HVPROLTHESTS“OFSCAUSE OF GLACIAL PERIODS. 555 
differential effects between the sea and land is lessened. On the 
other hand, in the absence of absorptive and diffusive effects, the 
tropical rays fall with full intensity upon the surface. On the 
land they promptly heat the immediate surface, and the heat is 
as promptly radiated away. On the sea, neglecting reflection, 
they penetrate deeply into the water until they are absorbed. 
The upper layer of the sea is therefore heated to a notable depth, 
and radiates its heat away with relative slowness. The result is 
‘an intensification of the differences in average temperature of 
the land and the sea. This action is quite familiar, but perhaps 
not the point here urged—that this difference is dependent on 
the atmospheric effects upon the incoming as well as outgoing 
rays. If the atmosphere were so far robbed of its absorbent 
factors, carbon dioxide and water, as to give great intensity to 
this differential effect, the result might be an average tempera- 
ture of the land below the freezing point, while that of the sea 
might be relatively warm. It seems clear that at some point 
short of an absolute thermal transparency a stage would be 
reached where the average temperature of the land would sink 
below zero, while yet the sea, barring convection in latitude, 
would retain a comparatively mild temperature. There would 
then apparently arise, even in low latitudes, the conditions of 
glaciation. 
Without following out these lines into greater detail, the 
more pertinent deductions may be summed up in the following 
propositions: A reduction of the thermal absorption of the 
atmosphere would intensify the differences of temperatures 
/ between (1) the basal and the upper portions of the atmosphere ; 
(2) low and high latitudes; (3) land and sea; (4) night and 
day; and (5) the seasons. In short, it would intensify tempera- 
ture differences generally, and would lead to (1) greater local 
heat, as well as greater local cold; (2) to greater local dryness, 
as well as greater local moisture; (3) to more intense move- 
ments of the atmosphere in the endeavor to maintain equilibrium ; 
and (4) to lower average temperature. The effect of reducing 
the absorbent factors is the intensification of differences. 
