FV POTHESES: OF CAUSE OF GLACIAL PERIODS 553 
are spread over greater space, and are subject to greater relative 
reflection because of the low angle of incidence. In the case of 
an earth swathed in a thermally absorptive mantle, the direct 
equatorial ray is absorbed in traversing the atmosphere to a less 
extent than the ray of higher latitudes because it penetrates a 
less depth of atmosphere, while the amount of reflection from 
the atmosphere is small in both cases, so far as the transparent 
elements are concerned. In so far therefore as the temperature 
effects are dependent upon the absorption of the incoming rays, 
the greater depth of atmosphere penetrated in the higher lati- 
tudes makes important compensation for obliquity of incidence. 
In the case of a naked earth, or an earth clothed with a non- 
absorptive atmosphere, the solar rays which are tangential to the 
polar regions have no heating influence upon the earth, while in 
the case of an earth clothed with an absorbent atmosphere, simi- 
lar rays are partially absorbed, and serve to warm the earth. In 
the polar regions, therefore there is a very radical difference 
between the effects of an absorbent and a _ non-absorbent 
atmosphere. The same holds true of the heating effects of the 
morning and evening sun. With a non-absorbent atmosphere, 
the tangential morning and evening rays pass through and are 
lost; in a thermally absorbent atmosphere, they are effectually 
retained. 
In so far as the incoming rays are absorbed in the atmosphere 
their immediate effects are chiefly felt in its upper strata and their 
influence upon the surface of the earth is lessened. This is due to 
the fact that the upper regions are penetrated by rays of all the 
various wave-lengths that emanate from the sun, while the lower 
portions are penetrated only by such rays as are left after the 
selective absorption of the upper atmosphere. The degree of 
this absorption of the rays of long wave-lengths is such that com- 
paratively little further absorption takes place in the basal por- 
tion of the atmosphere, according to the interpretations of 
Arrhenius. One effect of increasing the absorptive capacity of 
the upper air by increasing the amount of carbonic acid is an 
increase in the elevation of the strata chiefly heated by the 
