NATURAL PHILOSOPHY. 155 



This 'radiating heat, therefore, has little or no effect in heating the atmo- 

 sphere during its transmission to the earth's surface ; but after falling upon, 

 and heating terrestrial objects, it loses the power of radiating completely 

 through the atmosphere, and is transmitted back into space through the 

 atmosphere by conduction, conversion, and partial radiation to limited dis- 

 tances. But for any of these modes of transmission, it is essential that the 

 temperature of the atmosphere should be greater hi its lower than ha its upper 

 portions, and in a degree greater as the quantity of heat to be transmitted is 

 greater. The temperature (r v ) of the upper portion must be determined by 

 the condition, that in a given time a quantity of heat must radiate from it 

 into surrounding space equal to that which falls upon it from external sources 

 in the same time, and is transmitted back after reaching the surface of the 

 earth or objects near to it. Consequently r 2 must be independent of the 

 height of the earth's atmosphere. At lower points the temperature will 

 increase till we reach the surface of the Earth ; and if we denote the tempera- 

 ture there by T, , it is manifest that r, will be greater, the greater the 

 height of the earth's atmosphere. 



It must here be particularly observed, that T Z is the proper temperature of 

 the component particles of the atmosphere, and is probably widely different 

 from the temperature which would be indicated by a thermometer placed at 

 the upper extremity of the atmosphere, since the instrument would not only 

 be affected by the exchange of heat between its bulb and the atmospheric 

 particles, but also by the heat radiating upon its bulb from every source of 

 heat in surrounding space ; while the atmosphere, on account of its diather- 

 mancy, would remain unaffected by this radiating heat. 



Conceive now a thermometer to be placed at a point sufficiently above the 

 earth's atmosphere. If the bulb were sheltered from the direct influence of 

 the solar rays, the thermometer would indicate the temperature of that point 

 of space, independent of the effect of radiation from the central luminary of 

 the solar system, but dependent on the radiation from all other sources of heat 

 hi the universe. If the instrument thus sheltered were sufficiently remote 

 from the sun and every planet, it would indicate very nearly the same tem- 

 perature at every point within the solar system, assuming the absence of all 

 unknown centres of heat within that system or near to it. 



This is what may be understood by the general temperature of planetary 

 space. Let it be denoted by T. We shall then have T greater than r 2 ; 

 and therefore if we now conceive the thermometer to be transported to the 

 upper limit of the atmosphere, it will be affected by the lower temperature 

 there, and wall indicate a temperature intermediate to T and r 2 . If the 

 instrument be brought still lower within the atmosphere, it will indicate a 

 still lower temperature, from its being entirely surrounded by a portion of the 

 atmosphere more dense than at the extreme boundary, till this tendency to 

 lower the indications of the thermometer is counteracted by the greater 

 temperature of the atmospheric particles as we descend towards the earth's 

 surface. At some point, consequently, within the earth's atmosphere the 

 indication of the thermometer would attain its minimum; after which, in 

 descending continuously towards the earth, the temperature indicated 



