146 WRITINGS OF JOSEPH HENRY. [1855- 



heat removed from the earth, and the sun suddenly allowed 

 to shine upon it. In this case, all the rays which traversed 

 the atmosphere and reached the earth would be absorbed. 

 None would be radiated into space until the temperature of 

 the surface was so elevated that the rays emitted from it 

 could permeate the atmosphere. 



The surface of the earth at first would therefore receive 

 more rays than it gave off. Its temperature would increase 

 and with each increase of temperature a greater number of 

 rays would be produced of such intensity as would enable 

 them to permeate the atmospheric envelope, and finally an 

 equilibrium would be attained in which the rays sent off in a 

 given time would be just equal in number to those received. 



The point of temperature at which this equilibrium would 

 take place will depend on the height and permeability of the 

 atmosphere. If the aerial envelope offered no impediment 

 to the escape of heat of the lowest intensity, the equilibrium 

 would take place at so low a temperature that all bodies 

 capable of freezing would be perpetually in a solid state. If 

 on the other hand the atmosphere were more dense than it 

 is, or in other words, more impervious to rays of a higher 

 intensity than those which now pass through it, the tem- 

 perature of the surface of the earth would increase until the 

 heat given off would again be equal to that received. The 

 new equilibrium would be permanently retained, and the 

 whole average temperature of the surface of the globe would 

 be elevated. 



Heat from the Stars. — The temperature therefore of the sur- 

 face of a planet depends upon the nature of its atmosphere, 

 provided the heat which falls upon it is derived from a 

 source of high temperature : now radiations from the stars 

 are of this character, since they come from self-luminous 

 bodies, which are probably suns of other systems. The 

 radiations from them can therefore readily pass through 

 our atmosphere, and excite heat vibrations in the surface 

 materials of the earth. The intensity of these vibrations 

 must increase until it becomes so great that the radiations 

 produced can permeate the aerial covering, and in this way 



