142 WRITINGS OP JOSEPH HENRY. [1855- 



We cannot however explain the effect of the temperature 

 of planetary space upon our earth until we have further con- 

 sidered the subject of heat. 



Heat of the Earth. 



The temperature of the earth is derived from three 

 sources, namely, the original heat of the earth, the heat of 

 celestial space, and the heat of the sun. Before however 

 giving an account of the heat derived from these sources, 

 we shall consider the character of radiant heat, as developed 

 by the researches of Melloni and others. 



Radiant Heat. — The impulses which are received from the 

 sun, as we have seen, are far from being simple in their 

 nature. We know that a beam from this luminary consists 

 of at least four different classes of emanations, namely, of 

 light, of heat, of chemical action, and of phosphorogenic 

 effect. We also know that the first class, that of light, con- 

 sists of a number of different emanations which produce in 

 us the sensations of the different colors of the spectrum, and 

 from analogy we might have inferred that the heat emana- 

 tions also consist of a number of rays, possessing different 

 properties, and producing at the surface of the earth differ- 

 ent physical and perhaps physiological effects. 



Let us begin with heat of the lowest intensity, or that 

 which is supposed to be composed of waves of the greatest 

 length; for example, the radiation from a canister of hot 

 water suspended in mid-air. If this have a temperature 

 in the least degree above that of the surrounding bodies, 

 they will increase in temperature, while the vessel itself will 

 slowly cool. The rapidity of cooling will gradually dimin- 

 ish in a geometrical ratio, as the temperature of the canister 

 approaches that of the surrounding bodies, and they will 

 finally arrive at a state of dynamic equilibrium. The can- 

 ister at this point does not cease to radiate, but continues to 

 send impulses in every direction, receiving as many im- 

 pulses from the surrounding bodies, (including the air,) as it 

 sends off from its own surface. 



