656 LOWELL—MARS ON GLACIAL EPOCHS. [Nov. 16, 
18. With the minima the action is otherwise. Inasmuch as the 
greater heat received during the daylight hours by the southern 
hemisphere is exactly offset by the shortness of its season, it would 
seem at first as if there could be no difference in the total effect 
upon the two ice-caps. 
But further consideration shows a couple of factors which might, 
and possibly do, come in to qualify the effect. One is that the 
diurnal heat, being more intense though not so long continued, 
might work to more advantage. For the water would be the more 
likely to flow away the greater was the quantity of it manufac- 
tured ; or if it were caught up into the air more of it would be 
wafted away beyond reach of refreezing. It is the freshly acquired 
mobility that does the business. As ice the substance is chained to 
the spot ; as water or vapor it is free to roam; and natural condi- 
tions at once transport it out of the region and so out of the 
problem. 
The second factor is due to the action of the intervening nights. 
The vapor set free during the hours of sunshine is not all deposited 
during the night, as is witnessed by the presence of sunrise clouds. 
Such part as is not precipitated forms a blanket for the ground, pre- 
venting the heat of the surface from being radiated off into space. 
The greater the evaporation during the day the denser, other 
things equal, would be the cover-lid at night and thus the less heat 
be permitted to escape. This saving of heat is just so much to the 
good in the struggle of relative dissipation on the side of the 
southern hemisphere. ‘The next day does not find so much to undo 
before it can make its own advance. ‘Thus the whole effect in melt- 
ing the snow would be greater upon that hemisphere whose summer 
happens to be the more intense. 
19. It would appear then that on Mars not only has the eccen- 
tricity no tendency to foster the retention of an extensive ice-cap 
about the pole of that hemisphere which has its summer solstice 
near perihelion, but that the permanent accumulation there is 
actually less than at the opposite pole. 
20. Now suppose the total deposit of ice in winter to increase. 
Call 
a, the southern cap at its maximum. 
5, the same at its minimum. 
