TEMPERATUHE OF SrACE. 45 



It may at first sight seem hazardous to term the fearfully 

 low temperature of the regions of space (which varies 

 between the freezing point of mercury and that of spirits 

 of wine) even indirectly beneficial to the habitable climates 

 of the earth and to animal and vegetable life. But in proof 

 of the accuracy of the expression, we need only refer to 

 the action of the radiation of heat. The sun-warmed surface 

 of our planet, as well as the atmosphere to its outermost 

 strata, freely radiate heat into space. The loss of heat 

 which they experience arises from the difference of tem- 

 perature between the vault of heaven and the atmospheric 

 strata, and from the feebleness of the counter-radiation. How 

 enormous would be this loss of heat, 31 if the regions of space, 

 instead of the temperature they now possess, and which we 

 designate as 76 of a mercury thermometer, had a tempe- 

 rature of about 1400 or even many thousand times lower! 



It still remains for us to consider two hypotheses in relation 

 to the existence of a fluid filling the regions of space, of which 



si <( \Vere there no atmosphere, a thermometer freely ex- 

 posed (at sunset) to the heating influence of the earth's radia- 

 tion, and the cooling power of its own into space, would indicate 

 a medium temperature between that of the celestial spaces, 

 (132 Fahr.) and that of the earth's surface below it, 82 

 Fahr., at the equator, 3^ Fahr., in the Polar Sea. Under 

 the equator then it would stand, on the average, at 25 

 Fahr., and in the Polar Sea at 68 Fahr. The presence 

 of the atmosphere tends to prevent the thermometer so ex- 

 posed from attaining these extreme low temperatures : first, 

 by imparting heat by conduction ; secondly, by impeding 

 radiation outwards." Sir John Herschel, in the Edinburgh 

 keview, vol. 87, 1848, p. 222. " Si la chaleur des espaces 

 planetaires n'existait point, notre atmosphere eprouverait uii 

 refroidissement, dont on ne peut fixer la limite. Probable- 

 ment la vie des plantes et des animaux serait impossible a la 

 surface du globe, ou releguee dans une etroite zone de cette 

 surface." (Saigey, Physique du Globe, p. 77.) 



