658 LOWELL—MARS ON GLACIAL EPOCHS. [Nov. 16, 
So that the minimum increases relatively to the maximum faster in 
the southern hemisphere than in the northern one. And as this 
relative increase never changes sign, as the precipitation increases 
a time must come when the southern minimum will actually exceed 
the northern one in size and do so more and more, indefinitely. 
Whence we find ourselves facing the interesting conclusion that 
with precipitation increased equally over the whole planet the size 
of the perpetual ice-cap at the southern pole would finally surpass 
that about the northern one. Whereas, then, with moderate pre- 
cipitation the hemisphere with the extremes of summer and winter 
climate would have the less perpetual ice of the two; with more 
precipitation the result would be reversed. 
21. On the earth this greater precipitation is made possible by 
the greater amount of water on the surface. Thus a glacial period 
might be produced with us under the very same conditions which 
would bar it on Mars. It would come about in consequence of the 
eccentricity of the orbit, but not chiefly because of that eccentricity. 
Rather, we may say, because of the amount of moisture capable of 
being manufactured. For were the moisture to fall below a definite 
amount, not only would no glacial period result no matter what the 
eccentricity, but actually a sort of anti-glacial epoch would be 
brought about by that very same cause. 
Croll distinctly emphasizes the fact that it is the indirect not the 
direct effect of the eccentricity that causes a glacial period. This 
indirect effect he follows through increased precipitation to 
change of the winds and lastly to change of oceanic currents. What 
the present study of the problem appears to point out is that increased 
precipitation alone, from any cause whatever, is competent to the 
task. 
22. Here, then, we have a remarkable reversal in arctic conditions 
from glacial to something not unakin to its opposite, due directly, 
not to change in eccentricity, though it presupposes eccentricity in 
the process, but to a greater or less abundance of water. And 
water plays a part in the performance of this act in each of its three 
forms—as gas, as liquid and as solid. Asa gas, the lesser amount 
of it—for I believe it to be less in amount on Mars than with us, 
though relatively to the other two states of the substance greater— 
acts to keep the day-sky clear, the night-sky cloudy, and thus to 
foster the summer melting which results in a diminution of perma- 
nent ice. As a liquid, in which state it is certainly much scarcer 
