1900.] LOWELL—MAR3 ON GLACIAL EPOCHS. 649 
falls but lies below latitude 35° both in America and Asia, and if our 
sky were clear, as that of Mars is, would be distinctly perceived by 
an onlooker as part, even if a detached part, of the polar cap. 
From its maximum the cap then dwindles till at its least, about two 
months after the summer solstice, it has so shrunk as to measure 
only about 40° across. 
With regard to our southern cap, it is not possible to affirm so 
positively either the maximum or the minimum because of the 
presence there of surrounding oceans. Nevertheless it is apparent, 
from the greater cold of corresponding southern latitudes, that at its 
greatest the cap would exceed its northern fellow if duly given 
ground ; while the summer glaciers of Terra del Fuego show that 
it would probably be the greater of the two at its minimum as well. 
Now on Mars the northern cap attains at its height a width of 
70°, which event occurs about one hundred of our days, or fifty- 
three of its own, after its winter solstice. About the middle of 
February this date corresponds to on earth. It then decreases 
regularly to its minimum, which takes place about the same time 
after its summer solstice. Near the minimum it remains some time. 
At this, its smallest compass, it measures only some 3° across. 
Confining ourselves now for the moment to what we can directly 
compare, the northern caps of the two planets, we find a difference 
in relative size between them at both their extremes in the same 
diréction. That of the earth is bigger than that of Mars, both at 
maximum and at minimum. As to the maximum, this happens in 
spite of the fact that the Martian year, and therefore the Martian 
winter, is nearly twice as long as our own, so that for nearly double 
the time any given northern latitude there is tilted away from the 
sun. 
This is not all. If we express analytically the area of a zone em- 
bracing the pole of given breadth in degrees 9, we find for its value 
(@) ==) 
ios 
ieee 6 ¢ 
¢=o 
€) == @ 
Where ~ is the radius of the sphere, ¢g the azimuth and 6 the 
polar distance, 
(8) == (8) 
= — 277? cos 9 = 27 #* (I—COS OB) 
(6) == (0) 
