Change of Climate during Geological Epochs. 131 



corresponds with the summer in the other, it follows that while 

 the one hemisphere would be enduring the greatest extremes of 

 summer heat and winter cold, the other would be enjoying a 

 perpetual spring. 



It is quite true that whatever may be the excentricity of the 

 earth's orbit, the two hemispheres must receive equal quantities 

 of heat per annum ; for proximity to the sun is exactly com- 

 pensated by the effect of swifter motion. The total amount of 

 heat received from the sun between the vernal and autumnal 

 equinoxes is the same in both halves of the year, whatever the 

 excentricity of the orbit may be. For example, whatever extra 

 heat the southern hemisphere may at present receive from the 

 sun during its summer months in consequence of greater 

 proximity to the sun, is exactly compensated by a corresponding 

 loss arising from the shortness of the season ; and, on the 

 other hand, whatever deficiency of heat we in the northern 

 hemisphere may at present have during our summer half year 

 in consequence of the earth's distance from the sun, is exactly 

 compensated by a corresponding length of season. 



But the surface temperature of our globe depends as much 

 upon the amount of heat radiated into space as upon the 

 amount derived from the sun. It will be observed, however, that 

 this compensating principle holds only true in regard to the 

 heat directly received from the sun. In the case of the heat 

 lost by radiation the reverse takes place. The southern hemi- 

 sphere, for example, has not only a colder winter than the 

 northern, in consequence of greater distance from the sun, but 

 it has also a longer winter. And this extra loss of heat from 

 radiation is not compensated by its nearness to the sun during 

 summer months ; for, as we have already seen, it gains nothing 

 in consequence of proximity. And on the same principle our 

 winter in the northern hemisphere, in consequence of our 

 proximity to the sun, is not only warmer than that of the 

 southern hemisphere, but is also at the same time shorter. 

 Consequently our hemisphere is not cooled to such an extent as 

 the southern. It follows therefore, other things being equal, that 

 the mean temperature of the winter half year, as well as the 

 intensity of the sun's heat, must be inversely as the square of 

 the sun's distance. But it is not on this change in the mean 

 winter temperature, as we shall presently see, that the change of 

 climate chiefly depends. 



The Climate of the Carboniferous Epoch. 

 It is the generally received opinion among both geologists 

 and botanists that the flora of the coal-period does not indicate 

 the existence of a tropical, but a moist, equable, and temperate 



K2 



