28 HEAT 



also that the moving liquid or gas carries heat which it grad- 

 ually gives out to surrounding bodies. When a liquid or a gas 

 moves away from a hot object, carrying heat with it, the process 

 is called convection. 



Convection is responsible for winds and ocean currents, for 

 land and sea breezes, and other daily phenomena. The Gulf 

 Stream illustrates the transference of heat by convection. A 

 large body of water is strongly heated at the equator, and 

 then moves away, carrying heat with it to distant regions, 

 such as England and Norway. 



Owing to the shape of the earth and its position with respect 

 to the sun, different portions of the earth are unequally heated. 

 In those portions where the earth is greatly heated the air 

 likewise is heated, and there is a tendency for the air to rise, be- 

 cause the cold air from surrounding regions rushes in and forces 

 the less dense air upward. In this way winds are produced. 

 There are many circumstances which modify winds and cur- 

 rents, and it is not always easy to explain their direction and 

 velocity, but one very definite cause of winds and currents 

 is the unequal heating of the surface of the earth. 



Conduction. A poker used in stirring a fire becomes hot, 

 and heats the hand grasping the poker, although only the 

 opposite end of the poker has actually been in the fire. Heat 

 from the fire passes into the poker, travels along it, and 

 warms it. When heat flows in this way from a warm part 

 of a body to a colder part, the process is called conduc- 

 tion. A flatiron is heated by conduction, the heat from the 

 warm stove passing into the cold flatiron and gradually heat- 

 ing it. 



In convection, air and water circulate freely, carrying heat 

 with them ; in conduction, heat flows from a warm region 

 toward a cold region, but there is no apparent motion of any 

 kind. 



