352 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



is insufficient to explain completely the exceptionally favorable condi- 

 tions these polar regions have sometimes experienced. Other causes 

 must have concurred to produce this result. 



The change which the obliquity of the ecliptic undergoes has for a 

 long time attracted the attention of geologists and physicists, but it has 

 generally been concluded that this cosmic cause could not sensibly 

 modify the mean temperature of the terrestrial zones. M. Croll* has 

 examined this subject with unusual intelligence, and has come to the 

 conclusion that in fact the change of obliquity of the ecliptic will not 

 sensibly influence the temperate zone, but there will result a slight 

 change in the tropical climate, and that the effect will be considerable 

 upon the temperature of the polar regions. 



These conclusions have been drawn from the following considerations: 

 According to Laplace, the obliquity oscillates 1° 23' 34" from two sides 

 of the obliquity in 1801. The quantity of heat received by the equator 

 from the sun in our day is represented by 365.24 thermal days, by the 

 poles 151.59 thermal days. When the obliquity attains its maximum, 

 24° 50' 34", the quantity of heat received by the equator will be 363.51 

 days, and by the poles 160.04 days. The equator will then experience 

 a decrease in the quantity of heat represented by 1.73 thermal days, 

 and the poles an addition of 8.45. When the obliquity of the ecliptic 

 reaches the maximum, the quantity of heat received by the poles will be 

 greater by an eighteenth than that it now receives ; in other words, the 

 poles will then receive as much heat as the 76th degree of parallel re- 

 ceives at the present time. 



The greater obliquity would not change sensibly the mean tempera- 

 ture of the polar winters, although the cold zone (as well as the tropical 

 zone) might be more extended. For, after the disappearance of the sun 

 below the horizon, its rays are almost completely intercepted, so that a 

 decrease of 1° 20' 34" would not modify sensibly the temperature of the 

 poles. In the temperate regions the sun would to the same degree be 

 less elevated during the winter, which would slightly increase the cold. 

 The quantity of heat that the poles receive during the summer would 

 be, on the contrary, enough increased to modify the mean temperature of 

 the polar summer. In fact 8.45 thermal days in addition to an eighteenth 

 of the total quantity of heat received from the sun, would raise the 

 mean temperature 7° or 8° centigrade. 



If to this direct augmentation of the solar heat we add the constant 

 transfer of heat toward the poles and of cold toward the equator, we will 

 obtain results which will explain in a satisfactory manner the elevation 

 of temperature of certain geological periods. It is the fluid envelope of 

 the globe which most contributes to the equalization of temperature ; 

 the aerial currents transport to the poles a certain quantity of heat, but 

 much less than that carried by the oceanic currents. The Gulf Stream 

 alone transfers to the polar regions a quantity of heat equivalent to that 

 received by 320,000 square leagues at the surface of the equator. 



* Phil Magaz., June, 1867, p. 130. 



