432 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



According to Professor Dove* there is a difference of 7 0, 6 

 between the mean annual temperature of lat. 76° and the pole, — 

 the temperature of the former being 9° # 8, and that of the latter 

 2°*2. Since it follows that when the obliquity of the ecliptic is 

 at a maximum the poles would receive about as much heat per 

 annum as lat. 76° receives at present, it may be supposed that 

 the temperature of the poles at that period ought to be no higher 

 than that of lat. 76° at the present time. A little consideration 

 will, however, show that this by no means ought to be the case. 

 Professor Dove's Tables represent correctly the mean annual 

 temperature corresponding to every tenth degree of latitude from 

 the equator to the pole. But it must be observed that the rate 

 at which the temperature diminishes from the equator to the 

 pole is not proportionate to the decrease in the total quantity of 

 heat received from the sun as we pass from the equator to the 

 pole. Were the mean annual temperature of the various lati- 

 tudes proportionate to the amount of direct heat received, the 

 equator would be much warmer than it actually is at present, 

 and the poles much colder. The reason of this is perfectly ob- 

 vious. There is a constant transference of heat from the equator 

 to the poles, and of cold from the poles to the equator. The 

 warm water of the equator is constantly flowing towards the 

 poles, and the cold water at the poles is constantly flowing to 

 the equator. The same is the case in regard to the aerial cur- 

 rents. Consequently a great portion of the direct heat of the 

 sun goes, not to raise the temperature of the equator, but to 

 heat the poles. And, on the other hand, the cold materials at 

 the poles are transferred to the equator, and thus lower the tem- 

 perature of that part of the globe to a great extent. The pre- 

 sent difference of temperature between lat. 76° and the pole, 

 determined according to the rate at which the temperature is 

 found to diminish between the equator and the pole, amounts 

 to only about 7° or 8°. But were there no mutual transference 

 of warm and cold materials between the equatorial and polar 

 regions, and the temperature of each latitude to depend solely 

 upon the direct rays of the sun, the difference would far exceed 

 that amount. 



Now, when the obliquity of the ecliptic was at its superior 

 limit and the poles receiving about ^ more direct heat from 

 the sun than at present, the increase of temperature due to this 

 increase of heat would be far more than 7° or 8°. It would 

 probably be nearly double that amount. 



The enormous effect that ocean-currents have in equalizing 

 the temperature of our globe, by diminishing the difference be- 

 tween the temperature of the equator and the poles, has never 

 * Distribution of Heat on the Surface of the Globe, p. 14. 



