On the Cause of Mild Polar Climates. 269 



the temperature of space, then only 319° of the absolute tem- 

 perature of the equator are derived from the sun ; consequently 

 as the poles receive from the sun only ^ of this amount of 

 temperature, or 133°, this will give merely 186° as the dif- 

 ference which ought to exist between the equator and the 

 poles. There is, however, good reason for believing that the 

 temperature of space is far less than that assigned by Pouillet 

 and Herschel — that, in fact, it is probably not far above absolute 

 zero. Therefore by adopting so high a temperature as —239° 

 we make the difference between the temperature of the equator 

 and that of the poles too small. 



Second, it was necessary to know at what rate the tempe- 

 rature 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 temperature 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, neutralize that of the other. 



From such uncertain data it was, of course, impossible to 

 arrive at results which could in any way be regarded as accu- 

 rate. 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, however, that the difference of 

 temperature could not be less (although probably more) than 

 200°; but the present actual difference does not probably ex- 

 ceed 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° # 5, that of the poles is probably 

 not under 0°. If the present difference be 80°, it is then 120° 

 less than it would be did the temperature of each place depend 

 alone on the heat received directly from the sun. This great 

 reduction from about 200° to 80° can, of course, be due to no 

 other cause than to a transference of heat from the equator to 

 the poles. The question then arose, by what means was this 

 transference effected ? There were only two agencies avail- 

 able — the transference must be effected either by aerial or by 



