24 HEAT 



111 the preceding Section, we learned that many houses 

 heated by hot water are supplied with fresh-air pipes which 

 admit cold air into separate rooms or into suites of rooms. 

 In some cases the amount which enters is so great that the 

 air in a room is changed three or four times an hour. The 

 constant inflow of cold air and exit of warm air necessitates 

 larger radiators and more hot water and hence more coal to 

 heat the larger quantity of water, but the additional expense 

 is more than compensated by the gain in health. 



12. Winds and Currents. The gentlest summer breezes 

 and the fiercest blasts of winter are produced by the unequal 

 heating of air. We have seen that the air nearest to a stove 

 or hot object becomes hotter than the adjacent air, that it 

 tends to expand and is replaced and pushed upward and 

 outward by colder, heavier air falling downward. We have 

 learned also that the moving liquid or gas carries with it heat 

 which it gradually 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 dis- 

 tant 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 un- 

 equally heated. In those portions where the earth is greatly 

 heated, the air likewise will be heated ; there will be a 

 tendency for the air to rise, and for the cold air from sur- 

 rounding regions to rush in to fill its place. In this way 

 winds are produced. There are many circumstances which 

 modify winds and currents, and it is not always easy to ex- 



