400 Cambridge Philosophical Society : — 



rature there bj' Tj, it is manifest that r, will be greater, the greater 

 the height of the Earth's atmosphere. 



It must here be particularly observed, that r, is the proper tem- 

 perature of the component particles of the atmosphere, and is pro- 

 bably widely different from the temperature which would be indi- 

 cated by a thermometer placed at the upper extremity of the atmo- 

 sphere, since the instrument would not only be affected by the ex- 

 change 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 depen- 

 dent on the radiation from all other sources of heat in 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 pla- 

 netary space. Let it be denoted by T. We shall then have T greater 

 than Ta ; 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 will indicate a temperature in- 

 termediate to T and To. 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 that 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 ininimum ; after which, in descending continuously towards 

 the Earth, the temperature indicated would constantly increase, 

 omitting variations due to temporary or local causes. Thus it fol- 

 lows that the existence of an atmosphere like that of the Earth, en- 

 veloping a planet, may, according to its extent, either elevate the 

 superficial temperature of the planet above, or depress it below that 

 of surrounding space independently of the direct solar radiation. 

 With respect to our own globe, we are entirely ignorant of the height 

 to which the thermometer, in ascending, would continue to indicate 

 a decreasing temperature, but we are sure that such height is great. 

 This is important with reference to the ultimate object of this paper; 

 for if the height of a planet's atmosphere w'ere too small to allow a 

 thermometer descending in it to attain its minimum indication, it is 

 manifest that an increase of atmosphere would cause a decrease in the 

 planet's superficial temperature ; whereas if the height of the atmo- 

 sphere were great enough to allow the thermometer to attain the mini- 

 mum, any increase of atmosphere would necessarily cause an increase in 



