) by means 

 of convection, for these movements of liquids or gases 

 caused by their becoming warmer and therefore lighter 

 than their surroundings, or colder and therefore heavier, 

 are called convection movements or convection currents (see 

 Fig. 57). It is due to convection that the warmer air is 



