666 Mb W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, 



Hence we have 



hQ = 48° (C) at the Equator, 



= 49° in lat. 15°, 

 = 67°,6 at the Pole ; 

 and consequently, taking, as before, the mean temperature 



= - 39" ,5 + 2hQ, 

 we have the mean annual temperature of Venus 



= 56°,5 (C) at the Equator, 

 = 58°,5 in lat. 15°, 



= 95°,7 at the Pole. 



These would be the temperatures supposing the atmosphere to be exactly like that of the 

 Earth, and assuming the absence of all horizontal transference of heat. The greater heat, 

 however, would necessarily produce a greater evaporation, and the atmosphere would conse- 

 quently be far more laden with moisture, so that the absorption of the solar heat in its passage 

 to the surface of Venus would probably be considerably greater than in the case of the Earth. 

 The value of h would be thus reduced. Let it become 70' — t. Then will each of the values 

 of the above mean temperatures be reduced by 2Qt. Again, on account of the horizontal 

 transference of heat from the pole, now become the point of maximum temperature, the mean 

 temperature there would undoubtedly become considerably lowered. Let it be thus diminished 

 by i ; then the mean temperature at the pole would become 



= 95 u ,7 -2Qt-t' 

 = 95°,7 — 2t — t' very nearly. 

 The mean temperature at the equator would also be depressed 2Qt by the diminution of h, 

 and would probably be somewhat raised by the tendency to equalization of temperature. It 

 would therefore be 



56°,5 - 2Qt + t", 

 where Q = ,685. 



These results would further be subjected to the modification due to an increase or diminu- 

 tion of atmosphere as compared with that of the Earth. If the atmosphere of Venus be equal 

 to that of the Earth diminished by a quantity equal to that which forms a stratum resting on 

 the Earth's surface, and whose thickness = a, the mean temperature would be reduced gene- 

 rally by about l°,85a, the unit of length being = 1000 feet (Art. 15). Hence the mean tempe- 

 rature at the pole would 



= 95°,7 -U - ( - l°,85a, 

 and that at the equator would 



= 56°,5 - 1,37 « + t" - l°,85a. 

 If the atmosphere of Venus be supposed equal to that of the Earth, the value of t would, 

 I conceive, be necessarily positive ; but if we suppose that atmosphere to be diminished, as 

 implied by the last term in the above expressions, its value would be reduced and might 

 become negative, but it would seem extremely improbable that the transparency of Venus's 

 atmosphere should so far exceed that of the Earth as to render any negative value of t 

 otherwise than very small. On the contrary, a diminished transparency of the atmosphere of 



