RADIATION. 39 



equatorial regions, which are constantly receiving ex- 

 cess of heat from the sun, would soon become unin- 

 habitable by reason of its accumulation, and the polar 

 regions, from extreme cold. The currents or winds 

 thus produced are subject to great irregularities, which 

 are considered in works on natural philosophy. 



RADIATIOK 

 76. The general laws of radiation 



What are the 



laws of the are the same for heat as for light. Rays 

 eat? i0n f of heat divei 'g e constantly from all points 

 of the surface of all bodies, in straight 

 lines and in every direction ; and the intensity of heat 

 varies inversely as the square of the distance. The 

 latter point is explained in the chapter on light. 



77. HEAT is RADIATED FROM ALL BODIES. 



Illustrate the . . 



fact that heat It is to be observed that while light pro- 

 in alW radiatl~d cee( * s onlv ^ roni certain bodies, heat pro- 

 from bodies, ceeds from all points of all bodies without 

 exception. If the mercury in a thermometer were fro- 

 zen by extreme cold, and then hung in a cavity made 

 for the purpose in a block of ice, radiation of heat from 

 the ice would melt it, even if there were no air in the 

 cavity to help melt it by conduction. 



78. PROPORTION OF RADIATION TO TEM- 



What can be 



said of the pro- PERATURE. The hotter a stove is the more 



heat il S lves Ollt ' This is obvious, and we 

 might naturally suppose that a stove twice 

 as hot as another stove, compared with other objects 

 about it, would give out heat just twice as fast. It 



