AND THE OTHER PLANETS OP THE SOLAR SYSTEM. 635 



9. It has already been stated (Art. 5), that if the density of the shell (A) in our 

 hypothetical case were sufficiently small at its outer boundary, the maximum effect in 

 depressing a thermometer entering the upper region of the shell from the surrounding 

 space would not necessarily be at the extreme boundary of the shell ; and since the 

 density of the atmosphere must be extremely small in its upper regions, it is very 

 possible that the point of maximum depression of a thermometer moved gradually from 

 stellar space to the Earth's surface, would be found at a considerable distance within 

 the upper limit of the atmosphere, since the effect on the thermometer will depend on the 

 number of atmospheric particles within a certain distance of the bulb, as well as on their 

 proper temperature. Thus if the thickness of an atmosphere like the Earth's were 

 smaller than the distance of the point of maximum thermometric depression from 

 the external boundary of the atmosphere, an increase of the quantity of atmosphere might 

 depress the temperature of the planet's surface. If, on the contrary, the thickness of 

 the atmosphere be greater than the above-mentioned distance, an increase of atmosphere 

 must necessarily produce an increase in the planet's superficial temperature. We are 

 utterly ignorant respecting the lowest temperature which might thus be produced, or the 

 amount of atmosphere which might be required for the purpose. We are certain, however, 

 that the point of maximum thermometric depression in the Earth's atmosphere must be 

 at a great distance above the Earth's surface, and, therefore, that the quantity of atmosphere 

 which would produce the minimum temperature at that surface must be very much less 

 than that which actually envelopes our own globe. 



10. It appears then from this general reasoning that, assuming the absence of the 

 Sun, the atmosphere in which a planet is enveloped may, according to its extent, either 

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

 space. We have no means, however, of determining, even in the case of our own planet, 

 which of these effects is produced, on account of our imperfect acquaintance with the 

 conducting, radiating, and absorbing powers of the atmosphere, our ignorance of the state 

 in which it exists in its upper regions, and of the temperature of space beyond. But 

 whatever may be the actual effect of the existing atmosphere of our globe, we are sure 

 that an increase or diminution of its quantity would produce a corresponding increase or 

 diminution of the existing superficial terrestrial temperature, because we know that the 

 temperature indicated by the thermometer descends as we ascend from the Earth's surface ; 

 and this is the important point as regards our object of ascertaining what may be the 

 influence of an atmosphere enveloping any other planet according as that atmosphere may 

 be greater or less than our own. 



11. If the radiation of heat upon the Earth or other planets from different portions 

 of space should be very different, the same general reasoning would be applicable, and 

 would lead to similar conclusions. Whatever may be the quantity of heat which has to 

 be transmitted back into surrounding space through the atmosphere, and whatever may 

 be its original source, the greater the thickness of the atmosphere, the greater must be 



