104 Introduction to the Study of Science 



tion currents rise, and the air may be absolutely still. The 

 only means of conduction in this case is the air. 



How the sun's energy reaches the earth. Let us consider 

 this matter with reference to the way in which the sun's energy 

 is transmitted to our earth. Objects upon which the sun 

 shines are warmed. But is the air warmed through which the 

 sunlight passes? If the air transmits sunlight by conduction, 

 it must be warmed to at least the same degree as are the objects 

 upon the earth. Experience, however, affords many facts which 

 indicate that this is not the case. On the desert when the sun 

 shines directly upon the sand, the reflected heat from the sand 

 is intense. But the air is comparatively cool, as one realizes 

 on entering a shaded place. Mountain climbers and explorers 

 in the high latitude of the Polar regions quickly learn that the 

 sun's rays are cruelly hot, blistering face and hands when 

 exposed to them directly ; but the air may be at a temperature 

 very many degrees below zero. 



Aeronauts ascending in balloons about seven miles find the 

 cold unendurable. Registering thermometers and barometers 

 sent far higher in experimental balloons furnish evidence that 

 the temperature rapidly becomes lower with increasing alti- 

 tude, and the air itself exceedingly rare. The atmosphere 

 does not extend above the surface of the earth more than two 

 hundred miles (see pages 12 and 391 ff.). 



These facts seem to prove that the air cannot be a conductor 

 of the sun's energy to the earth, any more than it can be a con- 

 ductor of heat from range or fireplace to adjacent objects. The 

 air does not reach far enough toward the sun to be a conductor. 



In fact air is one of the worst conductors of heat. It is only 

 one twenty-fifth as good a conductor as water, and water is 

 about one hundred times worse than iron. Evidently the sun's 

 energy is not conducted by the air, but transmitted by a third 

 method. 



Glass is a better conductor than air or even water. The sun 

 shines through glass windows and warms objects upon which 



