the Distribution of Heat over the Globe. 93 



low that the error of assuming that the decrease of temperature 

 would be proportionate to the decrease in the intensity of the 

 sun's heat may not be great. 



It may be noticed here, although it does not bear directly on 

 this point, that although the air in a room, for example, or at 

 the earth's surface is principally cooled by convection rather than 

 by radiation, it is by radiation alone that the earth's atmo- 

 sphere parts with its heat to stellar space ; and this is the chief 

 matter with which we are at present concerned. Air, like 

 all other gases, is a bad radiator ; and this tends to protect it 

 from being cooled to such an extent as it would otherwise be, 

 were it a good radiator like solids. True, it is also a bad ab- 

 sorber ; but as it is cooled by radiation into space, and heated, 

 not altogether by absorption, but to a very large extent by con- 

 vection, it on the whole gains its heat more easily than it loses 

 it, and consequently must stand at a higher temperature than it 

 would do were it heated alone by absorption. 



But, to return ; the error of regarding the decrease of tempe- 

 rature as proportionate to the decrease in the amount of heat 

 received, is probably neutralized by one of an opposite nature, 

 viz. that of taking space at too high a temperature; for by so 

 doing we make the result too small. 



We know that absolute zero is at least 493° below the melt- 

 ing-point of ice. This is 222° below that of space. Conse- 

 quently, if the heat derived from the stars is able to maintain a 

 temperature of —239°, or 222° of absolute temperature, then 

 nearly as much heat is derived from the stars as from the sun. 

 But if so, why do the stars give so much heat and so very little 

 light ? If the radiation from the stars could maintain a thermo- 

 meter 222° above absolute zero, then space must be far more 

 transparent to heat-rays than to light-rays, or else the stars 

 give out a great amount of heat, but very little light, neither of 

 which suppositions is probably true. The probability is, I ven- 

 ture to presume, that the temperature of space is not very much 

 above absolute zero. At the time when these investigations into 

 the probable temperature of space were made, at least as n gards 

 that by Pouillet, the modern science of heat had no existence, 

 and little or nothing was then known with certainty regarding ab- 

 solute zero. In this case the whole matter would require to be re- 

 considered. The result of such an investigation in all probability 

 would be to assign a lower temperature to stellar space than —239°. 



Taking all these various considerations into account, it is pro- 

 bable that if we adopt —239° as the temperature of space^ we 

 shall not be far from the truth in assuming that the absolute 

 temperature of a place above that of space is proportionate to the 

 amount of heat received from the sun. 



