CAUSE OF MILD POLAR CLIMATES. 145 



as — 239°, we make the difference between the tempera- 

 ture of the equator and that of the poles too small. 



Second, it was necessary to know at what rate the 

 temperature increased or decreased with a given 

 increase or decrease in the amount of heat received. 

 It was well known that Newton's law — that the 

 change of temperature was directly proportionate to 

 the change in the quantity of heat received — was far 

 from being correct. The formula of Dulong and Petit 

 was found to give results pretty accurate within 

 ordinary limits of temperature. But it would not 

 have done, in making my estimate, to take that 

 formula, if I adopted Herschel's estimate of the tem- 

 perature of space ; for it would have made the 

 difference of temperature between the equator and 

 the poles by far too small. Newton's law, if we adopt 

 Herschel's estimate of the temperature of space, would 

 give results much nearer the truth ; for the error of 

 the one would, to a large extent at least, neutralise 

 that of the other. 



From such uncertain data it was, of course, impos- 

 sible to arrive at results which could in any way be 

 regarded as accurate. But it so happens that perfect 

 accuracy of results in the present case was not 

 essential ; all that really was required was a rough 

 estimate of what the difference of temperature 

 between the equator and the poles ought to be. 

 The method adopted showed pretty clearly, how- 

 ever, that the difference of temperature could not 

 be less (although probably more) than 200°; but the 

 present actual difference does not probably exceed 80°. 

 We have no means of ascertaining with certainty what 

 the mean annual temperature of the poles is ; but as 

 the temperature of lat. 80° N. is 4 0, 5, that of the poles 

 is probably not under 0°. If the present difference be 



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