RADIATION. 43 



of heat. If one is hotter than the rest, it gives faster 

 than it receives, until the equilibrium is reached. And 

 if, while they are thus coming to the same temperature, 

 one is a good reflector, and therefore slow to receive 

 the heat which comes to it, it is also slow to part with 

 what it gets ; thus the difference of reflecting power 

 is without influence. 



85. COOLING OF THE EARTH. Were it 



slid** of the not f r the SU11 > tne heat f tne eart h wolll d 

 cooling of the waste away very rapidly into space. It is, 



in fact, radiated into space now, as truly 

 as if there were no sun or stars, but these make tip for 

 the loss. At night, when the sun is below the horizon, 

 the waste by radiation takes place very rapidly, and 

 the earth and air grow colder in consequence. It 

 is not simply because of the absence of the direct 

 heat of the sun, for this is removed at once when the 

 sun sets, while the cooling proceeds until morning. 

 As the earth, being solid, is a better radiator than 

 the air, it cools most rapidly, sending out its heat 

 through the air into space. In this way the earth often 

 becomes cooled from ten to twenty degrees lower than 

 the air above it. 



86. ICE IN THE TROPICS. Advantage 



How is ice pro- ... 



duccd in the is taken of the cooling which occurs by ra- 

 trojncs? diation, to produce ice, in countries where 



the temperature of the air does not fall to the freezing 

 point. Water contained in shallow vessels, placed in 

 trenches dug in the ground, to protect it from currents 

 of warm air, becomes covered with ice by a night's ex- 



