AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 657 



25. Let us further consider what the superficial temperature of our globe would be 

 independently of the Sun's heat. Since h and * would then vanish, the corresponding value 

 of the temperature w would become equal to \//. Now we have, generally, 



J/ = u — hQ — 8 

 = u - 1,93 hQ, 

 an equation which gives \|/ when u and hQ are given for any latitude. Since \f/ is by 

 hypothesis constant, the same value would be obtained for every latitude. Thus taking any 

 of the above cases for which u and hQ are given, we find 



f = - 39°,5 (C), 

 which is also the required value of m . 



It must be borne in mind that this value of \f/ is not the temperature of that portion 

 of stellar space in which the solar system is placed (considered independently of the heat 

 radiating from the Sun), but that uniform temperature which, in the absence of solar heat, 

 would exist at every point of the Earth's surface. It would differ from the temperature of 

 surrounding space, either in excess or defect, in consequence of the Earth's atmosphere, as 

 already explained. The above value of *]/ expresses the limit towards which the mean 

 superfici.il temperature of our globe would ultimately approximate if the solar heat were 

 gradually to diminish and become finally extinct, supposing always, that all other external 

 conditions should remain unaltered. 



We have here been considering mean temperatures. Let us now proceed to the con- 

 sideration of annual variations of terrestrial temperature. 



26. Though the mean annual temperature of the lowest portion of the atmosphere as 

 given by a thermometer sheltered from the direct rays of the Sun, at any place, may be 

 considered the same as the mean annual temperature of the Earth's surface at that place, 

 it is manifest that there can be no such accordance between their respective variations 

 of temperature, where the periods of such variations are sufficiently short. The daily 

 changes of temperature in the atmosphere are frequently far greater than the corresponding 

 changes in that of the surface of the Earth at the same place. To make some approximate 

 comparison between the annual variations of the temperature of the surface of the Earth, and 

 those of its atmosphere, I have extracted the following table from M. Dove's Work, already 

 referred to. It gives the difference between the temperature of those two months in the year, 

 for one of which the temperature is greater, and for the other less, than for any other month, 

 under different parallels of latitude respectively, the monthly temperatures of each parallel 

 being taken as the mean of the monthly temperatures of different places situated upon that 

 parallel. Calling these differences D^.^, &c., we have 



D x = 2° (C) in latitude 10*, N. 



Z> 8 = 6°,5 20°, 



D, = 12°,22 30°, 



D t = 17°,90 40°, 



Z> 5 = 23°,75 50°, 



A = 29°,35 60°, 



84 — 8 



