82 



HEAT AND ENERGY 



Without convection, stoves and lamps would need to be 

 blown from beneath all the time. Practically all winds 

 are started by convection, heated air somewhere being 



set in motion by cold 

 air pressing upon it. 



93. Radiation. Ra- 

 diation is the transfer of 

 heat by vibrations of the 

 ether. This statement 

 may perhaps mean very 

 little as it stands, be- 

 cause it brings up an 

 idea which may be new 

 to most of us. The 

 transfer of energy by 

 as. radiation is so very im- 

 portant, however, that 

 a great effort should 

 be made in trying to 

 understand it. 

 First of all, an example of the transfer of heat by 

 radiation may be helpful. If a fire be kindled in a cold 

 room, objects in the room may become warm some time 

 before the air between them and the stove is equally 

 heated ; clearly the heat is not conducted by the air to 

 the objects, nor does it travel to them by convection. 

 Similarly the sun's heat warms the earth ; yet we know 

 that the space between the sun and the earth contains 

 no matter that is dense enough to carry heat by conduc- 

 tion or convection. 



FIG. 



